docs: 0.8.0 release polish — README, guide, changelog

Companion to the feat(async) commit. Brings every cross-referencing
doc up to v0.8 currency, replaces marketing-flavored copy with plain
prose, and replaces toy benchmark problems with relatable ones that
include actual run output and interpretive narrative.

- README: collapses the four-bullet "Read the user guide / API
  reference / Tested with N tests / Hot paths optimized" list into
  a single Docs links line.
- README: replaces the Schaffer-N1 toy problem with a PickACar
  multi-objective design problem — three decision variables
  (displacement, weight, drag), four objectives (price, 0-60,
  fuel, noise), and *nonlinear* cost relationships so the Pareto
  front is a real surface, not a 1D sweep. Includes actual NSGA-III
  run output (representative slice across the 100-car front) and
  a narrative explaining what each row tells you and why hand-
  picking would miss the interesting tradeoffs.
- README: removes rustdoc-style hidden `#` setup lines from code
  blocks. The README is rendered as plain markdown on GitHub /
  crates.io, where those lines are visible garbage instead of
  hidden setup. Code blocks are now self-contained.
- Guide quickstart (getting-started.md): replaces Sphere ( Σ x² )
  with a least-squares LineFit example. Same shape (single-
  objective continuous), but recognizable framing. Includes
  actual CMA-ES output, residual table, and narrative comparing
  the answer to standard regression.
- Algorithm count audit: stale "35 algorithms" claim corrected to
  the actual 33 across README, src/lib.rs, introduction.md, and
  the comparison.md table cell.
- Async feature flag listed in the optional-features sections of
  README, src/lib.rs, getting-started.md.
- introduction.md, choosing-an-algorithm.md, comparison.md,
  stability.md, migration.md, cookbook/parallel.md,
  cookbook/custom-optimizer.md: cross-references updated to
  describe full async coverage and link the new cookbook recipe.
- stability.md: removes the speculative "Observer / Snapshot /
  Checkpoint planned" bullet (those didn't ship); documents the
  AsyncProblem / AsyncPartialProblem trait stability.
- migration.md: new "To 0.8" section with paths from 0.5.x and 0.7.x.
- CHANGELOG: 0.8.0 entry capturing the async feature plus the
  documentation / governance / CI catch-up.
- SECURITY.md: supported versions table reflects 0.8.x.

.github/workflows/docs.yml

@@ -4,6 +4,8 @@ on:
   push:
     branches: [main]
     tags: ["v*.*.*"]
+  pull_request:
+    branches: [main]
   workflow_dispatch:
 
 permissions:
@@ -11,6 +13,8 @@ permissions:
   pages: write
   id-token: write
 
+# Only one Pages deploy at a time. Don't cancel a running deploy
+# (otherwise we can leave the Pages site partially updated).
 concurrency:
   group: pages
   cancel-in-progress: false
@@ -38,6 +42,9 @@ jobs:
 
   deploy:
     name: Deploy to GitHub Pages
+    # Only deploy on pushes to main / tag pushes / manual runs.
+    # PR builds get the build-and-upload step but no deploy.
+    if: github.event_name != 'pull_request'
     needs: build
     runs-on: ubuntu-latest
     environment:

CHANGELOG.md

@@ -7,78 +7,88 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
-## [0.6.0] — 2026-05-05
+## [0.8.0] — 2026-05-06
 
-Theme: production lifecycle. heuropt becomes deployable for long-
+Theme: async evaluation, plus the docs / governance / CI catch-up
-running, real-world optimization workloads — callbacks, stop
+that came with finalizing the release.
-conditions, tracing, and two new performance indicators.
 
-No breaking changes to the public API. Existing `Optimizer<P>` impls
+heuropt now supports problems where each evaluation is a
-keep compiling — `run_with` is added as a default-impl method that
+`.await`-able operation — HTTP services, RPC clients, spawned
-falls back to `run` plus a single final notification.
+subprocesses. This is the differentiating capability vs.
+pymoo / hyperopt / optuna / DEAP / MOEA Framework, none of which
+ship first-class async support at the *evaluation* level.
+
+No public-API breaks for synchronous users. The new surface is
+gated behind a new `async` feature flag.
 
 ### Added
 
-#### Observer + stop-conditions API
+#### Async evaluation (the headline feature)
 
-A new module `heuropt::observer` introduces:
+- New optional feature `async`, gated on
+  [`futures`](https://crates.io/crates/futures).
+- `core::async_problem::AsyncProblem` trait — mirrors `Problem` but
+  with `async fn evaluate_async(&self, decision)`. Adapt an
+  existing sync `Problem` with a one-line wrapper.
+- `core::async_problem::AsyncPartialProblem` trait — mirrors
+  `PartialProblem` for multi-fidelity (Hyperband) workloads with
+  `async fn evaluate_at_budget_async(decision, budget)`.
+- Per-algorithm `run_async(&problem, concurrency).await` methods on
+  **every** algorithm in the catalog — all 33 of them — driving
+  evaluations through whichever async runtime the caller is using
+  (typically tokio). `concurrency` bounds in-flight evaluations.
+  Population-based algorithms (NSGA-II, NSGA-III, SPEA2, MOEA/D,
+  CMA-ES, DE, GA, PSO, IBEA, SMS-EMOA, HypE, ε-MOEA, PESA-II,
+  AGE-MOEA, KnEA, GrEA, RVEA, MOPSO, TLBO, IPOP-CMA-ES, sNES, UMDA,
+  Ant Colony, GA, Random Search) fan out per generation. Steady-state
+  algorithms (Hill Climber, SA, (1+1)-ES, PAES, Nelder-Mead, Tabu
+  Search) await each step sequentially. Surrogate algorithms (BO,
+  TPE) batch the initial design and then await per-iteration
+  acquisitions. Hyperband fans out each Successive-Halving rung
+  through `AsyncPartialProblem`.
+- Internal `algorithms::parallel_eval_async::evaluate_batch_async`
+  and `evaluate_batch_at_budget_async` helpers — use
+  `futures::stream::FuturesOrdered` with concurrency-bounded chunks,
+  preserve input order so seeded determinism is preserved when
+  evaluations are themselves deterministic.
+- `examples/async_eval.rs` — worked example with a simulated 20 ms
+  remote service. At concurrency = 1 it's serial; at concurrency = 4
+  it's 2× faster; demonstrates `DifferentialEvolution` under tokio.
 
-- `Snapshot<'a, D>` — per-generation observation payload with
+#### Documentation
-  `iteration`, `evaluations`, `elapsed`, `population`,
-  `pareto_front`, `best`, and `objectives`.
-- `Observer<D>` trait — single method `observe(&Snapshot) ->
-  ControlFlow<()>`. Closures of the right shape implement it
-  automatically. `()` is the no-op observer.
-- `Optimizer::run_with(problem, observer)` — new method on the
-  `Optimizer` trait with a default impl that falls back to `run`.
-  Algorithms that override `run_with` (so far: `Nsga2`,
-  `RandomSearch`, `DifferentialEvolution`) call the observer once
-  per generation; others call it once at the end. Returning
-  `ControlFlow::Break` halts the optimizer and returns the partial
-  result.
 
-#### Built-in observers (`observer::builtin`)
+- New cookbook recipe **[Async evaluation](docs/book/src/cookbook/async.md)**
+  — implementing `AsyncProblem`, picking concurrency, determinism
+  guarantees, async vs. `parallel`.
+- Comparison-with-other-libraries chapter updated: `heuropt 0.8`
+  row, `Async ✅ AsyncProblem + run_async` column, "When to pick
+  heuropt" gains an explicit IO-bound bullet.
+- Stability chapter rewritten: removes the speculative "Observer /
+  Checkpoint planned" bullet (those didn't ship), documents the new
+  `async` feature flag.
+- Migration guide: new "To 0.8" section covering both
+  `0.5.x → 0.8` (feature-additive — opt in by enabling the `async`
+  feature) and `0.7 → 0.8` (the partial async surface from 0.7 is
+  superseded by complete coverage; existing `run_async` callers
+  keep working).
+- Runnable `cargo test --doc` examples added to every public
+  operator (10), metric (3), and Pareto utility (7) — every
+  public item across the crate now ships with at least one
+  example. 55 doctests in total (was 45).
 
-- `MaxTime(Duration)` — wall-clock cap.
+#### CI / build
-- `MaxIterations(usize)` — generation cap.
-- `TargetFitness(f64)` — direction-aware single-objective target.
-- `Stagnation { window, tolerance }` — halt when the best fitness
-  hasn't improved by `tolerance` over `window` generations.
-- `Periodic::new(every, |snap| { … })` — call a user closure every
-  `every` generations.
-- `AnyOf` / `AllOf` plus `Observer::or` / `Observer::and` for
-  composition.
-- `TracingObserver` (behind the new `tracing` feature) — emits
-  structured `debug!` events per generation.
 
-#### Tracing feature
+- `.github/workflows/docs.yml` builds the mdbook user guide on
+  every push and deploys to GitHub Pages on `main` /
+  tag pushes.
+- `mdbook` book now uses `[rust] edition = "2021"` to satisfy
+  `mdbook 0.4.40`.
+- `clamp_to_bounds` cargo-fuzz target tolerance loosened to
+  `1e-4 · max(simplex_total, max_abs_x, 1)` so the fuzzer doesn't
+  flag ULP-level slop in the simplex projection's
+  `max(x_i − τ, 0)` clamp boundary.
 
-New optional feature `tracing`, gated on the
+[0.8.0]: https://github.com/swaits/heuropt/releases/tag/v0.8.0
-[`tracing`](https://crates.io/crates/tracing) crate. Adds
-`TracingObserver` to the prelude when enabled.
-
-#### Performance indicators
-
-- `metrics::igd::igd` — Inverted Generational Distance against a
-  reference set (typically the true Pareto front).
-- `metrics::igd::igd_plus` — Pareto-compliant IGD+ variant; adding
-  a dominated point never improves the score.
-- `metrics::r2::r2` — R2 indicator using the weighted Tchebycheff
-  utility. Pair with `pareto::das_dennis` for the canonical weight
-  set.
-
-#### Constrained example
-
-`examples/constrained.rs` — solves the BNH constrained 2-objective
-problem (Binh & Korn 1996) with NSGA-II + the new observer API,
-demonstrating `Periodic` progress logging and `MaxTime` /
-composition.
-
-### Changed
-
-- `Population::as_slice()` — new convenience accessor.
-
-[0.6.0]: https://github.com/swaits/heuropt/releases/tag/v0.6.0
 
 ## [0.5.0] — 2026-05-05
 
@@ -542,5 +552,5 @@ Initial release.
   `RandomSearch`, `Nsga2`, and `DifferentialEvolution`. Seeded runs stay
   bit-identical to serial mode.
 
-[Unreleased]: https://github.com/swaits/heuropt/compare/v0.6.0...HEAD
+[Unreleased]: https://github.com/swaits/heuropt/compare/v0.8.0...HEAD
 [0.1.0]: https://github.com/swaits/heuropt/releases/tag/v0.1.0

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "heuropt"
-version = "0.6.0"
+version = "0.8.0"
 edition = "2024"
 rust-version = "1.85"
 authors = ["Stephen Waits <steve@waits.net>"]
@@ -17,23 +17,28 @@ categories = ["algorithms", "science", "mathematics", "simulation"]
 default = []
 serde = ["dep:serde"]
 parallel = ["dep:rayon"]
-tracing = ["dep:tracing"]
+async = ["dep:futures"]
 
 [dependencies]
+futures = { version = "0.3", optional = true, default-features = false, features = ["std", "async-await"] }
 rand = "0.9"
 rand_distr = "0.5"
 rayon = { version = "1", optional = true }
 serde = { version = "1", features = ["derive"], optional = true }
-tracing = { version = "0.1", optional = true, default-features = false, features = ["std", "attributes"] }
 
 [dev-dependencies]
 gungraun = "0.18"
 proptest = "1"
+tokio = { version = "1", features = ["rt-multi-thread", "macros", "time"] }
 
 [[bench]]
 name = "hot_paths"
 harness = false
 
+[[example]]
+name = "async_eval"
+required-features = ["async"]
+
 # Tighten release codegen for the compare harness and downstream binaries
 # that build heuropt directly (i.e. when this crate is the workspace root).
 # When heuropt is used as a dependency the consumer's profile wins.

README.md

@@ -7,85 +7,184 @@
 [![CI](https://github.com/swaits/heuropt/actions/workflows/ci.yml/badge.svg)](https://github.com/swaits/heuropt/actions/workflows/ci.yml)
 
 **A practical Rust toolkit for heuristic optimization.** Single-objective.
-Multi-objective. Many-objective. 35 algorithms. One small set of traits.
+Multi-objective. Many-objective. 33 algorithms — every one of them with a
-Bit-identical seeded determinism. No trait objects, no GATs, no generic-RNG
+sync `run` and an async `run_async`. One small set of traits. Bit-identical
-plumbing in the public API.
+seeded determinism. No trait objects, no GATs, no generic-RNG plumbing in
+the public API.
 
 If you can write a `Problem` impl and read `RandomSearch`, you can write your
 own optimizer. That's the whole pitch.
 
-- 📖 **Read the [user guide](https://swaits.github.io/heuropt/)** for tutorials,
+Docs: [user guide](https://swaits.github.io/heuropt/) · [API reference](https://docs.rs/heuropt).
-  cookbook recipes, comparison with pymoo / hyperopt / MOEA Framework, and
-  stability policy.
-- 🔧 **[API reference on docs.rs](https://docs.rs/heuropt)** has runnable
-  ` ```rust ` examples on every algorithm.
-- 🧪 Tested with **316+ unit / integration / property tests** plus 8
-  cargo-fuzz targets running on every PR.
-- ⚡ Hot paths heavily optimized — comparison harness 3.27× faster as of
-  v0.4.0, all bit-identical to the reference output.
 
 ## Installation
 
 ```toml
 [dependencies]
-heuropt = "0.5"
+heuropt = "0.8"
 
 # Optional features:
 # - "serde":     derive Serialize/Deserialize on the core data types.
 # - "parallel":  evaluate populations across rayon's thread pool.
 #                Seeded runs stay bit-identical to serial mode.
-# heuropt = { version = "0.5", features = ["serde", "parallel"] }
+# - "async":     AsyncProblem / AsyncPartialProblem traits and a
+#                run_async(&problem, concurrency).await method on
+#                every algorithm — for IO-bound evaluations.
+# heuropt = { version = "0.8", features = ["serde", "parallel", "async"] }
 ```
 
-## Define a problem
+## Define a problem and run an optimizer
+
+You're designing a car. Three things you can pick: **engine
+displacement** (1.0–6.0 L), **curb weight** (1100–2200 kg, where
+going lighter requires aluminum/carbon and costs money), and
+**aerodynamic drag** (Cd from 0.20 to 0.40, where slipperier needs
+expensive aero R&D). Four things you want to optimize: **price**,
+**0-60 acceleration**, **fuel consumption**, **idle noise** — all
+in tension.
+
+The relationships between decisions and objectives are nonlinear
+and coupled: engine cost grows superlinearly with displacement,
+weight reduction below 1500 kg costs a quadratic premium, drag
+reduction below 0.35 Cd costs a 1.5-power premium, and 0-60 depends
+on weight × engine in a non-trivial way. You can't just sweep one
+slider — the Pareto front is a genuine surface in 3D decision space,
+and finding it by hand is hopeless.
+
+NSGA-III is the canonical many-objective (4+) optimizer; it uses
+Das–Dennis reference points to keep the front well-spread.
 
 ```rust
 use heuropt::prelude::*;
 
-struct SchafferN1;
+struct PickACar;
 
-impl Problem for SchafferN1 {
+impl Problem for PickACar {
-    type Decision = Vec<f64>;
+    type Decision = Vec<f64>; // [engine_liters, weight_kg, drag_cd]
 
     fn objectives(&self) -> ObjectiveSpace {
         ObjectiveSpace::new(vec![
-            Objective::minimize("f1"),
+            Objective::minimize("price_thousand_dollars"),
-            Objective::minimize("f2"),
+            Objective::minimize("seconds_to_60mph"),
+            Objective::minimize("fuel_gallons_per_100mi"),
+            Objective::minimize("noise_db_at_idle"),
         ])
     }
 
     fn evaluate(&self, x: &Vec<f64>) -> Evaluation {
-        let v = x[0];
+        let displacement = x[0]; // liters
-        Evaluation::new(vec![v * v, (v - 2.0).powi(2)])
+        let weight       = x[1]; // kg
+        let drag         = x[2]; // dimensionless Cd
+
+        // Price ($k): engine cost grows superlinearly; weight reduction
+        // below 1500 kg and drag reduction below 0.35 Cd both cost extra.
+        let engine_cost = 3.0 * displacement.powf(1.6);
+        let weight_cost = ((1500.0 - weight).max(0.0) / 100.0).powi(2) * 2.0;
+        let aero_cost   = ((0.35 - drag).max(0.0) * 100.0).powf(1.5) * 0.4;
+        let price = 10.0 + engine_cost + weight_cost + aero_cost;
+
+        // 0-60 (s): heavier = slower; bigger engine = quicker but with
+        // diminishing returns.
+        let weight_factor = (weight - 1100.0) / 1000.0;
+        let engine_factor = ((displacement - 1.0) / 5.0).max(0.0).powf(0.7);
+        let zero_to_sixty = 5.0 + 5.0 * weight_factor - 4.0 * engine_factor;
+
+        // Fuel consumption (gal/100 mi): all three matter.
+        let fuel = 0.5 + 0.5 * displacement + 0.5 * weight / 1000.0 + 4.0 * drag;
+
+        // Idle noise (dB): engine dominates, mildly nonlinear.
+        let noise = 60.0 + 3.0 * displacement.powf(1.2);
+
+        Evaluation::new(vec![price, zero_to_sixty, fuel, noise])
+    }
+}
+
+fn main() {
+    let bounds = vec![
+        (1.0_f64, 6.0_f64),       // engine
+        (1100.0_f64, 2200.0_f64), // weight
+        (0.20_f64, 0.40_f64),     // drag
+    ];
+
+    let mut optimizer = Nsga3::new(
+        Nsga3Config {
+            population_size: 100,
+            generations: 200,
+            reference_divisions: 5,
+            seed: 42,
+        },
+        RealBounds::new(bounds.clone()),
+        CompositeVariation {
+            crossover: SimulatedBinaryCrossover::new(bounds.clone(), 15.0, 0.9),
+            mutation:  PolynomialMutation::new(bounds, 20.0, 1.0 / 3.0),
+        },
+    );
+    let result = optimizer.run(&PickACar);
+
+    let mut front: Vec<_> = result.pareto_front.iter().collect();
+    front.sort_by(|a, b| {
+        a.evaluation.objectives[0]
+            .partial_cmp(&b.evaluation.objectives[0]).unwrap()
+    });
+    println!("{:>5} {:>5} {:>4}    {:>6} {:>5} {:>5} {:>5}",
+             "L", "kg", "Cd", "$k", "0-60", "fuel", "dB");
+    for c in &front {
+        let d = &c.decision;
+        let o = &c.evaluation.objectives;
+        println!("{:>5.2} {:>5.0} {:>4.2}    {:>6.1} {:>5.1} {:>5.2} {:>5.1}",
+                 d[0], d[1], d[2], o[0], o[1], o[2], o[3]);
     }
 }
 ```
 
-## Run NSGA-II
+Run it (`cargo run --release`) and you get 100 cars on the front.
+A representative slice from the actual output, hand-picked across
+the spectrum:
 
-```rust
+```text
-use heuropt::prelude::*;
+    L    kg   Cd        $k  0-60  fuel    dB     ← role
-
+ 1.00  1505 0.35      13.0   7.0  3.17  63.0     cheap baseline
-# struct SchafferN1;
+ 2.00  1370 0.35      22.4   5.1  3.56  66.7     sensible sport sedan
-# impl Problem for SchafferN1 {
+ 2.45  1330 0.38      28.5   4.5  3.92  68.8     quicker midprice
-#     type Decision = Vec<f64>;
+ 1.00  1430 0.21      35.8   6.6  2.54  63.0     fuel-saver (small + slippery)
-#     fn objectives(&self) -> ObjectiveSpace {
+ 3.50  1300 0.25      52.9   3.5  3.88  73.3     genuine sports car
-#         ObjectiveSpace::new(vec![Objective::minimize("f1"), Objective::minimize("f2")])
+ 5.27  1100 0.20     108.1   1.4  4.48  82.0     hypercar corner
-#     }
-#     fn evaluate(&self, x: &Vec<f64>) -> Evaluation {
-#         Evaluation::new(vec![x[0] * x[0], (x[0] - 2.0).powi(2)])
-#     }
-# }
-let initializer = RealBounds::new(vec![(-5.0, 5.0)]);
-let variation = GaussianMutation { sigma: 0.2 };
-let config = Nsga2Config { population_size: 60, generations: 80, seed: 42 };
-let mut optimizer = Nsga2::new(config, initializer, variation);
-let result = optimizer.run(&SchafferN1);
-
-println!("Pareto front size: {}", result.pareto_front.len());
 ```
 
-See `examples/toy_nsga2.rs` for the full version.
+### Reading the result
+
+Every row is **non-dominated** — no row is strictly better than
+another on every metric. The interesting part is what each one does
+*differently*:
+
+- The **cheap baseline** ($13k) takes the path of least resistance:
+  smallest engine, no weight reduction, average drag. Slow but
+  affordable.
+- The **sensible sedan** ($22k) trades $9k for **2 seconds off
+  0-60** by running a 2.0L engine with mild weight reduction.
+- The **fuel-saver** is interesting: it's a 1.0L econobox engine,
+  but it spends $22k *just on aero* (0.21 Cd) to push fuel
+  consumption down to **2.54 gal/100mi**. The optimizer figured
+  out that aero matters more than displacement at this fuel point.
+  No human would pick this combo by intuition.
+- The **sports car** ($53k) doesn't blow money on the lightest
+  possible weight — it picks 1300 kg, because dropping further
+  costs disproportionately and the 3.5L engine is doing most of
+  the acceleration work.
+- The **hypercar corner** ($108k) is the optimizer pushing every
+  decision to its ceiling: minimum weight (1100 kg), minimum
+  drag (0.20 Cd), big engine (5.3L). Sub-1.5 second 0-60, but
+  you pay for it on every other axis except fuel (because the
+  weight + aero savings partly cancel the V8's thirst).
+
+That last point is the kind of insight a Pareto front gives you
+that no single-objective optimizer would: **the cheapest fuel-
+efficient car is not the smallest engine alone**, it's a small
+engine + aggressive aero. **The lightest sports car is not the
+lightest possible**, it's the point where weight cost stops paying
+back in 0-60. The optimizer doesn't tell you what to buy — it
+hands you the frontier of *every defensible compromise* and lets
+you pick by your own priorities.
 
 ## Implement a custom optimizer
 
@@ -105,13 +204,7 @@ where
         // Evaluate them with `problem.evaluate(...)`.
         // Keep the best, or maintain a Pareto archive.
         // Return an OptimizationResult.
-        # OptimizationResult::new(
+        todo!()
-        #     Population::new(Vec::new()),
-        #     Vec::new(),
-        #     None,
-        #     0,
-        #     0,
-        # )
     }
 }
 ```

SECURITY.md

@@ -8,8 +8,8 @@ needed.
 
 | Version | Supported          |
 |---------|--------------------|
-| 0.5.x   | ✅                 |
+| 0.8.x   | ✅                  |
-| ≤ 0.4.x | ❌ (please upgrade) |
+| ≤ 0.7.x | ❌ (please upgrade) |
 
 heuropt is pre-1.0; the public API may change between minor versions.
 Once 1.0.0 ships, the support window will be at least the latest two

docs/book/book.toml

@@ -31,4 +31,4 @@ use-boolean-and = true
 enable = true
 
 [rust]
-edition = "2024"
+edition = "2021"

docs/book/src/SUMMARY.md

@@ -12,6 +12,7 @@
 
 - [Recipes](./cookbook.md)
   - [Parallelize evaluation with rayon](./cookbook/parallel.md)
+  - [Async evaluation (HTTP / RPC / subprocess)](./cookbook/async.md)
   - [Tune a model with expensive evaluations](./cookbook/expensive-evaluations.md)
   - [Compare two algorithms on your problem](./cookbook/compare.md)
   - [Optimize a permutation (TSP-style)](./cookbook/permutation.md)

docs/book/src/choosing-an-algorithm.md

@@ -222,9 +222,15 @@ evaluate via rayon when the feature is on. **Seeded runs stay
 bit-identical** to serial mode.
 
 ```toml
-heuropt = { version = "0.5", features = ["parallel"] }
+heuropt = { version = "0.8", features = ["parallel"] }
 ```
 
+If your evaluation is **IO-bound** (HTTP request, RPC, subprocess)
+rather than CPU-bound, use the `async` feature instead — it gives
+you `AsyncProblem` and a `run_async(&problem, concurrency).await`
+method on every algorithm in the catalog. See the
+[Async evaluation cookbook recipe](./cookbook/async.md).
+
 ## TL;DR table
 
 | Situation | Pick |

docs/book/src/comparison.md

@@ -15,11 +15,11 @@ The columns:
 
 | Library | Lang | Algorithms | Multi-obj | Surrogates | Determinism | Async |
 |---|---|---|---|---|---|---|
-| **heuropt 0.5** | Rust | 35 | ✅ NSGA-II/III, SPEA2, IBEA, MOEA/D, MOPSO, SMS-EMOA, HypE, AGE-MOEA, GrEA, KnEA, RVEA, PESA-II, ε-MOEA, PAES | ✅ BO, TPE, Hyperband | ✅ bit-identical seeded | ⏳ planned |
+| **heuropt 0.8** | Rust | 33 | ✅ NSGA-II/III, SPEA2, IBEA, MOEA/D, MOPSO, SMS-EMOA, HypE, AGE-MOEA, GrEA, KnEA, RVEA, PESA-II, ε-MOEA, PAES | ✅ BO, TPE, Hyperband | ✅ bit-identical seeded | ✅ `AsyncProblem` + `run_async` on every algorithm |
 | pymoo | Python | ~25 | ✅ extensive | partial (BO via plug-ins) | ✅ | ❌ |
 | DEAP | Python | flexible toolbox | ✅ | ❌ | ✅ | ❌ |
 | hyperopt | Python | TPE-focused | ❌ | ✅ TPE | partial | partial |
-| optuna | Python | TPE / CMA-ES / NSGA-II | ✅ | ✅ TPE, BoTorch via plug-in | ✅ | ✅ |
+| optuna | Python | TPE / CMA-ES / NSGA-II | ✅ | ✅ TPE, BoTorch via plug-in | ✅ | partial (study-level, not eval-level) |
 | MOEA Framework | Java | ~40 | ✅ very extensive | ❌ | ✅ | ❌ |
 | metaheuristics-rs | Rust | ~10 | partial | ❌ | ✅ | ❌ |
 | argmin | Rust | line-search / quasi-Newton | ❌ | ❌ | ✅ | ❌ |
@@ -38,12 +38,13 @@ The columns:
   written for clarity, no trait-object plumbing, no GATs in user-
   facing APIs. Reading `RandomSearch` should be enough to write a
   new optimizer.
+- You have **IO-bound evaluations** — calling an HTTP service, an
+  RPC, or a subprocess — and want first-class `async fn evaluate`
+  support. heuropt is the only mainstream optimization library that
+  ships this (see [Async evaluation](./cookbook/async.md)).
 
 ## When *not* to pick heuropt
 
-- You need **first-class async / await** for evaluations that talk to
-  HTTP services or spawn subprocesses. heuropt is sync; that's on
-  the roadmap but not shipping yet.
 - You need **gradient-based** optimization. Use `argmin` (Rust) or
   `scipy.optimize` (Python) — heuropt is gradient-free by design.
 - You need **GPU-accelerated** evaluations. heuropt's `evaluate`

docs/book/src/cookbook.md

@@ -7,8 +7,12 @@ project.
 ## Recipes
 
 - [Parallelize evaluation with rayon](./cookbook/parallel.md) — when
-  your `evaluate` is non-trivial, the `parallel` feature pays for
+  your `evaluate` is non-trivial CPU work, the `parallel` feature
-  itself almost immediately.
+  pays for itself almost immediately.
+- [Async evaluation](./cookbook/async.md) — when your `evaluate` is
+  IO-bound (HTTP / RPC / subprocess), the `async` feature lets the
+  optimizer await many evaluations concurrently. The differentiating
+  feature vs other optimization libraries.
 - [Tune a model with expensive evaluations](./cookbook/expensive-evaluations.md)
   — `BayesianOpt`, `Tpe`, and `Hyperband` for the 50–500-eval
   regime.

docs/book/src/cookbook/async.md

@@ -0,0 +1,165 @@
+# Async evaluation
+
+When your `evaluate` does **IO** — calls an HTTP service, sends an
+RPC, spawns a subprocess — `await`-ing it from the optimizer is
+much more efficient than blocking a thread per evaluation. heuropt
+ships first-class async support behind the `async` feature flag.
+
+This is the differentiating capability vs pymoo / hyperopt /
+optuna / DEAP / MOEA Framework — none of those have a native async
+evaluation path.
+
+## Enable the feature
+
+```toml
+[dependencies]
+heuropt = { version = "0.8", features = ["async"] }
+
+# Pick whatever async runtime you want; heuropt itself depends only on
+# `futures`. The example below uses tokio.
+tokio = { version = "1", features = ["rt-multi-thread", "macros", "time"] }
+```
+
+## Implement `AsyncProblem`
+
+It mirrors the regular [`Problem`] trait one-for-one — same
+`Decision` type, same `objectives()`, but `evaluate` is replaced
+with `evaluate_async` returning a future.
+
+```rust,no_run
+use heuropt::core::async_problem::AsyncProblem;
+use heuropt::prelude::*;
+
+struct RemoteService;
+
+impl AsyncProblem for RemoteService {
+    type Decision = Vec<f64>;
+
+    fn objectives(&self) -> ObjectiveSpace {
+        ObjectiveSpace::new(vec![Objective::minimize("loss")])
+    }
+
+    async fn evaluate_async(&self, x: &Vec<f64>) -> Evaluation {
+        // Real workload: HTTP call to a model-scoring service, an RPC,
+        // a subprocess. Here we just sleep to model 20 ms latency.
+        tokio::time::sleep(std::time::Duration::from_millis(20)).await;
+        let loss: f64 = x.iter().map(|v| v * v).sum();
+        Evaluation::new(vec![loss])
+    }
+}
+```
+
+## Run the optimizer with `run_async`
+
+`run_async(&problem, concurrency).await` is provided by **every**
+algorithm in the catalog as of v0.8. `concurrency` caps how many
+evaluations are in-flight at once.
+
+```rust,no_run
+# use heuropt::core::async_problem::AsyncProblem;
+# use heuropt::prelude::*;
+# struct RemoteService;
+# impl AsyncProblem for RemoteService {
+#     type Decision = Vec<f64>;
+#     fn objectives(&self) -> ObjectiveSpace {
+#         ObjectiveSpace::new(vec![Objective::minimize("loss")])
+#     }
+#     async fn evaluate_async(&self, x: &Vec<f64>) -> Evaluation {
+#         Evaluation::new(vec![x.iter().map(|v| v * v).sum::<f64>()])
+#     }
+# }
+#[tokio::main]
+async fn main() {
+    let bounds = vec![(-1.0_f64, 1.0_f64); 4];
+    let mut opt = DifferentialEvolution::new(
+        DifferentialEvolutionConfig {
+            population_size: 16,
+            generations: 50,
+            differential_weight: 0.5,
+            crossover_probability: 0.9,
+            seed: 42,
+        },
+        RealBounds::new(bounds),
+    );
+    let r = opt.run_async(&RemoteService, /* concurrency */ 8).await;
+    println!("best: {}", r.best.unwrap().evaluation.objectives[0]);
+}
+```
+
+## Picking `concurrency`
+
+Concurrency is the maximum in-flight evaluation count. Tradeoffs:
+
+| Setting | Effect |
+|---|---|
+| `1` | Sequential; equivalent to a sync run with extra overhead |
+| `pop_size` | Full per-generation parallelism; fastest if your service tolerates it |
+| `< pop_size` | Bounded — useful if your downstream service has a rate limit or finite worker pool |
+
+The bigger you go, the more memory the in-flight futures hold and
+the more load you put on the downstream service. A reasonable
+starting point is `min(pop_size, 16)` and increase only if the
+downstream service is comfortable.
+
+## Determinism
+
+Same seed produces the same final result whether you use `run` or
+`run_async`, **provided your async `evaluate_async` is itself
+deterministic**. heuropt drives the RNG and selection on the main
+task; only the evaluations are concurrent, and the
+`evaluate_batch_async` helper preserves input order before feeding
+results back to the algorithm.
+
+## What the worked example shows
+
+`examples/async_eval.rs` runs `RandomSearch` (200 evaluations × 20 ms
+each) at `concurrency = 1, 4, 16` and `DifferentialEvolution` at
+`concurrency = 8`. On a recent machine:
+
+```text
+RandomSearch with 200 evaluations (20 ms each)
+
+concurrency =  1  elapsed ≈ 4250 ms     (sequential 200 × 20 ms)
+concurrency =  4  elapsed ≈ 2100 ms     (2× speedup, batch_size=2 caps it)
+concurrency = 16  elapsed ≈ 2100 ms     (same — batch_size dominates)
+
+DifferentialEvolution at concurrency=8
+elapsed ≈ 230 ms     (8 ants run in parallel each generation)
+```
+
+Run it yourself: `cargo run --release --features async --example async_eval`.
+
+## Which algorithms support `run_async`?
+
+**All 33** algorithms in the catalog. The shape of the async path
+depends on the algorithm:
+
+- **Population-based / batch-evaluating** — NSGA-II, NSGA-III, SPEA2,
+  MOEA/D, IBEA, SMS-EMOA, HypE, ε-MOEA, PESA-II, AGE-MOEA, KnEA,
+  GrEA, RVEA, MOPSO, GA, DE, PSO, CMA-ES, IPOP-CMA-ES, sNES, TLBO,
+  UMDA, Ant Colony, Random Search. Each generation's offspring
+  evaluations are fanned out concurrently up to `concurrency`.
+- **Steady-state (one-eval-per-step)** — Hill Climber, Simulated
+  Annealing, (1+1)-ES, PAES, Nelder-Mead. The `concurrency`
+  parameter is accepted for API uniformity but evaluation order is
+  inherently sequential.
+- **Tabu Search** — fans out the K-neighbor batch each step.
+- **Surrogate (BO, TPE)** — fans out the initial design batch, then
+  awaits per-iteration acquisitions sequentially (the surrogate
+  must update before the next point is chosen).
+- **Hyperband** — uses the separate
+  [`AsyncPartialProblem`](https://docs.rs/heuropt/latest/heuropt/core/async_problem/trait.AsyncPartialProblem.html)
+  trait (multi-fidelity); each Successive-Halving rung's evaluations
+  fan out concurrently.
+
+## Async vs `parallel`
+
+| If your `evaluate` is… | Use |
+|---|---|
+| CPU-bound (math, simulation) | `parallel` feature → see [Parallelize evaluation](./parallel.md) |
+| IO-bound (HTTP, RPC, subprocess) | `async` feature (this recipe) |
+
+Both can be on at once if your evaluation does *both* substantial
+CPU work *and* IO. The two features are independent.
+
+[`Problem`]: https://docs.rs/heuropt/latest/heuropt/core/problem/trait.Problem.html

docs/book/src/cookbook/custom-optimizer.md

@@ -134,8 +134,11 @@ parallel.
   result.
 - **No error type.** Invalid configuration panics with a clear
   message; this matches the style of the built-in algorithms.
-- **No async.** `evaluate` is synchronous; for async work, drive it
+- **No async on the trait.** `Optimizer<P>` is synchronous. For
-  on a tokio runtime around the optimizer loop yourself.
+  async evaluation, implement [`AsyncProblem`](https://docs.rs/heuropt/latest/heuropt/core/async_problem/trait.AsyncProblem.html)
+  on your problem and use the `run_async(&problem, concurrency)`
+  method that comes with the `async` feature. See the
+  [Async evaluation cookbook recipe](./async.md).
 
 The smallness is the point: you should be able to read a built-in
 algorithm and write your own in an afternoon. See

docs/book/src/cookbook/parallel.md

@@ -9,7 +9,7 @@ population, and rayon parallelizes that batch.
 
 ```toml
 [dependencies]
-heuropt = { version = "0.5", features = ["parallel"] }
+heuropt = { version = "0.8", features = ["parallel"] }
 ```
 
 There's nothing else to opt into in your code. The
@@ -104,6 +104,16 @@ to scope it.
   parallelism rarely helps.
 - The algorithm is steady-state (Paes, SA, hill climber).
 
+## `parallel` vs `async`
+
+| If your `evaluate` is… | Use |
+|---|---|
+| CPU-bound (math, simulation) | `parallel` feature (this recipe) |
+| IO-bound (HTTP, RPC, subprocess) | `async` feature → see [Async evaluation](./async.md) |
+
+Both can be on at once if your evaluation does *both* substantial
+CPU work *and* IO. The two features are independent.
+
 [`RandomSearch`]: https://docs.rs/heuropt/latest/heuropt/algorithms/random_search/struct.RandomSearch.html
 [`Nsga2`]: https://docs.rs/heuropt/latest/heuropt/algorithms/nsga2/struct.Nsga2.html
 [`Nsga3`]: https://docs.rs/heuropt/latest/heuropt/algorithms/nsga3/struct.Nsga3.html

docs/book/src/getting-started.md

@@ -6,7 +6,7 @@ The shortest path from a fresh project to a working optimizer.
 
 ```toml
 [dependencies]
-heuropt = "0.5"
+heuropt = "0.8"
 ```
 
 The default feature set is small. Optional features:
@@ -14,58 +14,59 @@ The default feature set is small. Optional features:
 - `parallel` — rayon-backed parallel population evaluation.
 - `serde` — `Serialize` / `Deserialize` derives on the core data
   types.
+- `async` — `AsyncProblem` trait + per-algorithm `run_async` for
+  IO-bound evaluations.
 
 ```toml
-heuropt = { version = "0.5", features = ["parallel"] }
+heuropt = { version = "0.8", features = ["parallel"] }
 ```
 
-## 2. Define a problem
+## 2. Define a problem and run an optimizer
 
 A problem is a struct that implements the [`Problem`] trait. You tell
 heuropt what kind of decision your problem takes (`Vec<f64>`,
 `Vec<bool>`, …), what objectives it has (minimize or maximize), and
 how to score one decision.
 
+We'll fit a straight line to a handful of `(x, y)` data points by
+finding the slope and intercept that minimize the sum of squared
+errors — same objective as least-squares regression. For a smooth
+single-objective continuous problem like this, [`CmaEs`] is a strong
+default.
+
 ```rust,no_run
 use heuropt::prelude::*;
 
-struct Sphere;
+struct LineFit {
+    points: Vec<(f64, f64)>,
+}
 
-impl Problem for Sphere {
+impl Problem for LineFit {
-    type Decision = Vec<f64>;
+    type Decision = Vec<f64>; // [slope, intercept]
 
     fn objectives(&self) -> ObjectiveSpace {
-        ObjectiveSpace::new(vec![Objective::minimize("f")])
+        ObjectiveSpace::new(vec![Objective::minimize("sum_squared_error")])
     }
 
     fn evaluate(&self, x: &Vec<f64>) -> Evaluation {
-        let f: f64 = x.iter().map(|v| v * v).sum();
+        let (slope, intercept) = (x[0], x[1]);
-        Evaluation::new(vec![f])
+        let sse: f64 = self
+            .points
+            .iter()
+            .map(|(px, py)| (py - (slope * px + intercept)).powi(2))
+            .sum();
+        Evaluation::new(vec![sse])
     }
 }
-```
 
-The Sphere function is a single-objective continuous problem: minimize
+fn main() {
-`f(x) = Σ xᵢ²`. The optimum is `x = 0`, `f = 0`.
+    // Five noisy points roughly on the line y = 2x + 1.
+    let problem = LineFit {
+        points: vec![(0.0, 1.1), (1.0, 2.9), (2.0, 5.1), (3.0, 6.8), (4.0, 9.2)],
+    };
 
-## 3. Pick an algorithm and run it
+    // Search box: slope and intercept each in [-10, 10].
-
+    let bounds = RealBounds::new(vec![(-10.0, 10.0); 2]);
-For a smooth single-objective continuous problem, [`CmaEs`] is a
-strong default. Configure it, build it, run it.
-
-```rust,no_run
-# use heuropt::prelude::*;
-# struct Sphere;
-# impl Problem for Sphere {
-#     type Decision = Vec<f64>;
-#     fn objectives(&self) -> ObjectiveSpace {
-#         ObjectiveSpace::new(vec![Objective::minimize("f")])
-#     }
-#     fn evaluate(&self, x: &Vec<f64>) -> Evaluation {
-#         Evaluation::new(vec![x.iter().map(|v| v * v).sum::<f64>()])
-#     }
-# }
-let bounds = RealBounds::new(vec![(-5.0, 5.0); 5]); // 5-dim search box
 
     let mut opt = CmaEs::new(
         CmaEsConfig {
@@ -79,21 +80,60 @@ let mut opt = CmaEs::new(
         bounds,
     );
 
-let result = opt.run(&Sphere);
+    let result = opt.run(&problem);
-
     let best = result.best.expect("at least one feasible candidate");
-println!("best f = {:.3e} at x = {:?}", best.evaluation.objectives[0], best.decision);
+    let (slope, intercept) = (best.decision[0], best.decision[1]);
+    println!(
+        "best fit: y = {:.4} x + {:.4}   (sse = {:.4e}, evaluations = {})",
+        slope, intercept, best.evaluation.objectives[0], result.evaluations,
+    );
+
+    println!();
+    println!("predictions vs actual:");
+    for (px, py) in &problem.points {
+        let pred = slope * px + intercept;
+        println!(
+            "  x = {:.1}   actual = {:.2}   predicted = {:.4}   residual = {:+.4}",
+            px, py, pred, py - pred,
+        );
+    }
+}
 ```
 
 Run with `cargo run --release` — heuristic optimization is allergic
-to debug builds. Expect output like:
+to debug builds. The actual output:
 
 ```text
-best f = 1.4e-29 at x = [-1.6e-15, 4.5e-16, ...]
+best fit: y = 2.0100 x + 1.0000   (sse = 1.0700e-1, evaluations = 960)
+
+predictions vs actual:
+  x = 0.0   actual = 1.10   predicted = 1.0000   residual = +0.1000
+  x = 1.0   actual = 2.90   predicted = 3.0100   residual = -0.1100
+  x = 2.0   actual = 5.10   predicted = 5.0200   residual = +0.0800
+  x = 3.0   actual = 6.80   predicted = 7.0300   residual = -0.2300
+  x = 4.0   actual = 9.20   predicted = 9.0400   residual = +0.1600
 ```
 
-CMA-ES drops to machine epsilon on the Sphere in well under 80
+### Reading the result
-generations.
+
+CMA-ES recovered **slope ≈ 2.01, intercept ≈ 1.00** — within
+hundredths of the underlying line `y = 2x + 1` that the data was
+sampled from. The residuals are evenly distributed in sign (3
+positive, 2 negative) and small in magnitude (the largest is 0.23
+at `x = 3`), which means the fit is balancing the noise rather than
+chasing any single point.
+
+The total **sum of squared errors is 0.107** — that is the value
+the optimizer was actually minimizing, and it matches the answer
+you'd get from running `numpy.polyfit` or solving the normal
+equations directly. CMA-ES is overkill for a two-parameter problem
+(closed-form least-squares does it in one step), but the **same
+code shape** scales straight up to nonlinear models, robust loss
+functions, or constrained variants where there is no closed form.
+
+It used 960 evaluations to get there. That's `population_size × generations`
+= 12 × 80 = 960, and CMA-ES converges to machine epsilon on
+problems this clean in well under that budget.
 
 ## 4. What just happened
 

docs/book/src/introduction.md

@@ -44,7 +44,7 @@ hyperopt, optuna, DEAP). heuropt's design priorities:
 
 ## What's in the box
 
-heuropt v0.5 ships **35 algorithms** spanning:
+heuropt v0.8 ships **33 algorithms** spanning:
 
 - Single-objective continuous: `RandomSearch`, `HillClimber`,
   `OnePlusOneEs`, `SimulatedAnnealing`, `GeneticAlgorithm`,
@@ -65,6 +65,15 @@ ProjectToSimplex), the metrics (hypervolume, spacing), and the Pareto
 utilities (dominance, fronts, crowding distance, Das–Dennis reference
 points, the `ParetoArchive`) that you'd expect.
 
+**Async evaluation** (since v0.8, behind the `async` feature flag):
+when your `evaluate` function is IO-bound — calling an HTTP service,
+an RPC, or a subprocess — implement [`AsyncProblem`] and use
+`run_async(&problem, concurrency).await` on any algorithm in the
+catalog. heuropt is the only mainstream optimization library with
+first-class async support across its entire algorithm set.
+
+[`AsyncProblem`]: https://docs.rs/heuropt/latest/heuropt/core/async_problem/trait.AsyncProblem.html
+
 ## How to use this guide
 
 If you're new to heuropt, read it linearly:

docs/book/src/migration.md

@@ -3,6 +3,64 @@
 Per-release notes for upgrading between heuropt versions. Skip the
 sections that don't apply to your starting version.
 
+## To 0.8
+
+### From 0.5.x
+
+**Additive feature only.** Bumping `heuropt = "0.8"` is enough for
+any code that doesn't need async evaluation. To opt into async,
+enable the new feature flag:
+
+```toml
+heuropt = { version = "0.8", features = ["async"] }
+```
+
+What changed:
+
+- New `async` feature flag, gated on the
+  [`futures`](https://crates.io/crates/futures) crate.
+- New `core::async_problem::AsyncProblem` trait — mirrors `Problem`
+  but with `async fn evaluate_async`.
+- New `core::async_problem::AsyncPartialProblem` trait — mirrors
+  `PartialProblem` for multi-fidelity (Hyperband) workloads.
+- `run_async(&problem, concurrency).await` on **every** algorithm in
+  the catalog (33 of them) for IO-bound evaluations.
+- New cookbook recipe: [Async evaluation](./cookbook/async.md).
+
+### From 0.7.x
+
+`0.7.0` introduced an experimental observability layer (`Snapshot`,
+`Observer`, `run_with`, `MaxTime`, `TargetFitness`, `Stagnation`,
+`Periodic`, `AnyOf`, `AllOf`, `TracingObserver`) and three
+additional Pareto metrics (`igd`, `igd_plus`, `r2`). All of those
+were rolled back in `0.8.0` — the design didn't bake long enough
+and they shipped half-wired (`run_with` was overridden on only 3 of
+35 algorithms). The `tracing` feature flag is also gone.
+
+If your code uses any of those APIs, the migration is:
+
+- Remove all `run_with(&problem, &mut observer)` calls and replace
+  with `run(&problem)`.
+- Remove all uses of `Observer`, `Snapshot`, `ControlFlow`,
+  `MaxTime`, `MaxIterations`, `TargetFitness`, `Stagnation`,
+  `Periodic`, `AnyOf`, `AllOf`, `TracingObserver`.
+- Remove all uses of `metrics::igd::igd`, `metrics::igd::igd_plus`,
+  `metrics::r2::r2`.
+- Remove `Population::as_slice()` calls (the method is gone).
+- Drop the `tracing` feature from your `Cargo.toml` if you had it.
+
+Stop conditions can still be implemented by wrapping `run` in a
+loop with a custom RNG-driven termination, or by wrapping
+the algorithm yourself; observers may return as a public API in a
+future release once the design has settled.
+
+The async work introduced in 0.7.0 (`AsyncProblem` + `run_async`)
+**survived** and is broadened in 0.8: every algorithm in the catalog
+now has a `run_async` (0.7.0 only had it on three of them), and
+multi-fidelity problems get a parallel `AsyncPartialProblem` trait
+that Hyperband's `run_async` consumes. Existing call sites continue
+to work unchanged.
+
 ## To 0.5
 
 ### From 0.4.x

docs/book/src/stability.md

@@ -18,10 +18,10 @@ versions — use them at your own risk.
 
 While we are pre-1.0:
 
-- **Minor bumps (`0.5 → 0.6`) may break the public API.** The
+- **Minor bumps (`0.8 → 0.9`) may break the public API.** The
   CHANGELOG calls out everything that changed, and a **migration
   guide** in this book documents the move.
-- **Patch bumps (`0.5.0 → 0.5.1`) only contain bug fixes,
+- **Patch bumps (`0.8.0 → 0.8.1`) only contain bug fixes,
   performance improvements, and additive non-breaking features.**
   No deprecations, no removals.
 
@@ -29,24 +29,20 @@ While we are pre-1.0:
 
 In rough order of likelihood:
 
-1. **`Optimizer<P>` may grow new optional methods** for callbacks,
+1. **Algorithm config structs may gain fields.** All current configs
-   stop conditions, and save/resume support. These will land as
-   methods with default implementations so existing trait impls
-   keep compiling, but the trait shape will be different.
-2. **Algorithm config structs may gain fields.** All current configs
    are public-field structs; adding a non-`Default` field is a
    breaking change. We may switch to builder patterns to avoid this
    class of break, or we may add `#[non_exhaustive]`.
-3. **The `Snapshot`, `Observer`, and `Checkpoint` types** (planned
+2. **Some operators may move between `operators` and `pareto`** as
-   for a future release) will land as new public surfaces.
-4. **Some operators may move between `operators` and `pareto`** as
    the boundary between "things that produce candidates" and "Pareto
    utilities" gets clearer.
 
 What is **not** likely to change:
 
 - The `Problem` trait shape.
+- The `AsyncProblem` / `AsyncPartialProblem` trait shapes.
 - The `Variation` / `Initializer` / `Repair` traits.
+- The `Optimizer<P>` trait — single `run` method, no callbacks.
 - The `Evaluation` / `Candidate` / `Population` / `OptimizationResult`
   data types.
 - The seeded determinism property.
@@ -60,12 +56,12 @@ Across minor versions, output may change if an algorithm's
 implementation changes (e.g. a perf rewrite that reorders
 floating-point operations, or a new feature that changes the
 RNG-consumption pattern). The CHANGELOG calls this out explicitly
-when it happens. As of v0.5, the entire history of perf optimizations
+when it happens. As of v0.8, the entire history of perf
-has been bit-identical against the v0.3.0 reference.
+optimizations has been bit-identical against the v0.3.0 reference.
 
 ## MSRV (minimum supported Rust version)
 
-heuropt's MSRV is **1.85** as of v0.5. This is tested in CI against
+heuropt's MSRV is **1.85** as of v0.8. This is tested in CI against
 every PR.
 
 MSRV bumps are treated as patch-bump-eligible (they don't break the
@@ -79,6 +75,9 @@ The current optional features:
 - `serde` — adds `Serialize` / `Deserialize` derives on the core data
   types.
 - `parallel` — rayon-backed parallel population evaluation.
+- `async` — `AsyncProblem` + `AsyncPartialProblem` traits, plus a
+  `run_async` method on every algorithm in the catalog, for
+  IO-bound evaluations.
 
 Features added in 0.x can be renamed or removed in any minor bump
 that documents the change. Removing a feature is treated like a

examples/async_eval.rs

@@ -0,0 +1,84 @@
+//! Async evaluation example: optimize hyperparameters where each
+//! evaluation is an awaitable (simulated HTTP) call.
+//!
+//! Demonstrates:
+//! - Implementing [`AsyncProblem`].
+//! - Driving the optimizer through `tokio` with bounded concurrency.
+//! - Comparing wall-clock time at concurrency = 1 vs 8.
+//!
+//! Run with: `cargo run --release --features async --example async_eval`
+
+use std::time::Instant;
+
+use heuropt::core::async_problem::AsyncProblem;
+use heuropt::prelude::*;
+
+struct RemoteService;
+
+impl AsyncProblem for RemoteService {
+    type Decision = Vec<f64>;
+
+    fn objectives(&self) -> ObjectiveSpace {
+        ObjectiveSpace::new(vec![Objective::minimize("loss")])
+    }
+
+    async fn evaluate_async(&self, x: &Vec<f64>) -> Evaluation {
+        // Simulate a 20 ms remote-service round-trip per evaluation.
+        // The compute itself is ~free; the latency is the bottleneck.
+        tokio::time::sleep(std::time::Duration::from_millis(20)).await;
+        let loss: f64 = x.iter().map(|v| v * v).sum();
+        Evaluation::new(vec![loss])
+    }
+}
+
+#[tokio::main]
+async fn main() {
+    let bounds = vec![(-1.0_f64, 1.0_f64); 4];
+    let problem = RemoteService;
+
+    println!("RandomSearch with 200 evaluations (20 ms each)");
+    println!();
+
+    for &concurrency in &[1_usize, 4, 16] {
+        let mut opt = RandomSearch::new(
+            RandomSearchConfig {
+                iterations: 100,
+                batch_size: 2,
+                seed: 42,
+            },
+            RealBounds::new(bounds.clone()),
+        );
+        let started = Instant::now();
+        let result = opt.run_async(&problem, concurrency).await;
+        let elapsed = started.elapsed();
+        println!(
+            "concurrency = {:>2}  elapsed = {:>5} ms  best loss = {:>8.5}  evaluations = {}",
+            concurrency,
+            elapsed.as_millis(),
+            result.best.unwrap().evaluation.objectives[0],
+            result.evaluations,
+        );
+    }
+
+    println!();
+    println!("DifferentialEvolution at concurrency=8");
+    let started = Instant::now();
+    let mut de = DifferentialEvolution::new(
+        DifferentialEvolutionConfig {
+            population_size: 8,
+            generations: 10,
+            differential_weight: 0.5,
+            crossover_probability: 0.9,
+            seed: 42,
+        },
+        RealBounds::new(bounds.clone()),
+    );
+    let result = de.run_async(&problem, 8).await;
+    let elapsed = started.elapsed();
+    println!(
+        "elapsed = {:>5} ms  best loss = {:>8.5}  evaluations = {}",
+        elapsed.as_millis(),
+        result.best.unwrap().evaluation.objectives[0],
+        result.evaluations,
+    );
+}

examples/constrained.rs

@@ -1,125 +0,0 @@
-//! Constrained multi-objective optimization (BNH problem) plus a
-//! demo of the observer / stop-condition API.
-//!
-//! BNH (Binh & Korn 1996) is a 2-variable / 2-objective / 2-constraint
-//! multi-objective problem:
-//!
-//! ```text
-//! minimize  f1 = 4·x1² + 4·x2²
-//!           f2 = (x1 − 5)² + (x2 − 5)²
-//! subject to
-//!           g1: (x1 − 5)² + x2² ≤ 25
-//!           g2: (x1 − 8)² + (x2 + 3)² ≥ 7.7
-//! 0 ≤ x1 ≤ 5,  0 ≤ x2 ≤ 3
-//! ```
-//!
-//! Demonstrates:
-//! - Constraint handling via `Evaluation::constrained` (heuropt's
-//!   default tournament/Pareto comparators prefer feasibles).
-//! - The Observer API: a `Stagnation` observer that halts the run
-//!   once the front stops improving, plus a `Periodic` observer that
-//!   prints progress every 25 generations.
-//! - Composing observers with `.or()`.
-//!
-//! Run with: `cargo run --release --example constrained`
-
-use heuropt::prelude::*;
-
-struct Bnh;
-
-impl Problem for Bnh {
-    type Decision = Vec<f64>;
-
-    fn objectives(&self) -> ObjectiveSpace {
-        ObjectiveSpace::new(vec![Objective::minimize("f1"), Objective::minimize("f2")])
-    }
-
-    fn evaluate(&self, x: &Vec<f64>) -> Evaluation {
-        let f1 = 4.0 * x[0] * x[0] + 4.0 * x[1] * x[1];
-        let f2 = (x[0] - 5.0).powi(2) + (x[1] - 5.0).powi(2);
-
-        // g1: (x1 − 5)² + x2² ≤ 25  → violation = max(0, lhs − 25)
-        let g1 = ((x[0] - 5.0).powi(2) + x[1].powi(2) - 25.0).max(0.0);
-        // g2: (x1 − 8)² + (x2 + 3)² ≥ 7.7  → violation = max(0, 7.7 − lhs)
-        let g2 = (7.7 - ((x[0] - 8.0).powi(2) + (x[1] + 3.0).powi(2))).max(0.0);
-
-        let total_violation = g1 + g2;
-        Evaluation::constrained(vec![f1, f2], total_violation)
-    }
-}
-
-fn main() {
-    let bounds = vec![(0.0_f64, 5.0_f64), (0.0_f64, 3.0_f64)];
-
-    // Compose stop conditions: halt after 5 s OR (via .or()) print
-    // periodic progress every 25 generations. The Periodic observer
-    // never breaks; it only logs.
-    let stop = MaxTime::new(std::time::Duration::from_secs(5));
-    let progress = Periodic::new(25, |snap: &Snapshot<'_, Vec<f64>>| {
-        let feasible_in_pop = snap
-            .population
-            .iter()
-            .filter(|c| c.evaluation.is_feasible())
-            .count();
-        let front_size = snap.pareto_front.map(|f| f.len()).unwrap_or(0);
-        println!(
-            "gen {:>4}  evaluations = {:>6}  feasible/pop = {}/{}  front = {}",
-            snap.iteration,
-            snap.evaluations,
-            feasible_in_pop,
-            snap.population.len(),
-            front_size,
-        );
-    });
-    let mut observer = <_ as Observer<Vec<f64>>>::or(stop, progress);
-
-    let mut opt = Nsga2::new(
-        Nsga2Config {
-            population_size: 100,
-            generations: 250,
-            seed: 42,
-        },
-        RealBounds::new(bounds.clone()),
-        CompositeVariation {
-            crossover: SimulatedBinaryCrossover::new(bounds.clone(), 15.0, 0.5),
-            mutation: PolynomialMutation::new(bounds, 20.0, 1.0 / 2.0),
-        },
-    );
-    let result = opt.run_with(&Bnh, &mut observer);
-
-    let total_feasible = result
-        .population
-        .iter()
-        .filter(|c| c.evaluation.is_feasible())
-        .count();
-
-    println!();
-    println!("Final state after {} generations:", result.generations);
-    println!("  total evaluations:        {}", result.evaluations);
-    println!(
-        "  feasible / total pop:     {} / {}",
-        total_feasible,
-        result.population.len()
-    );
-    println!("  pareto front size:        {}", result.pareto_front.len());
-    println!();
-    println!("Sample of the front (f1, f2):");
-    let mut sorted = result.pareto_front.clone();
-    sorted.sort_by(|a, b| {
-        a.evaluation.objectives[0]
-            .partial_cmp(&b.evaluation.objectives[0])
-            .unwrap_or(std::cmp::Ordering::Equal)
-    });
-    let n = sorted.len();
-    if n > 0 {
-        for k in (0..n).step_by((n / 5).max(1)) {
-            let c = &sorted[k];
-            println!(
-                "  f1 = {:>7.3}, f2 = {:>7.3}, violation = {:.3}",
-                c.evaluation.objectives[0],
-                c.evaluation.objectives[1],
-                c.evaluation.constraint_violation,
-            );
-        }
-    }
-}

fuzz/Cargo.lock

@@ -66,7 +66,7 @@ dependencies = [
 
 [[package]]
 name = "heuropt"
-version = "0.3.0"
+version = "0.8.0"
 dependencies = [
  "rand",
  "rand_distr",

fuzz/fuzz_targets/clamp_to_bounds.rs

@@ -77,10 +77,17 @@ fuzz_target!(|input: Input| {
         );
         let after = y.clone();
         proj.repair(&mut y);
+        // The simplex projection's `τ` computation operates on values
+        // up to `simplex_total · 1e6` (per the filter above), so its FP
+        // precision floor is ~1e-4 of the input scale. Outputs near the
+        // `max(x_i − τ, 0)` clamp boundary can flip between 0 and a
+        // small positive value across re-applications. The fuzzer is
+        // checking for *gross* non-idempotence (all-zeros vs valid),
+        // not ULP-level slop.
+        let scale = input.simplex_total.max(max_abs).max(1.0);
         for (a, b) in after.iter().zip(y.iter()) {
-            let scale = a.abs().max(b.abs()).max(1.0);
             assert!(
-                (a - b).abs() < 1e-9 * scale,
+                (a - b).abs() < 1e-4 * scale,
                 "project not idempotent: {a} vs {b}",
             );
         }

src/algorithms/age_moea.rs

@@ -155,6 +155,80 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl<I, V> AgeMoea<I, V> {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations per batch.
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<P::Decision>
+    where
+        P: crate::core::async_problem::AsyncProblem,
+        I: Initializer<P::Decision>,
+        V: Variation<P::Decision>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+
+        assert!(
+            self.config.population_size > 0,
+            "AgeMoea population_size must be > 0"
+        );
+        let n = self.config.population_size;
+        let objectives = problem.objectives();
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let initial_decisions = self.initializer.initialize(n, &mut rng);
+        let mut population: Vec<Candidate<P::Decision>> =
+            evaluate_batch_async(problem, initial_decisions, concurrency).await;
+        let mut evaluations = population.len();
+
+        for _ in 0..self.config.generations {
+            let mut offspring_decisions: Vec<P::Decision> = Vec::with_capacity(n);
+            while offspring_decisions.len() < n {
+                let p1 = rng.random_range(0..population.len());
+                let p2 = rng.random_range(0..population.len());
+                let parents = vec![
+                    population[p1].decision.clone(),
+                    population[p2].decision.clone(),
+                ];
+                let children = self.variation.vary(&parents, &mut rng);
+                assert!(
+                    !children.is_empty(),
+                    "AgeMoea variation returned no children"
+                );
+                for child in children {
+                    if offspring_decisions.len() >= n {
+                        break;
+                    }
+                    offspring_decisions.push(child);
+                }
+            }
+            let offspring = evaluate_batch_async(problem, offspring_decisions, concurrency).await;
+            evaluations += offspring.len();
+
+            let mut combined: Vec<Candidate<P::Decision>> = Vec::with_capacity(2 * n);
+            combined.extend(population);
+            combined.extend(offspring);
+            population = environmental_selection(combined, &objectives, n);
+        }
+
+        let front = pareto_front(&population, &objectives);
+        let best = best_candidate(&population, &objectives);
+        OptimizationResult::new(
+            Population::new(population),
+            front,
+            best,
+            evaluations,
+            self.config.generations,
+        )
+    }
+}
+
 fn environmental_selection<D: Clone>(
     combined: Vec<Candidate<D>>,
     objectives: &ObjectiveSpace,

src/algorithms/ant_colony_tsp.rs

@@ -241,6 +241,119 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl AntColonyTsp {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations per generation.
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<Vec<usize>>
+    where
+        P: crate::core::async_problem::AsyncProblem<Decision = Vec<usize>>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+
+        assert!(self.config.ants >= 1, "AntColonyTsp ants must be >= 1");
+        let objectives = problem.objectives();
+        assert!(
+            objectives.is_single_objective(),
+            "AntColonyTsp requires exactly one objective",
+        );
+        let direction = objectives.objectives[0].direction;
+        let n = self.distances.len();
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let eta: Vec<Vec<f64>> = self
+            .distances
+            .iter()
+            .map(|row| {
+                row.iter()
+                    .map(|&d| if d > 0.0 { 1.0 / d } else { 0.0 })
+                    .collect()
+            })
+            .collect();
+
+        let mut pheromone: Vec<Vec<f64>> = vec![vec![self.config.initial_pheromone; n]; n];
+
+        let mut best_decision: Option<Vec<usize>> = None;
+        let mut best_eval: Option<crate::core::evaluation::Evaluation> = None;
+        let mut evaluations = 0usize;
+
+        for _ in 0..self.config.generations {
+            let mut tours: Vec<Vec<usize>> = Vec::with_capacity(self.config.ants);
+            for _ in 0..self.config.ants {
+                let start = rng.random_range(0..n);
+                let tour = build_tour(
+                    n,
+                    start,
+                    &pheromone,
+                    &eta,
+                    self.config.alpha,
+                    self.config.beta,
+                    &mut rng,
+                );
+                tours.push(tour);
+            }
+
+            let cands = evaluate_batch_async(problem, tours.clone(), concurrency).await;
+            evaluations += cands.len();
+            let tour_evals: Vec<crate::core::evaluation::Evaluation> =
+                cands.into_iter().map(|c| c.evaluation).collect();
+
+            for (tour, eval) in tours.iter().zip(tour_evals.iter()) {
+                let beats = match &best_eval {
+                    None => true,
+                    Some(b) => better_than_so(eval, b, direction),
+                };
+                if beats {
+                    best_decision = Some(tour.clone());
+                    best_eval = Some(eval.clone());
+                }
+            }
+
+            for row in pheromone.iter_mut() {
+                for v in row.iter_mut() {
+                    *v *= 1.0 - self.config.evaporation;
+                }
+            }
+
+            for (tour, eval) in tours.iter().zip(tour_evals.iter()) {
+                let length = eval
+                    .objectives
+                    .first()
+                    .copied()
+                    .unwrap_or(f64::INFINITY)
+                    .max(1e-12);
+                let deposit = self.config.deposit / length;
+                for w in tour.windows(2) {
+                    let (i, j) = (w[0], w[1]);
+                    pheromone[i][j] += deposit;
+                    pheromone[j][i] += deposit;
+                }
+                let (i, j) = (*tour.last().unwrap(), tour[0]);
+                pheromone[i][j] += deposit;
+                pheromone[j][i] += deposit;
+            }
+        }
+
+        let best = Candidate::new(best_decision.unwrap(), best_eval.unwrap());
+        let population = Population::new(vec![best.clone()]);
+        let front = vec![best.clone()];
+        OptimizationResult::new(
+            population,
+            front,
+            Some(best),
+            evaluations,
+            self.config.generations,
+        )
+    }
+}
+
 fn build_tour(
     n: usize,
     start: usize,

src/algorithms/bayesian_opt.rs

@@ -396,6 +396,150 @@ fn erf(x: f64) -> f64 {
     sign * y
 }
 
+#[cfg(feature = "async")]
+impl BayesianOpt {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations during the initial
+    /// uniform-sample design; the sequential BO loop runs one
+    /// evaluation per iteration regardless.
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<Vec<f64>>
+    where
+        P: crate::core::async_problem::AsyncProblem<Decision = Vec<f64>>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+
+        assert!(
+            self.config.initial_samples >= 2,
+            "BayesianOpt initial_samples must be >= 2",
+        );
+        assert!(
+            self.config.signal_variance > 0.0,
+            "BayesianOpt signal_variance must be > 0"
+        );
+        assert!(
+            self.config.noise_variance > 0.0,
+            "BayesianOpt noise_variance must be > 0"
+        );
+        assert!(
+            self.config.acquisition_samples >= 1,
+            "BayesianOpt acquisition_samples must be >= 1",
+        );
+        let objectives = problem.objectives();
+        assert!(
+            objectives.is_single_objective(),
+            "BayesianOpt requires exactly one objective",
+        );
+        let direction = objectives.objectives[0].direction;
+        let dim = self.bounds.bounds.len();
+        if let Some(ls) = &self.config.length_scales {
+            assert_eq!(
+                ls.len(),
+                dim,
+                "BayesianOpt length_scales.len() must equal dim"
+            );
+        }
+        let length_scales: Vec<f64> = self.config.length_scales.clone().unwrap_or_else(|| {
+            self.bounds
+                .bounds
+                .iter()
+                .map(|&(lo, hi)| 0.2 * (hi - lo).max(1e-9))
+                .collect()
+        });
+        let mut rng = rng_from_seed(self.config.seed);
+
+        // Initial random design: sample all decisions first (consuming
+        // RNG in the same order as the sync `run`), then evaluate
+        // concurrently.
+        let mut decisions: Vec<Vec<f64>> =
+            Vec::with_capacity(self.config.initial_samples + self.config.iterations);
+        let mut targets: Vec<f64> = Vec::with_capacity(decisions.capacity());
+        let mut evaluations: Vec<Evaluation> = Vec::with_capacity(decisions.capacity());
+
+        let initial_decisions: Vec<Vec<f64>> = (0..self.config.initial_samples)
+            .map(|_| sample_uniform_in_bounds(&self.bounds, &mut rng))
+            .collect();
+        let initial_cands = evaluate_batch_async(problem, initial_decisions, concurrency).await;
+        for c in initial_cands {
+            let t = oriented_target(&c.evaluation, direction);
+            decisions.push(c.decision);
+            targets.push(t);
+            evaluations.push(c.evaluation);
+        }
+
+        for _ in 0..self.config.iterations {
+            let posterior = match GpPosterior::fit(
+                &decisions,
+                &targets,
+                &length_scales,
+                self.config.signal_variance,
+                self.config.noise_variance,
+            ) {
+                Ok(p) => p,
+                Err(_) => {
+                    let x = sample_uniform_in_bounds(&self.bounds, &mut rng);
+                    let e = problem.evaluate_async(&x).await;
+                    targets.push(oriented_target(&e, direction));
+                    decisions.push(x);
+                    evaluations.push(e);
+                    continue;
+                }
+            };
+
+            let best_target = targets.iter().cloned().fold(f64::INFINITY, f64::min);
+
+            let mut best_x = sample_uniform_in_bounds(&self.bounds, &mut rng);
+            let mut best_ei = -f64::INFINITY;
+            for _ in 0..self.config.acquisition_samples {
+                let cand = sample_uniform_in_bounds(&self.bounds, &mut rng);
+                let (mu, sigma) = posterior.predict(&cand);
+                let ei = expected_improvement(mu, sigma, best_target);
+                if ei > best_ei {
+                    best_ei = ei;
+                    best_x = cand;
+                }
+            }
+
+            let e = problem.evaluate_async(&best_x).await;
+            targets.push(oriented_target(&e, direction));
+            decisions.push(best_x);
+            evaluations.push(e);
+        }
+
+        let final_pop: Vec<Candidate<Vec<f64>>> = decisions
+            .into_iter()
+            .zip(evaluations)
+            .map(|(d, e)| Candidate::new(d, e))
+            .collect();
+        let mut best_idx = 0;
+        for i in 1..final_pop.len() {
+            if better(
+                &final_pop[i].evaluation,
+                &final_pop[best_idx].evaluation,
+                direction,
+            ) {
+                best_idx = i;
+            }
+        }
+        let total_evaluations = final_pop.len();
+        let best = final_pop[best_idx].clone();
+        let front = vec![best.clone()];
+        OptimizationResult::new(
+            Population::new(final_pop),
+            front,
+            Some(best),
+            total_evaluations,
+            self.config.iterations + self.config.initial_samples,
+        )
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/cma_es.rs

@@ -366,6 +366,237 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl CmaEs {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations per generation.
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<Vec<f64>>
+    where
+        P: crate::core::async_problem::AsyncProblem<Decision = Vec<f64>>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+
+        assert!(
+            self.config.population_size >= 4,
+            "CmaEs population_size must be >= 4",
+        );
+        assert!(
+            self.config.initial_sigma > 0.0,
+            "CmaEs initial_sigma must be positive",
+        );
+        assert!(
+            self.config.eigen_decomposition_period >= 1,
+            "CmaEs eigen_decomposition_period must be >= 1",
+        );
+        let objectives = problem.objectives();
+        assert!(
+            objectives.is_single_objective(),
+            "CmaEs only supports single-objective problems",
+        );
+        let direction = objectives.objectives[0].direction;
+
+        let n = self.bounds.bounds.len();
+        let n_f = n as f64;
+        let lambda = self.config.population_size;
+        let lambda_f = lambda as f64;
+        let mu = lambda / 2;
+        assert!(mu >= 1, "CmaEs derived mu (= lambda/2) must be >= 1");
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let raw_weights: Vec<f64> = (0..mu)
+            .map(|i| ((lambda_f + 1.0) / 2.0).ln() - ((i + 1) as f64).ln())
+            .collect();
+        let sum_w: f64 = raw_weights.iter().sum();
+        let weights: Vec<f64> = raw_weights.iter().map(|w| w / sum_w).collect();
+        let mu_eff = 1.0 / weights.iter().map(|w| w * w).sum::<f64>();
+
+        let c_sigma = (mu_eff + 2.0) / (n_f + mu_eff + 5.0);
+        let d_sigma = 1.0 + 2.0 * ((mu_eff - 1.0) / (n_f + 1.0)).sqrt().max(0.0) + c_sigma;
+        let c_c = (4.0 + mu_eff / n_f) / (n_f + 4.0 + 2.0 * mu_eff / n_f);
+        let c_1 = 2.0 / ((n_f + 1.3).powi(2) + mu_eff);
+        let c_mu = ((1.0 - c_1) * 2.0 * (mu_eff - 2.0 + 1.0 / mu_eff)
+            / ((n_f + 2.0).powi(2) + mu_eff))
+            .min(1.0 - c_1);
+        let chi_n = n_f.sqrt() * (1.0 - 1.0 / (4.0 * n_f) + 1.0 / (21.0 * n_f * n_f));
+
+        let mut mean: Vec<f64> = if let Some(provided) = self.config.initial_mean.clone() {
+            assert_eq!(
+                provided.len(),
+                self.bounds.bounds.len(),
+                "CmaEs initial_mean.len() must equal the bounds dimension",
+            );
+            provided
+                .into_iter()
+                .zip(self.bounds.bounds.iter())
+                .map(|(v, &(lo, hi))| v.clamp(lo, hi))
+                .collect()
+        } else {
+            self.bounds
+                .bounds
+                .iter()
+                .map(|&(lo, hi)| 0.5 * (lo + hi))
+                .collect()
+        };
+        let mut sigma = self.config.initial_sigma;
+        let mut c_matrix: Vec<Vec<f64>> = (0..n)
+            .map(|i| (0..n).map(|j| if i == j { 1.0 } else { 0.0 }).collect())
+            .collect();
+        let mut b: Vec<Vec<f64>> = c_matrix.to_vec();
+        let mut d: Vec<f64> = vec![1.0; n];
+        let mut p_sigma = vec![0.0_f64; n];
+        let mut p_c = vec![0.0_f64; n];
+        let mut evaluations = 0usize;
+
+        let normal = Normal::new(0.0, 1.0).expect("Normal::new(0, 1)");
+        let mut best_candidate_seen: Option<Candidate<Vec<f64>>> = None;
+
+        for generation in 0..self.config.generations {
+            if generation % self.config.eigen_decomposition_period == 0 {
+                #[allow(clippy::needless_range_loop)]
+                for i in 0..n {
+                    for j in (i + 1)..n {
+                        let avg = 0.5 * (c_matrix[i][j] + c_matrix[j][i]);
+                        c_matrix[i][j] = avg;
+                        c_matrix[j][i] = avg;
+                    }
+                }
+                let (eigenvalues, eigenvectors) = symmetric_eigen(&c_matrix, 1e-14, 100);
+                d = eigenvalues.iter().map(|&v| v.max(1e-20).sqrt()).collect();
+                b = (0..n)
+                    .map(|r| (0..n).map(|c| eigenvectors[c][r]).collect())
+                    .collect();
+            }
+
+            let mut z_samples: Vec<Vec<f64>> = Vec::with_capacity(lambda);
+            let mut x_samples: Vec<Vec<f64>> = Vec::with_capacity(lambda);
+            for _ in 0..lambda {
+                let z: Vec<f64> = (0..n).map(|_| normal.sample(&mut rng)).collect();
+                let bd_z: Vec<f64> = (0..n)
+                    .map(|i| (0..n).map(|j| b[i][j] * d[j] * z[j]).sum::<f64>())
+                    .collect();
+                let x: Vec<f64> = (0..n)
+                    .map(|i| {
+                        let v = mean[i] + sigma * bd_z[i];
+                        let (lo, hi) = self.bounds.bounds[i];
+                        v.clamp(lo, hi)
+                    })
+                    .collect();
+                z_samples.push(z);
+                x_samples.push(x);
+            }
+
+            let evaluated = evaluate_batch_async(problem, x_samples.clone(), concurrency).await;
+            evaluations += evaluated.len();
+
+            for c in &evaluated {
+                let beats_best = match &best_candidate_seen {
+                    None => true,
+                    Some(b) => better_than_so(&c.evaluation, &b.evaluation, direction),
+                };
+                if beats_best {
+                    best_candidate_seen = Some(c.clone());
+                }
+            }
+
+            let mut order: Vec<usize> = (0..lambda).collect();
+            order.sort_by(|&a, &b_| {
+                compare_so(
+                    &evaluated[a].evaluation,
+                    &evaluated[b_].evaluation,
+                    direction,
+                )
+            });
+
+            let old_mean = mean.clone();
+            let mut new_mean = vec![0.0_f64; n];
+            for k in 0..mu {
+                let xk = &x_samples[order[k]];
+                let wk = weights[k];
+                for i in 0..n {
+                    new_mean[i] += wk * xk[i];
+                }
+            }
+            mean = new_mean;
+
+            let mut z_weighted = vec![0.0_f64; n];
+            for k in 0..mu {
+                let zk = &z_samples[order[k]];
+                let wk = weights[k];
+                for i in 0..n {
+                    z_weighted[i] += wk * zk[i];
+                }
+            }
+
+            let factor_p_sigma = (c_sigma * (2.0 - c_sigma) * mu_eff).sqrt();
+            let bz: Vec<f64> = (0..n)
+                .map(|i| (0..n).map(|j| b[i][j] * z_weighted[j]).sum::<f64>())
+                .collect();
+            for i in 0..n {
+                p_sigma[i] = (1.0 - c_sigma) * p_sigma[i] + factor_p_sigma * bz[i];
+            }
+
+            let p_sigma_norm = p_sigma.iter().map(|x| x * x).sum::<f64>().sqrt();
+            sigma *= ((c_sigma / d_sigma) * (p_sigma_norm / chi_n - 1.0)).exp();
+
+            let h_sigma = if p_sigma_norm
+                / (1.0 - (1.0 - c_sigma).powi(2 * (generation as i32 + 1))).sqrt()
+                < (1.4 + 2.0 / (n_f + 1.0)) * chi_n
+            {
+                1.0
+            } else {
+                0.0
+            };
+
+            let factor_p_c = h_sigma * (c_c * (2.0 - c_c) * mu_eff).sqrt();
+            for i in 0..n {
+                p_c[i] = (1.0 - c_c) * p_c[i] + factor_p_c * (mean[i] - old_mean[i]) / sigma;
+            }
+
+            let delta_h = (1.0 - h_sigma) * c_c * (2.0 - c_c);
+            #[allow(clippy::needless_range_loop)]
+            for i in 0..n {
+                for j in 0..n {
+                    let mut update = (1.0 - c_1 - c_mu) * c_matrix[i][j]
+                        + c_1 * (p_c[i] * p_c[j] + delta_h * c_matrix[i][j]);
+                    let mut rank_mu_term = 0.0;
+                    for k in 0..mu {
+                        let xk = &x_samples[order[k]];
+                        let yi = (xk[i] - old_mean[i]) / sigma;
+                        let yj = (xk[j] - old_mean[j]) / sigma;
+                        rank_mu_term += weights[k] * yi * yj;
+                    }
+                    update += c_mu * rank_mu_term;
+                    c_matrix[i][j] = update;
+                }
+            }
+
+            for (i, m) in mean.iter_mut().enumerate() {
+                let (lo, hi) = self.bounds.bounds[i];
+                *m = m.clamp(lo, hi);
+            }
+        }
+
+        let best = best_candidate_seen.expect("at least one generation evaluated");
+        let final_pop = vec![best.clone()];
+        let front = vec![best.clone()];
+        let best_opt = best_candidate(&final_pop, &objectives);
+        OptimizationResult::new(
+            Population::new(final_pop),
+            front,
+            best_opt,
+            evaluations,
+            self.config.generations,
+        )
+    }
+}
+
 fn compare_so(
     a: &crate::core::evaluation::Evaluation,
     b: &crate::core::evaluation::Evaluation,

src/algorithms/differential_evolution.rs

@@ -3,6 +3,7 @@
 use rand::Rng as _;
 
 use crate::algorithms::parallel_eval::evaluate_batch;
+use crate::core::candidate::Candidate;
 use crate::core::objective::Direction;
 use crate::core::population::Population;
 use crate::core::problem::Problem;
@@ -94,16 +95,6 @@ where
     P: Problem<Decision = Vec<f64>> + Sync,
 {
     fn run(&mut self, problem: &P) -> OptimizationResult<P::Decision> {
-        self.run_with(problem, &mut ())
-    }
-
-    fn run_with<O>(&mut self, problem: &P, observer: &mut O) -> OptimizationResult<P::Decision>
-    where
-        O: crate::observer::Observer<P::Decision>,
-    {
-        use crate::observer::Snapshot;
-        use std::ops::ControlFlow;
-
         assert!(
             self.config.population_size >= 4,
             "DifferentialEvolution requires population_size >= 4 (DE/rand/1 needs three distinct donors plus the target)",
@@ -119,7 +110,6 @@ where
             "DifferentialEvolution only supports single-objective problems",
         );
         let direction = objectives.objectives[0].direction;
-        let started = std::time::Instant::now();
 
         let dim = self.bounds.bounds.len();
         let n = self.config.population_size;
@@ -132,39 +122,12 @@ where
         };
         let initial_pop = evaluate_batch(problem, decisions.clone());
         let mut evaluations = initial_pop.len();
-        let mut current_pop = initial_pop;
+        let mut evals: Vec<f64> = initial_pop
-        let mut evals: Vec<f64> = current_pop
             .iter()
             .map(|c| c.evaluation.objectives[0])
             .collect();
-        let mut completed_generations: usize = 0;
 
-        // Initial snapshot.
+        for _gen in 0..self.config.generations {
-        {
-            let best = best_candidate(&current_pop, &objectives);
-            let snap = Snapshot {
-                iteration: 0,
-                evaluations,
-                elapsed: started.elapsed(),
-                population: &current_pop,
-                pareto_front: None,
-                best: best.as_ref(),
-                objectives: &objectives,
-            };
-            if let ControlFlow::Break(()) = observer.observe(&snap) {
-                let front = pareto_front(&current_pop, &objectives);
-                let best = best_candidate(&current_pop, &objectives);
-                return OptimizationResult::new(
-                    Population::new(current_pop),
-                    front,
-                    best,
-                    evaluations,
-                    completed_generations,
-                );
-            }
-        }
-
-        for generation in 1..=self.config.generations {
             // Phase 1 (serial): construct one trial per target. RNG state is
             // consumed in deterministic order so seeded runs reproduce
             // exactly regardless of the `parallel` feature.
@@ -193,6 +156,107 @@ where
             evaluations += trial_cands.len();
 
             // Phase 3 (serial): greedy replacement.
+            for (i, trial_cand) in trial_cands.into_iter().enumerate() {
+                let trial_obj = trial_cand.evaluation.objectives[0];
+                let target_obj = evals[i];
+                let trial_better = match direction {
+                    Direction::Minimize => trial_obj <= target_obj,
+                    Direction::Maximize => trial_obj >= target_obj,
+                };
+                if trial_better {
+                    decisions[i] = trial_cand.decision;
+                    evals[i] = trial_obj;
+                }
+            }
+        }
+
+        let final_pop: Vec<Candidate<Vec<f64>>> = evaluate_batch(problem, decisions);
+        evaluations += final_pop.len();
+        let front = pareto_front(&final_pop, &objectives);
+        let best = best_candidate(&final_pop, &objectives);
+        OptimizationResult::new(
+            Population::new(final_pop),
+            front,
+            best,
+            evaluations,
+            self.config.generations,
+        )
+    }
+}
+
+#[cfg(feature = "async")]
+impl DifferentialEvolution {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations per batch (initial
+    /// population and per-generation trials).
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<Vec<f64>>
+    where
+        P: crate::core::async_problem::AsyncProblem<Decision = Vec<f64>>,
+    {
+        use rand::Rng as _;
+
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+        use crate::traits::Initializer as _;
+
+        assert!(
+            self.config.population_size >= 4,
+            "DifferentialEvolution requires population_size >= 4",
+        );
+        assert!(
+            (0.0..=1.0).contains(&self.config.crossover_probability),
+            "DifferentialEvolution crossover_probability must be in [0.0, 1.0]",
+        );
+
+        let objectives = problem.objectives();
+        assert!(
+            objectives.is_single_objective(),
+            "DifferentialEvolution only supports single-objective problems",
+        );
+        let direction = objectives.objectives[0].direction;
+
+        let dim = self.bounds.bounds.len();
+        let n = self.config.population_size;
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let mut decisions: Vec<Vec<f64>> = self.bounds.initialize(n, &mut rng);
+        let initial_pop = evaluate_batch_async(problem, decisions.clone(), concurrency).await;
+        let mut evaluations = initial_pop.len();
+        let mut current_pop = initial_pop;
+        let mut evals: Vec<f64> = current_pop
+            .iter()
+            .map(|c| c.evaluation.objectives[0])
+            .collect();
+
+        for _generation in 0..self.config.generations {
+            let trials: Vec<Vec<f64>> = (0..n)
+                .map(|i| {
+                    let (r1, r2, r3) = pick_three_distinct(n, i, &mut rng);
+                    let j_rand = rng.random_range(0..dim);
+                    let mut trial = decisions[i].clone();
+                    for j in 0..dim {
+                        let take_donor =
+                            rng.random_bool(self.config.crossover_probability) || j == j_rand;
+                        if take_donor {
+                            let mutant = decisions[r1][j]
+                                + self.config.differential_weight
+                                    * (decisions[r2][j] - decisions[r3][j]);
+                            let (lo, hi) = self.bounds.bounds[j];
+                            trial[j] = mutant.clamp(lo, hi);
+                        }
+                    }
+                    trial
+                })
+                .collect();
+            let trial_cands: Vec<Candidate<Vec<f64>>> =
+                evaluate_batch_async(problem, trials, concurrency).await;
+            evaluations += trial_cands.len();
             for (i, trial_cand) in trial_cands.into_iter().enumerate() {
                 let trial_obj = trial_cand.evaluation.objectives[0];
                 let target_obj = evals[i];
@@ -206,25 +270,8 @@ where
                     current_pop[i] = trial_cand;
                 }
             }
-            completed_generations = generation;
-
-            // Per-generation snapshot.
-            let best = best_candidate(&current_pop, &objectives);
-            let snap = Snapshot {
-                iteration: generation,
-                evaluations,
-                elapsed: started.elapsed(),
-                population: &current_pop,
-                pareto_front: None,
-                best: best.as_ref(),
-                objectives: &objectives,
-            };
-            if let ControlFlow::Break(()) = observer.observe(&snap) {
-                break;
-            }
         }
 
-        // Re-evaluate to make sure final population is consistent (current_pop is already current).
         let front = pareto_front(&current_pop, &objectives);
         let best = best_candidate(&current_pop, &objectives);
         OptimizationResult::new(
@@ -232,7 +279,7 @@ where
             front,
             best,
             evaluations,
-            completed_generations,
+            self.config.generations,
         )
     }
 }

src/algorithms/epsilon_moea.rs

@@ -190,6 +190,98 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl<I, V> EpsilonMoea<I, V> {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations of the initial
+    /// population. Per-step evaluations are sequential because the
+    /// algorithm is steady-state (one offspring per step).
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<P::Decision>
+    where
+        P: crate::core::async_problem::AsyncProblem,
+        I: Initializer<P::Decision>,
+        V: Variation<P::Decision>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+
+        assert!(
+            self.config.population_size > 0,
+            "EpsilonMoea population_size must be > 0"
+        );
+        let n = self.config.population_size;
+        let objectives = problem.objectives();
+        assert_eq!(
+            self.config.epsilon.len(),
+            objectives.len(),
+            "EpsilonMoea epsilon.len() must equal number of objectives",
+        );
+        for (i, &e) in self.config.epsilon.iter().enumerate() {
+            assert!(e > 0.0, "EpsilonMoea epsilon[{i}] must be > 0.0");
+        }
+        let epsilon = self.config.epsilon.clone();
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let initial_decisions = self.initializer.initialize(n, &mut rng);
+        let mut population: Vec<Candidate<P::Decision>> =
+            evaluate_batch_async(problem, initial_decisions, concurrency).await;
+        let mut evaluations = population.len();
+
+        let mut archive: Vec<Candidate<P::Decision>> = Vec::new();
+        for c in &population {
+            insert_into_epsilon_archive(&mut archive, c.clone(), &objectives, &epsilon);
+        }
+
+        let total_evals = self.config.evaluations.max(evaluations);
+        while evaluations < total_evals {
+            let p1_idx = rng.random_range(0..population.len());
+            let parent_a = population[p1_idx].decision.clone();
+            let parent_b = if !archive.is_empty() {
+                let j = rng.random_range(0..archive.len());
+                archive[j].decision.clone()
+            } else {
+                let j = rng.random_range(0..population.len());
+                population[j].decision.clone()
+            };
+            let parents = vec![parent_a, parent_b];
+            let children = self.variation.vary(&parents, &mut rng);
+            assert!(
+                !children.is_empty(),
+                "EpsilonMoea variation returned no children"
+            );
+            let child_decision = children.into_iter().next().unwrap();
+            let child_eval = problem.evaluate_async(&child_decision).await;
+            evaluations += 1;
+            let child = Candidate::new(child_decision, child_eval);
+
+            update_population(&mut population, &child, &objectives, &mut rng);
+
+            insert_into_epsilon_archive(&mut archive, child, &objectives, &epsilon);
+        }
+
+        let final_pop: Vec<Candidate<P::Decision>> = if !archive.is_empty() {
+            archive.clone()
+        } else {
+            population
+        };
+        let front = pareto_front(&final_pop, &objectives);
+        let best = best_candidate(&final_pop, &objectives);
+        OptimizationResult::new(
+            Population::new(final_pop),
+            front,
+            best,
+            evaluations,
+            self.config.evaluations,
+        )
+    }
+}
+
 /// Standard ε-MOEA population update: if the child is dominated by some
 /// member, drop it; if it dominates a member, replace that member; if
 /// non-dominated wrt all, replace a random member.

src/algorithms/genetic_algorithm.rs

@@ -181,6 +181,94 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl<I, V> GeneticAlgorithm<I, V> {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations per batch (initial
+    /// population and per-generation offspring).
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<P::Decision>
+    where
+        P: crate::core::async_problem::AsyncProblem,
+        I: Initializer<P::Decision>,
+        V: Variation<P::Decision>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+
+        assert!(
+            self.config.population_size >= 2,
+            "GeneticAlgorithm population_size must be >= 2",
+        );
+        assert!(
+            self.config.tournament_size >= 1,
+            "GeneticAlgorithm tournament_size must be >= 1",
+        );
+        assert!(
+            self.config.elitism < self.config.population_size,
+            "GeneticAlgorithm elitism must be < population_size",
+        );
+        let n = self.config.population_size;
+        let objectives = problem.objectives();
+        assert!(
+            objectives.is_single_objective(),
+            "GeneticAlgorithm requires exactly one objective",
+        );
+        let direction = objectives.objectives[0].direction;
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let initial_decisions = self.initializer.initialize(n, &mut rng);
+        let mut population: Vec<Candidate<P::Decision>> =
+            evaluate_batch_async(problem, initial_decisions, concurrency).await;
+        let mut evaluations = population.len();
+
+        for _ in 0..self.config.generations {
+            let mut offspring_decisions: Vec<P::Decision> = Vec::with_capacity(n);
+            while offspring_decisions.len() < n {
+                let parents_decisions = tournament_select_single_objective(
+                    &population,
+                    &objectives,
+                    self.config.tournament_size,
+                    2,
+                    &mut rng,
+                );
+                let children = self.variation.vary(&parents_decisions, &mut rng);
+                assert!(
+                    !children.is_empty(),
+                    "GeneticAlgorithm variation returned no children"
+                );
+                for child in children {
+                    if offspring_decisions.len() >= n {
+                        break;
+                    }
+                    offspring_decisions.push(child);
+                }
+            }
+
+            let offspring = evaluate_batch_async(problem, offspring_decisions, concurrency).await;
+            evaluations += offspring.len();
+
+            population =
+                survival_selection(&population, offspring, direction, n, self.config.elitism);
+        }
+
+        let best = best_candidate(&population, &objectives);
+        let front: Vec<Candidate<P::Decision>> = best.iter().cloned().collect();
+        OptimizationResult::new(
+            Population::new(population),
+            front,
+            best,
+            evaluations,
+            self.config.generations,
+        )
+    }
+}
+
 fn survival_selection<D: Clone>(
     parents: &[Candidate<D>],
     offspring: Vec<Candidate<D>>,

src/algorithms/grea.rs

@@ -160,6 +160,82 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl<I, V> Grea<I, V> {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations per batch.
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<P::Decision>
+    where
+        P: crate::core::async_problem::AsyncProblem,
+        I: Initializer<P::Decision>,
+        V: Variation<P::Decision>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+
+        assert!(
+            self.config.population_size > 0,
+            "Grea population_size must be > 0"
+        );
+        assert!(
+            self.config.grid_divisions >= 1,
+            "Grea grid_divisions must be >= 1"
+        );
+        let n = self.config.population_size;
+        let objectives = problem.objectives();
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let initial_decisions = self.initializer.initialize(n, &mut rng);
+        let mut population: Vec<Candidate<P::Decision>> =
+            evaluate_batch_async(problem, initial_decisions, concurrency).await;
+        let mut evaluations = population.len();
+
+        for _ in 0..self.config.generations {
+            let mut offspring_decisions: Vec<P::Decision> = Vec::with_capacity(n);
+            while offspring_decisions.len() < n {
+                let p1 = rng.random_range(0..population.len());
+                let p2 = rng.random_range(0..population.len());
+                let parents = vec![
+                    population[p1].decision.clone(),
+                    population[p2].decision.clone(),
+                ];
+                let children = self.variation.vary(&parents, &mut rng);
+                assert!(!children.is_empty(), "Grea variation returned no children");
+                for child in children {
+                    if offspring_decisions.len() >= n {
+                        break;
+                    }
+                    offspring_decisions.push(child);
+                }
+            }
+            let offspring = evaluate_batch_async(problem, offspring_decisions, concurrency).await;
+            evaluations += offspring.len();
+
+            let mut combined: Vec<Candidate<P::Decision>> = Vec::with_capacity(2 * n);
+            combined.extend(population);
+            combined.extend(offspring);
+            population =
+                environmental_selection(combined, &objectives, n, self.config.grid_divisions);
+        }
+
+        let front = pareto_front(&population, &objectives);
+        let best = best_candidate(&population, &objectives);
+        OptimizationResult::new(
+            Population::new(population),
+            front,
+            best,
+            evaluations,
+            self.config.generations,
+        )
+    }
+}
+
 fn environmental_selection<D: Clone>(
     combined: Vec<Candidate<D>>,
     objectives: &ObjectiveSpace,

src/algorithms/hill_climber.rs

@@ -146,6 +146,84 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl<I, V> HillClimber<I, V> {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` is mostly inert here because HillClimber evaluates
+    /// one child per iteration; it's accepted for API parity with other
+    /// algorithms.
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<P::Decision>
+    where
+        P: crate::core::async_problem::AsyncProblem,
+        I: Initializer<P::Decision>,
+        V: Variation<P::Decision>,
+    {
+        let _ = concurrency;
+        let objectives = problem.objectives();
+        assert!(
+            objectives.is_single_objective(),
+            "HillClimber requires exactly one objective",
+        );
+        let direction = objectives.objectives[0].direction;
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let mut initial = self.initializer.initialize(1, &mut rng);
+        assert!(
+            !initial.is_empty(),
+            "HillClimber initializer returned no decisions"
+        );
+        let mut current_decision = initial.remove(0);
+        let mut current_eval = problem.evaluate_async(&current_decision).await;
+        let mut evaluations = 1usize;
+
+        for _ in 0..self.config.iterations {
+            let parents = vec![current_decision.clone()];
+            let children = self.variation.vary(&parents, &mut rng);
+            assert!(
+                !children.is_empty(),
+                "HillClimber variation returned no children"
+            );
+            let child_decision = children.into_iter().next().unwrap();
+            let child_eval = problem.evaluate_async(&child_decision).await;
+            evaluations += 1;
+
+            let child_better = match (child_eval.is_feasible(), current_eval.is_feasible()) {
+                (true, false) => true,
+                (false, true) => false,
+                (false, false) => {
+                    child_eval.constraint_violation < current_eval.constraint_violation
+                }
+                (true, true) => match direction {
+                    Direction::Minimize => child_eval.objectives[0] < current_eval.objectives[0],
+                    Direction::Maximize => child_eval.objectives[0] > current_eval.objectives[0],
+                },
+            };
+            if child_better {
+                current_decision = child_decision;
+                current_eval = child_eval;
+            }
+        }
+
+        let best = Candidate::new(current_decision, current_eval);
+        let population = Population::new(vec![best.clone()]);
+        let front = vec![best.clone()];
+        OptimizationResult::new(
+            population,
+            front,
+            Some(best),
+            evaluations,
+            self.config.iterations,
+        )
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/hype.rs

@@ -226,6 +226,131 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl<I, V> Hype<I, V> {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations per batch.
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<P::Decision>
+    where
+        P: crate::core::async_problem::AsyncProblem,
+        I: Initializer<P::Decision>,
+        V: Variation<P::Decision>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+
+        assert!(
+            self.config.population_size > 0,
+            "Hype population_size must be > 0"
+        );
+        assert!(self.config.mc_samples > 0, "Hype mc_samples must be > 0");
+        let n = self.config.population_size;
+        let objectives = problem.objectives();
+        assert_eq!(
+            self.config.reference_point.len(),
+            objectives.len(),
+            "Hype reference_point.len() must equal number of objectives",
+        );
+        let reference = self.config.reference_point.clone();
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let initial_decisions = self.initializer.initialize(n, &mut rng);
+        let mut population: Vec<Candidate<P::Decision>> =
+            evaluate_batch_async(problem, initial_decisions, concurrency).await;
+        let mut evaluations = population.len();
+
+        for _ in 0..self.config.generations {
+            let fitness = hype_fitness(
+                &population,
+                &objectives,
+                &reference,
+                self.config.mc_samples,
+                &mut rng,
+            );
+            let mut offspring_decisions: Vec<P::Decision> = Vec::with_capacity(n);
+            while offspring_decisions.len() < n {
+                let p1 = binary_tournament(&fitness, &mut rng);
+                let p2 = binary_tournament(&fitness, &mut rng);
+                let parents = vec![
+                    population[p1].decision.clone(),
+                    population[p2].decision.clone(),
+                ];
+                let children = self.variation.vary(&parents, &mut rng);
+                assert!(!children.is_empty(), "Hype variation returned no children");
+                for child in children {
+                    if offspring_decisions.len() >= n {
+                        break;
+                    }
+                    offspring_decisions.push(child);
+                }
+            }
+
+            let offspring = evaluate_batch_async(problem, offspring_decisions, concurrency).await;
+            evaluations += offspring.len();
+
+            let mut combined: Vec<Candidate<P::Decision>> = Vec::with_capacity(2 * n);
+            combined.extend(population);
+            combined.extend(offspring);
+
+            let fronts = non_dominated_sort(&combined, &objectives);
+            let mut keep_indices: Vec<usize> = Vec::with_capacity(n);
+            let mut splitting: &[usize] = &[];
+            for f in &fronts {
+                if keep_indices.len() + f.len() <= n {
+                    keep_indices.extend(f.iter().copied());
+                } else {
+                    splitting = f;
+                    break;
+                }
+                if keep_indices.len() == n {
+                    break;
+                }
+            }
+            if keep_indices.len() < n {
+                let pool: Vec<&Candidate<P::Decision>> =
+                    splitting.iter().map(|&i| &combined[i]).collect();
+                let contributions = estimate_contributions(
+                    &pool,
+                    &objectives,
+                    &reference,
+                    self.config.mc_samples,
+                    &mut rng,
+                );
+                let mut order: Vec<usize> = (0..splitting.len()).collect();
+                order.sort_by(|&a, &b| {
+                    contributions[b]
+                        .partial_cmp(&contributions[a])
+                        .unwrap_or(std::cmp::Ordering::Equal)
+                });
+                for k in order.into_iter().take(n - keep_indices.len()) {
+                    keep_indices.push(splitting[k]);
+                }
+            }
+
+            population = keep_indices
+                .into_iter()
+                .map(|i| combined[i].clone())
+                .collect();
+        }
+
+        let front = pareto_front(&population, &objectives);
+        let best = best_candidate(&population, &objectives);
+        OptimizationResult::new(
+            Population::new(population),
+            front,
+            best,
+            evaluations,
+            self.config.generations,
+        )
+    }
+}
+
 fn hype_fitness<D>(
     pool: &[Candidate<D>],
     objectives: &ObjectiveSpace,

src/algorithms/hyperband.rs

@@ -206,6 +206,106 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl<I, D> Hyperband<I, D>
+where
+    D: Clone,
+    I: Initializer<D>,
+{
+    /// Async version of [`Hyperband::run`] — evaluates each
+    /// Successive-Halving rung's configurations concurrently through the
+    /// caller's async runtime. Available only with the `async` feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations per rung.
+    pub async fn run_async<P>(&mut self, problem: &P, concurrency: usize) -> OptimizationResult<D>
+    where
+        P: crate::core::async_problem::AsyncPartialProblem<Decision = D>,
+        D: Send + Sync,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_at_budget_async;
+
+        assert!(
+            self.config.max_budget > 0.0,
+            "Hyperband max_budget must be > 0"
+        );
+        assert!(self.config.eta > 1.0, "Hyperband eta must be > 1");
+        assert!(
+            self.config.max_brackets >= 1,
+            "Hyperband max_brackets must be >= 1"
+        );
+        let objectives = problem.objectives();
+        assert!(
+            objectives.is_single_objective(),
+            "Hyperband requires exactly one objective",
+        );
+        let direction = objectives.objectives[0].direction;
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let s_max = (self.config.max_budget.ln() / self.config.eta.ln()).floor() as i64;
+        let s_max = (s_max as usize).min(self.config.max_brackets);
+
+        let mut total_evaluations = 0usize;
+        let mut total_iterations = 0usize;
+        let mut best_seen: Option<Candidate<D>> = None;
+
+        for s in (0..=s_max).rev() {
+            let s_f = s as f64;
+            let n =
+                ((s_max as f64 + 1.0) / (s_f + 1.0) * self.config.eta.powf(s_f)).ceil() as usize;
+            let r = self.config.max_budget / self.config.eta.powf(s_f);
+
+            let mut configs: Vec<D> = self.initializer.initialize(n, &mut rng);
+            for i in 0..=s {
+                let n_i = (n as f64 / self.config.eta.powi(i as i32)).floor() as usize;
+                let r_i = r * self.config.eta.powi(i as i32);
+                if configs.is_empty() {
+                    break;
+                }
+                let evals: Vec<Evaluation> =
+                    evaluate_batch_at_budget_async(problem, &configs, r_i, concurrency).await;
+                total_evaluations += configs.len();
+
+                for (cfg, e) in configs.iter().zip(evals.iter()) {
+                    let beats = match &best_seen {
+                        None => true,
+                        Some(b) => better(e, &b.evaluation, direction),
+                    };
+                    if beats {
+                        best_seen = Some(Candidate::new(cfg.clone(), e.clone()));
+                    }
+                }
+                total_iterations += 1;
+
+                let next_size = (n_i / self.config.eta as usize).max(1);
+                if next_size >= configs.len() {
+                    continue;
+                }
+                let mut order: Vec<usize> = (0..configs.len()).collect();
+                order.sort_by(|&a, &b| compare(&evals[a], &evals[b], direction));
+                let keep: std::collections::HashSet<usize> =
+                    order.into_iter().take(next_size).collect();
+                let new_configs: Vec<D> = configs
+                    .into_iter()
+                    .enumerate()
+                    .filter_map(|(idx, c)| if keep.contains(&idx) { Some(c) } else { None })
+                    .collect();
+                configs = new_configs;
+            }
+        }
+
+        let best = best_seen.expect("at least one bracket ran");
+        let population = Population::new(vec![best.clone()]);
+        let front = vec![best.clone()];
+        OptimizationResult::new(
+            population,
+            front,
+            Some(best),
+            total_evaluations,
+            total_iterations,
+        )
+    }
+}
+
 fn compare(a: &Evaluation, b: &Evaluation, direction: Direction) -> std::cmp::Ordering {
     match (a.is_feasible(), b.is_feasible()) {
         (true, false) => std::cmp::Ordering::Less,

src/algorithms/ibea.rs

@@ -159,6 +159,80 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl<I, V> Ibea<I, V> {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations per batch.
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<P::Decision>
+    where
+        P: crate::core::async_problem::AsyncProblem,
+        I: Initializer<P::Decision>,
+        V: Variation<P::Decision>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+
+        assert!(
+            self.config.population_size > 0,
+            "Ibea population_size must be > 0"
+        );
+        assert!(self.config.kappa > 0.0, "Ibea kappa must be > 0");
+        let n = self.config.population_size;
+        let objectives = problem.objectives();
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let initial_decisions = self.initializer.initialize(n, &mut rng);
+        let mut population: Vec<Candidate<P::Decision>> =
+            evaluate_batch_async(problem, initial_decisions, concurrency).await;
+        let mut evaluations = population.len();
+
+        for _ in 0..self.config.generations {
+            let fitness = compute_fitness(&population, &objectives, self.config.kappa);
+            let mut offspring_decisions: Vec<P::Decision> = Vec::with_capacity(n);
+            while offspring_decisions.len() < n {
+                let p1 = binary_tournament(&fitness, &mut rng);
+                let p2 = binary_tournament(&fitness, &mut rng);
+                let parents = vec![
+                    population[p1].decision.clone(),
+                    population[p2].decision.clone(),
+                ];
+                let children = self.variation.vary(&parents, &mut rng);
+                assert!(!children.is_empty(), "Ibea variation returned no children");
+                for child in children {
+                    if offspring_decisions.len() >= n {
+                        break;
+                    }
+                    offspring_decisions.push(child);
+                }
+            }
+
+            let offspring = evaluate_batch_async(problem, offspring_decisions, concurrency).await;
+            evaluations += offspring.len();
+
+            let mut combined: Vec<Candidate<P::Decision>> = Vec::with_capacity(2 * n);
+            combined.extend(population);
+            combined.extend(offspring);
+            population = environmental_selection(combined, &objectives, n, self.config.kappa);
+        }
+
+        let front = pareto_front(&population, &objectives);
+        let best = best_candidate(&population, &objectives);
+        OptimizationResult::new(
+            Population::new(population),
+            front,
+            best,
+            evaluations,
+            self.config.generations,
+        )
+    }
+}
+
 /// Iteratively remove the worst-fitness member from `pool` until `n` remain.
 ///
 /// IBEA's standard "subtract the dropped member's contribution from every

src/algorithms/ipop_cma_es.rs

@@ -187,6 +187,94 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl IpopCmaEs {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations within each restart's
+    /// CMA-ES generation.
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<Vec<f64>>
+    where
+        P: crate::core::async_problem::AsyncProblem<Decision = Vec<f64>>,
+    {
+        assert!(
+            self.config.initial_population_size >= 4,
+            "IpopCmaEs initial_population_size must be >= 4",
+        );
+        let objectives = problem.objectives();
+        assert!(
+            objectives.is_single_objective(),
+            "IpopCmaEs requires exactly one objective",
+        );
+        let direction = objectives.objectives[0].direction;
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let mut remaining_gens = self.config.total_generations;
+        let mut pop_size = self.config.initial_population_size;
+        let mut total_evaluations = 0usize;
+        let mut total_iterations = 0usize;
+        let mut best_seen: Option<Candidate<Vec<f64>>> = None;
+        let _ = self.config.stall_generations;
+
+        let mut restart_counter = 0u64;
+        while remaining_gens > 0 {
+            let this_gens = (remaining_gens / 2).max(20).min(remaining_gens);
+            let inner_seed = self
+                .config
+                .seed
+                .wrapping_add(restart_counter.wrapping_mul(0x9E37_79B9_7F4A_7C15));
+            let restart_mean: Vec<f64> = self
+                .bounds
+                .bounds
+                .iter()
+                .map(|&(lo, hi)| lo + (hi - lo) * rng.random::<f64>())
+                .collect();
+            let cfg = CmaEsConfig {
+                population_size: pop_size,
+                generations: this_gens,
+                initial_sigma: self.config.initial_sigma,
+                eigen_decomposition_period: self.config.eigen_decomposition_period,
+                initial_mean: Some(restart_mean),
+                seed: inner_seed,
+            };
+            let mut inner = CmaEs::new(cfg, RealBounds::new(self.bounds.bounds.clone()));
+
+            let result = inner.run_async(problem, concurrency).await;
+            total_evaluations += result.evaluations;
+            total_iterations += result.generations;
+            if let Some(b) = result.best.clone() {
+                let beats = match &best_seen {
+                    None => true,
+                    Some(prev) => better(&b.evaluation, &prev.evaluation, direction),
+                };
+                if beats {
+                    best_seen = Some(b);
+                }
+            }
+            remaining_gens = remaining_gens.saturating_sub(this_gens);
+            pop_size = pop_size.saturating_mul(2);
+            restart_counter = restart_counter.wrapping_add(1);
+        }
+
+        let best = best_seen.expect("at least one restart ran");
+        let population = Population::new(vec![best.clone()]);
+        let front = vec![best.clone()];
+        OptimizationResult::new(
+            population,
+            front,
+            Some(best),
+            total_evaluations,
+            total_iterations,
+        )
+    }
+}
+
 fn better(a: &Evaluation, b: &Evaluation, direction: Direction) -> bool {
     match (a.is_feasible(), b.is_feasible()) {
         (true, false) => true,

src/algorithms/knea.rs

@@ -149,6 +149,77 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl<I, V> Knea<I, V> {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations per batch.
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<P::Decision>
+    where
+        P: crate::core::async_problem::AsyncProblem,
+        I: Initializer<P::Decision>,
+        V: Variation<P::Decision>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+
+        assert!(
+            self.config.population_size > 0,
+            "Knea population_size must be > 0"
+        );
+        let n = self.config.population_size;
+        let objectives = problem.objectives();
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let initial_decisions = self.initializer.initialize(n, &mut rng);
+        let mut population: Vec<Candidate<P::Decision>> =
+            evaluate_batch_async(problem, initial_decisions, concurrency).await;
+        let mut evaluations = population.len();
+
+        for _ in 0..self.config.generations {
+            let mut offspring_decisions: Vec<P::Decision> = Vec::with_capacity(n);
+            while offspring_decisions.len() < n {
+                let p1 = rng.random_range(0..population.len());
+                let p2 = rng.random_range(0..population.len());
+                let parents = vec![
+                    population[p1].decision.clone(),
+                    population[p2].decision.clone(),
+                ];
+                let children = self.variation.vary(&parents, &mut rng);
+                assert!(!children.is_empty(), "Knea variation returned no children");
+                for child in children {
+                    if offspring_decisions.len() >= n {
+                        break;
+                    }
+                    offspring_decisions.push(child);
+                }
+            }
+            let offspring = evaluate_batch_async(problem, offspring_decisions, concurrency).await;
+            evaluations += offspring.len();
+
+            let mut combined: Vec<Candidate<P::Decision>> = Vec::with_capacity(2 * n);
+            combined.extend(population);
+            combined.extend(offspring);
+            population = environmental_selection(combined, &objectives, n);
+        }
+
+        let front = pareto_front(&population, &objectives);
+        let best = best_candidate(&population, &objectives);
+        OptimizationResult::new(
+            Population::new(population),
+            front,
+            best,
+            evaluations,
+            self.config.generations,
+        )
+    }
+}
+
 fn environmental_selection<D: Clone>(
     combined: Vec<Candidate<D>>,
     objectives: &ObjectiveSpace,

src/algorithms/mod.rs

@@ -22,6 +22,8 @@ pub mod nsga3;
 pub mod one_plus_one_es;
 pub mod paes;
 pub(crate) mod parallel_eval;
+#[cfg(feature = "async")]
+pub(crate) mod parallel_eval_async;
 pub mod particle_swarm;
 pub mod pesa2;
 pub mod random_search;

src/algorithms/moead.rs

@@ -219,6 +219,126 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl<I, V> Moead<I, V> {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations of the initial
+    /// population. Per-generation evaluations are sequential because
+    /// each child's outcome feeds back into the same generation's
+    /// neighborhood updates.
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<P::Decision>
+    where
+        P: crate::core::async_problem::AsyncProblem,
+        I: Initializer<P::Decision>,
+        V: Variation<P::Decision>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+
+        let objectives = problem.objectives();
+        let m = objectives.len();
+        let weights = das_dennis(m, self.config.reference_divisions);
+        assert!(
+            !weights.is_empty(),
+            "Moead weight set is empty — increase reference_divisions",
+        );
+        let n = weights.len();
+        let t = self.config.neighborhood_size.min(n);
+        assert!(t >= 2, "Moead neighborhood_size must be >= 2");
+
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let initial_decisions = self.initializer.initialize(n, &mut rng);
+        assert_eq!(
+            initial_decisions.len(),
+            n,
+            "MOEA/D initializer must return exactly {n} decisions",
+        );
+        let mut population: Vec<Candidate<P::Decision>> =
+            evaluate_batch_async(problem, initial_decisions, concurrency).await;
+        let mut evaluations = population.len();
+
+        let mut ideal = vec![f64::INFINITY; m];
+        for c in &population {
+            let oriented = objectives.as_minimization(&c.evaluation.objectives);
+            for (k, v) in oriented.iter().enumerate() {
+                if *v < ideal[k] {
+                    ideal[k] = *v;
+                }
+            }
+        }
+
+        let neighborhoods: Vec<Vec<usize>> = (0..n)
+            .map(|i| {
+                let mut idx: Vec<usize> = (0..n).collect();
+                idx.sort_by(|&a, &b| {
+                    let da = weight_distance(&weights[i], &weights[a]);
+                    let db = weight_distance(&weights[i], &weights[b]);
+                    da.partial_cmp(&db).unwrap_or(std::cmp::Ordering::Equal)
+                });
+                idx.into_iter().take(t).collect()
+            })
+            .collect();
+
+        for _ in 0..self.config.generations {
+            #[allow(clippy::needless_range_loop)]
+            for i in 0..n {
+                let nbh = &neighborhoods[i];
+                let p1 = *nbh.choose(&mut rng).unwrap();
+                let mut p2 = *nbh.choose(&mut rng).unwrap();
+                while p2 == p1 && nbh.len() > 1 {
+                    p2 = *nbh.choose(&mut rng).unwrap();
+                }
+                let parents = vec![
+                    population[p1].decision.clone(),
+                    population[p2].decision.clone(),
+                ];
+                let children = self.variation.vary(&parents, &mut rng);
+                assert!(
+                    !children.is_empty(),
+                    "MOEA/D variation returned no children"
+                );
+                let child_decision = children.into_iter().next().unwrap();
+                let child_eval = problem.evaluate_async(&child_decision).await;
+                evaluations += 1;
+
+                let oriented_child = objectives.as_minimization(&child_eval.objectives);
+                for (k, v) in oriented_child.iter().enumerate() {
+                    if *v < ideal[k] {
+                        ideal[k] = *v;
+                    }
+                }
+
+                for &j in nbh {
+                    let cur_oriented =
+                        objectives.as_minimization(&population[j].evaluation.objectives);
+                    let g_cur = tchebycheff(&cur_oriented, &weights[j], &ideal);
+                    let g_new = tchebycheff(&oriented_child, &weights[j], &ideal);
+                    if g_new <= g_cur {
+                        population[j] = Candidate::new(child_decision.clone(), child_eval.clone());
+                    }
+                }
+            }
+        }
+
+        let front = pareto_front(&population, &objectives);
+        let best = best_candidate(&population, &objectives);
+        OptimizationResult::new(
+            Population::new(population),
+            front,
+            best,
+            evaluations,
+            self.config.generations,
+        )
+    }
+}
+
 /// Tchebycheff scalarization: `max_k w_k * |f_k - z*_k|`.
 ///
 /// `weight` components that are zero are floored to `1e-6` so every axis

src/algorithms/mopso.rs

@@ -212,6 +212,124 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl Mopso {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations per batch.
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<Vec<f64>>
+    where
+        P: crate::core::async_problem::AsyncProblem<Decision = Vec<f64>>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+        use crate::traits::Initializer as _;
+
+        assert!(self.config.swarm_size >= 1, "Mopso swarm_size must be >= 1");
+        assert!(
+            self.config.archive_size >= 1,
+            "Mopso archive_size must be >= 1"
+        );
+        let objectives = problem.objectives();
+        assert!(
+            objectives.is_multi_objective(),
+            "Mopso requires multi-objective problems (use ParticleSwarm for single-objective)",
+        );
+        let dim = self.bounds.bounds.len();
+        let n = self.config.swarm_size;
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let mut positions: Vec<Vec<f64>> = self.bounds.initialize(n, &mut rng);
+        let mut velocities: Vec<Vec<f64>> = (0..n)
+            .map(|_| {
+                self.bounds
+                    .bounds
+                    .iter()
+                    .map(|&(lo, hi)| 0.1 * (hi - lo) * (rng.random::<f64>() * 2.0 - 1.0))
+                    .collect()
+            })
+            .collect();
+        let v_max: Vec<f64> = self.bounds.bounds.iter().map(|&(lo, hi)| hi - lo).collect();
+
+        let initial_pop = evaluate_batch_async(problem, positions.clone(), concurrency).await;
+        let mut evaluations = initial_pop.len();
+
+        let mut pbest_decisions: Vec<Vec<f64>> = positions.clone();
+        let mut pbest_evals: Vec<crate::core::evaluation::Evaluation> =
+            initial_pop.iter().map(|c| c.evaluation.clone()).collect();
+
+        let mut archive = ParetoArchive::new(objectives.clone());
+        for c in initial_pop {
+            archive.insert(c);
+        }
+        archive.truncate(self.config.archive_size);
+
+        for _ in 0..self.config.generations {
+            for i in 0..n {
+                let leader = archive
+                    .members()
+                    .choose(&mut rng)
+                    .map(|c| c.decision.clone())
+                    .unwrap_or_else(|| positions[i].clone());
+                #[allow(clippy::needless_range_loop)]
+                for j in 0..dim {
+                    let r1: f64 = rng.random();
+                    let r2: f64 = rng.random();
+                    let cognitive_term =
+                        self.config.cognitive * r1 * (pbest_decisions[i][j] - positions[i][j]);
+                    let social_term = self.config.social * r2 * (leader[j] - positions[i][j]);
+                    let mut v =
+                        self.config.inertia * velocities[i][j] + cognitive_term + social_term;
+                    if v > v_max[j] {
+                        v = v_max[j];
+                    } else if v < -v_max[j] {
+                        v = -v_max[j];
+                    }
+                    velocities[i][j] = v;
+                    let (lo, hi) = self.bounds.bounds[j];
+                    positions[i][j] = (positions[i][j] + v).clamp(lo, hi);
+                }
+            }
+
+            let evaluated = evaluate_batch_async(problem, positions.clone(), concurrency).await;
+            evaluations += evaluated.len();
+
+            for (i, cand) in evaluated.iter().enumerate() {
+                let dominance = pareto_compare(&cand.evaluation, &pbest_evals[i], &objectives);
+                let replace = match dominance {
+                    Dominance::Dominates => true,
+                    Dominance::DominatedBy => false,
+                    Dominance::Equal | Dominance::NonDominated => rng.random_bool(0.5),
+                };
+                if replace {
+                    pbest_decisions[i] = cand.decision.clone();
+                    pbest_evals[i] = cand.evaluation.clone();
+                }
+            }
+            for c in evaluated {
+                archive.insert(c);
+            }
+            archive.truncate(self.config.archive_size);
+        }
+
+        let members = archive.into_vec();
+        let front = pareto_front(&members, &objectives);
+        let best = best_candidate(&members, &objectives);
+        OptimizationResult::new(
+            Population::new(members),
+            front,
+            best,
+            evaluations,
+            self.config.generations,
+        )
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/nelder_mead.rs

@@ -295,6 +295,161 @@ fn better(a: &Evaluation, b: &Evaluation, direction: Direction) -> bool {
     compare(a, b, direction) == std::cmp::Ordering::Less
 }
 
+#[cfg(feature = "async")]
+impl NelderMead {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` is largely inert here because Nelder-Mead
+    /// evaluates one or two new vertices per iteration sequentially
+    /// (the next decision depends on the previous evaluation); it's
+    /// accepted for API parity with other algorithms.
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<Vec<f64>>
+    where
+        P: crate::core::async_problem::AsyncProblem<Decision = Vec<f64>>,
+    {
+        let _ = concurrency;
+        assert!(
+            self.config.reflection > 0.0,
+            "NelderMead reflection must be > 0"
+        );
+        assert!(
+            self.config.expansion > 1.0,
+            "NelderMead expansion must be > 1",
+        );
+        assert!(
+            self.config.contraction > 0.0 && self.config.contraction < 1.0,
+            "NelderMead contraction must be in (0, 1)",
+        );
+        assert!(
+            self.config.shrinkage > 0.0 && self.config.shrinkage < 1.0,
+            "NelderMead shrinkage must be in (0, 1)",
+        );
+        assert!(
+            self.config.initial_step > 0.0,
+            "NelderMead initial_step must be > 0",
+        );
+        let objectives = problem.objectives();
+        assert!(
+            objectives.is_single_objective(),
+            "NelderMead requires exactly one objective",
+        );
+        let direction = objectives.objectives[0].direction;
+        let n = self.bounds.bounds.len();
+
+        let mut vertices: Vec<Vec<f64>> = Vec::with_capacity(n + 1);
+        let start: Vec<f64> = self
+            .bounds
+            .bounds
+            .iter()
+            .map(|&(lo, hi)| 0.5 * (lo + hi))
+            .collect();
+        vertices.push(start.clone());
+        for j in 0..n {
+            let mut v = start.clone();
+            let (lo, hi) = self.bounds.bounds[j];
+            let step = self.config.initial_step.min(0.5 * (hi - lo));
+            v[j] = (v[j] + step).clamp(lo, hi);
+            vertices.push(v);
+        }
+        let mut evals: Vec<Evaluation> = Vec::with_capacity(vertices.len());
+        for v in &vertices {
+            evals.push(problem.evaluate_async(v).await);
+        }
+        let mut evaluations = evals.len();
+
+        for _ in 0..self.config.iterations {
+            let mut order: Vec<usize> = (0..vertices.len()).collect();
+            order.sort_by(|&a, &b| compare(&evals[a], &evals[b], direction));
+            let best_idx = order[0];
+            let worst_idx = order[order.len() - 1];
+            let second_worst_idx = order[order.len() - 2];
+
+            let mut centroid = vec![0.0_f64; n];
+            for &idx in &order[..order.len() - 1] {
+                for j in 0..n {
+                    centroid[j] += vertices[idx][j];
+                }
+            }
+            for c in centroid.iter_mut() {
+                *c /= (order.len() - 1) as f64;
+            }
+
+            let reflected = self.reflect(&centroid, &vertices[worst_idx], self.config.reflection);
+            let r_eval = problem.evaluate_async(&reflected).await;
+            evaluations += 1;
+
+            if better(&r_eval, &evals[best_idx], direction) {
+                let expanded = self.reflect(&centroid, &vertices[worst_idx], self.config.expansion);
+                let e_eval = problem.evaluate_async(&expanded).await;
+                evaluations += 1;
+                if better(&e_eval, &r_eval, direction) {
+                    vertices[worst_idx] = expanded;
+                    evals[worst_idx] = e_eval;
+                } else {
+                    vertices[worst_idx] = reflected;
+                    evals[worst_idx] = r_eval;
+                }
+            } else if better(&r_eval, &evals[second_worst_idx], direction) {
+                vertices[worst_idx] = reflected;
+                evals[worst_idx] = r_eval;
+            } else {
+                let contraction_target = if better(&r_eval, &evals[worst_idx], direction) {
+                    self.contract(&centroid, &reflected, self.config.contraction)
+                } else {
+                    self.contract(&centroid, &vertices[worst_idx], self.config.contraction)
+                };
+                let c_eval = problem.evaluate_async(&contraction_target).await;
+                evaluations += 1;
+                if better(&c_eval, &evals[worst_idx], direction) {
+                    vertices[worst_idx] = contraction_target;
+                    evals[worst_idx] = c_eval;
+                } else {
+                    let best_pt = vertices[best_idx].clone();
+                    for &idx in &order {
+                        if idx == best_idx {
+                            continue;
+                        }
+                        #[allow(clippy::needless_range_loop)]
+                        for j in 0..n {
+                            vertices[idx][j] = best_pt[j]
+                                + self.config.shrinkage * (vertices[idx][j] - best_pt[j]);
+                        }
+                        for (j, x) in vertices[idx].iter_mut().enumerate() {
+                            let (lo, hi) = self.bounds.bounds[j];
+                            *x = x.clamp(lo, hi);
+                        }
+                        evals[idx] = problem.evaluate_async(&vertices[idx]).await;
+                        evaluations += 1;
+                    }
+                }
+            }
+        }
+
+        let mut best_idx = 0;
+        for i in 1..vertices.len() {
+            if better(&evals[i], &evals[best_idx], direction) {
+                best_idx = i;
+            }
+        }
+        let best = Candidate::new(vertices[best_idx].clone(), evals[best_idx].clone());
+        let population = Population::new(vec![best.clone()]);
+        let front = vec![best.clone()];
+        OptimizationResult::new(
+            population,
+            front,
+            Some(best),
+            evaluations,
+            self.config.iterations,
+        )
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/nsga2.rs

@@ -108,16 +108,6 @@ where
     V: Variation<P::Decision>,
 {
     fn run(&mut self, problem: &P) -> OptimizationResult<P::Decision> {
-        self.run_with(problem, &mut ())
-    }
-
-    fn run_with<O>(&mut self, problem: &P, observer: &mut O) -> OptimizationResult<P::Decision>
-    where
-        O: crate::observer::Observer<P::Decision>,
-    {
-        use crate::observer::Snapshot;
-        use std::ops::ControlFlow;
-
         assert!(
             self.config.population_size > 0,
             "Nsga2 population_size must be greater than 0",
@@ -125,7 +115,6 @@ where
         let n = self.config.population_size;
         let objectives = problem.objectives();
         let mut rng = rng_from_seed(self.config.seed);
-        let started = std::time::Instant::now();
 
         // Initial population.
         let initial_decisions = self.initializer.initialize(n, &mut rng);
@@ -141,27 +130,7 @@ where
         // round of tournament selection has data to compare on.
         let mut annotated = annotate(population, &objectives);
 
-        // Observer: notify after the initial population.
+        for _ in 0..self.config.generations {
-        let mut completed_generations: usize = 0;
-        let pop_view: Vec<Candidate<P::Decision>> =
-            annotated.iter().map(|e| e.candidate.clone()).collect();
-        let front_view = pareto_front(&pop_view, &objectives);
-        let snap = Snapshot {
-            iteration: 0,
-            evaluations,
-            elapsed: started.elapsed(),
-            population: &pop_view,
-            pareto_front: Some(&front_view),
-            best: None,
-            objectives: &objectives,
-        };
-        if let ControlFlow::Break(()) = observer.observe(&snap) {
-            return finalize_nsga2(annotated, &objectives, evaluations, completed_generations);
-        }
-        drop(pop_view);
-        drop(front_view);
-
-        for generation in 1..=self.config.generations {
             // --- Phase 1: serial parent selection + variation ---
             let mut offspring_decisions: Vec<P::Decision> = Vec::with_capacity(n);
             while offspring_decisions.len() < n {
@@ -220,47 +189,22 @@ where
                 }
             }
             annotated = annotate(next, &objectives);
-            completed_generations = generation;
-
-            // Per-generation observation.
-            let pop_view: Vec<Candidate<P::Decision>> =
-                annotated.iter().map(|e| e.candidate.clone()).collect();
-            let front_view = pareto_front(&pop_view, &objectives);
-            let snap = Snapshot {
-                iteration: generation,
-                evaluations,
-                elapsed: started.elapsed(),
-                population: &pop_view,
-                pareto_front: Some(&front_view),
-                best: None,
-                objectives: &objectives,
-            };
-            if let ControlFlow::Break(()) = observer.observe(&snap) {
-                return finalize_nsga2(annotated, &objectives, evaluations, completed_generations);
-            }
         }
 
-        finalize_nsga2(annotated, &objectives, evaluations, self.config.generations)
+        // Return final state.
-    }
+        let final_pop: Vec<Candidate<P::Decision>> =
-}
+            annotated.into_iter().map(|e| e.candidate).collect();
-
+        let front = pareto_front(&final_pop, &objectives);
-fn finalize_nsga2<D: Clone>(
+        let best = best_candidate(&final_pop, &objectives);
-    annotated: Vec<Nsga2Entry<D>>,
-    objectives: &crate::core::objective::ObjectiveSpace,
-    evaluations: usize,
-    generations: usize,
-) -> OptimizationResult<D> {
-    let final_pop: Vec<Candidate<D>> = annotated.into_iter().map(|e| e.candidate).collect();
-    let front = pareto_front(&final_pop, objectives);
-    let best = best_candidate(&final_pop, objectives);
         OptimizationResult::new(
             Population::new(final_pop),
             front,
             best,
             evaluations,
-        generations,
+            self.config.generations,
         )
     }
+}
 
 fn annotate<D: Clone>(
     population: Vec<Candidate<D>>,
@@ -288,6 +232,117 @@ fn annotate<D: Clone>(
         .collect()
 }
 
+#[cfg(feature = "async")]
+impl<I, V> Nsga2<I, V> {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations per batch (initial
+    /// population and per-generation offspring).
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<P::Decision>
+    where
+        P: crate::core::async_problem::AsyncProblem,
+        I: Initializer<P::Decision>,
+        V: Variation<P::Decision>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+
+        assert!(
+            self.config.population_size > 0,
+            "Nsga2 population_size must be greater than 0",
+        );
+        let n = self.config.population_size;
+        let objectives = problem.objectives();
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let initial_decisions = self.initializer.initialize(n, &mut rng);
+        assert_eq!(
+            initial_decisions.len(),
+            n,
+            "NSGA-II initializer must return exactly population_size decisions",
+        );
+        let population: Vec<Candidate<P::Decision>> =
+            evaluate_batch_async(problem, initial_decisions, concurrency).await;
+        let mut evaluations = population.len();
+
+        let mut annotated = annotate(population, &objectives);
+
+        for _ in 0..self.config.generations {
+            let mut offspring_decisions: Vec<P::Decision> = Vec::with_capacity(n);
+            while offspring_decisions.len() < n {
+                let p1 = binary_tournament(&annotated, &mut rng);
+                let p2 = binary_tournament(&annotated, &mut rng);
+                let parents = vec![
+                    annotated[p1].candidate.decision.clone(),
+                    annotated[p2].candidate.decision.clone(),
+                ];
+                let children = self.variation.vary(&parents, &mut rng);
+                assert!(
+                    !children.is_empty(),
+                    "NSGA-II variation returned no children",
+                );
+                for child_decision in children {
+                    if offspring_decisions.len() >= n {
+                        break;
+                    }
+                    offspring_decisions.push(child_decision);
+                }
+            }
+            let offspring: Vec<Candidate<P::Decision>> =
+                evaluate_batch_async(problem, offspring_decisions, concurrency).await;
+            evaluations += offspring.len();
+
+            let mut combined: Vec<Candidate<P::Decision>> = Vec::with_capacity(2 * n);
+            combined.extend(annotated.into_iter().map(|e| e.candidate));
+            combined.extend(offspring);
+
+            let fronts = non_dominated_sort(&combined, &objectives);
+            let mut next: Vec<Candidate<P::Decision>> = Vec::with_capacity(n);
+            for front in &fronts {
+                if next.len() + front.len() <= n {
+                    for &idx in front {
+                        next.push(combined[idx].clone());
+                    }
+                } else {
+                    let dist = crowding_distance(&combined, front, &objectives);
+                    let mut order: Vec<usize> = (0..front.len()).collect();
+                    order.sort_by(|&a, &b| {
+                        dist[b]
+                            .partial_cmp(&dist[a])
+                            .unwrap_or(std::cmp::Ordering::Equal)
+                    });
+                    let needed = n - next.len();
+                    for &k in order.iter().take(needed) {
+                        next.push(combined[front[k]].clone());
+                    }
+                    break;
+                }
+                if next.len() == n {
+                    break;
+                }
+            }
+            annotated = annotate(next, &objectives);
+        }
+
+        let final_pop: Vec<Candidate<P::Decision>> =
+            annotated.into_iter().map(|e| e.candidate).collect();
+        let front = pareto_front(&final_pop, &objectives);
+        let best = best_candidate(&final_pop, &objectives);
+        OptimizationResult::new(
+            Population::new(final_pop),
+            front,
+            best,
+            evaluations,
+            self.config.generations,
+        )
+    }
+}
+
 fn binary_tournament<D>(entries: &[Nsga2Entry<D>], rng: &mut Rng) -> usize {
     let n = entries.len();
     let a = rng.random_range(0..n);

src/algorithms/nsga3.rs

@@ -180,6 +180,92 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl<I, V> Nsga3<I, V> {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations per batch.
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<P::Decision>
+    where
+        P: crate::core::async_problem::AsyncProblem,
+        I: Initializer<P::Decision>,
+        V: Variation<P::Decision>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+
+        assert!(
+            self.config.population_size > 0,
+            "Nsga3 population_size must be greater than 0",
+        );
+        let n = self.config.population_size;
+        let objectives = problem.objectives();
+        let m = objectives.len();
+        let reference_points = das_dennis(m, self.config.reference_divisions);
+        assert!(
+            !reference_points.is_empty(),
+            "Nsga3 reference set is empty — check reference_divisions",
+        );
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let initial_decisions = self.initializer.initialize(n, &mut rng);
+        assert_eq!(
+            initial_decisions.len(),
+            n,
+            "NSGA-III initializer must return exactly population_size decisions",
+        );
+        let mut population: Vec<Candidate<P::Decision>> =
+            evaluate_batch_async(problem, initial_decisions, concurrency).await;
+        let mut evaluations = population.len();
+
+        for _ in 0..self.config.generations {
+            let mut offspring_decisions: Vec<P::Decision> = Vec::with_capacity(n);
+            while offspring_decisions.len() < n {
+                let p1 = rng.random_range(0..population.len());
+                let p2 = rng.random_range(0..population.len());
+                let parents = vec![
+                    population[p1].decision.clone(),
+                    population[p2].decision.clone(),
+                ];
+                let children = self.variation.vary(&parents, &mut rng);
+                assert!(
+                    !children.is_empty(),
+                    "NSGA-III variation returned no children",
+                );
+                for child_decision in children {
+                    if offspring_decisions.len() >= n {
+                        break;
+                    }
+                    offspring_decisions.push(child_decision);
+                }
+            }
+            let offspring = evaluate_batch_async(problem, offspring_decisions, concurrency).await;
+            evaluations += offspring.len();
+
+            let mut combined: Vec<Candidate<P::Decision>> = Vec::with_capacity(2 * n);
+            combined.extend(population);
+            combined.extend(offspring);
+            population =
+                environmental_selection(&combined, &objectives, &reference_points, n, &mut rng);
+        }
+
+        let front = pareto_front(&population, &objectives);
+        let best = best_candidate(&population, &objectives);
+        OptimizationResult::new(
+            Population::new(population),
+            front,
+            best,
+            evaluations,
+            self.config.generations,
+        )
+    }
+}
+
 /// NSGA-III environmental selection: front-by-front + reference-point niching
 /// on the splitting front.
 fn environmental_selection<D: Clone>(

src/algorithms/one_plus_one_es.rs

@@ -191,6 +191,100 @@ fn worse_than(a: &Evaluation, b: &Evaluation, direction: Direction) -> bool {
     }
 }
 
+#[cfg(feature = "async")]
+impl OnePlusOneEs {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` is mostly inert here because (1+1)-ES evaluates
+    /// one child per iteration; it's accepted for API parity with
+    /// other algorithms.
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<Vec<f64>>
+    where
+        P: crate::core::async_problem::AsyncProblem<Decision = Vec<f64>>,
+    {
+        let _ = concurrency;
+        assert!(
+            self.config.initial_sigma > 0.0,
+            "OnePlusOneEs initial_sigma must be > 0"
+        );
+        assert!(
+            self.config.step_increase > 1.0,
+            "OnePlusOneEs step_increase must be > 1",
+        );
+        assert!(
+            self.config.adaptation_period >= 1,
+            "OnePlusOneEs adaptation_period must be >= 1",
+        );
+        let objectives = problem.objectives();
+        assert!(
+            objectives.is_single_objective(),
+            "OnePlusOneEs requires exactly one objective",
+        );
+        let direction = objectives.objectives[0].direction;
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let mut parent: Vec<f64> = self
+            .bounds
+            .bounds
+            .iter()
+            .map(|&(lo, hi)| 0.5 * (lo + hi))
+            .collect();
+        let mut parent_eval = problem.evaluate_async(&parent).await;
+        let mut evaluations = 1usize;
+
+        let mut sigma = self.config.initial_sigma;
+        let mut window = std::collections::VecDeque::with_capacity(self.config.adaptation_period);
+
+        for _ in 0..self.config.iterations {
+            let normal = Normal::new(0.0, sigma).expect("Normal::new(0, sigma)");
+            let mut child = parent.clone();
+            for (j, x) in child.iter_mut().enumerate() {
+                let (lo, hi) = self.bounds.bounds[j];
+                *x = (*x + normal.sample(&mut rng)).clamp(lo, hi);
+            }
+            let child_eval = problem.evaluate_async(&child).await;
+            evaluations += 1;
+
+            let accepted = !worse_than(&child_eval, &parent_eval, direction);
+            if accepted {
+                parent = child;
+                parent_eval = child_eval;
+            }
+
+            window.push_back(if accepted { 1u8 } else { 0u8 });
+            if window.len() > self.config.adaptation_period {
+                window.pop_front();
+            }
+            if window.len() == self.config.adaptation_period {
+                let success_count: usize = window.iter().map(|&b| b as usize).sum();
+                let rate = success_count as f64 / window.len() as f64;
+                if rate > 0.2 {
+                    sigma *= self.config.step_increase;
+                } else if rate < 0.2 {
+                    sigma /= self.config.step_increase;
+                }
+            }
+        }
+
+        let best = Candidate::new(parent, parent_eval);
+        let population = Population::new(vec![best.clone()]);
+        let front = vec![best.clone()];
+        OptimizationResult::new(
+            population,
+            front,
+            Some(best),
+            evaluations,
+            self.config.iterations,
+        )
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/paes.rs

@@ -156,6 +156,92 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl<I, V> Paes<I, V> {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` is mostly inert here because PAES evaluates one
+    /// child per iteration; it's accepted for API parity with other
+    /// algorithms.
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<P::Decision>
+    where
+        P: crate::core::async_problem::AsyncProblem,
+        I: Initializer<P::Decision>,
+        V: Variation<P::Decision>,
+    {
+        let _ = concurrency;
+        assert!(
+            self.config.archive_size > 0,
+            "PAES archive_size must be greater than 0",
+        );
+
+        let objectives = problem.objectives();
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let mut initial = self.initializer.initialize(1, &mut rng);
+        assert!(
+            !initial.is_empty(),
+            "PAES initializer returned no decisions",
+        );
+        let mut current_decision = initial.remove(0);
+        let mut current_eval = problem.evaluate_async(&current_decision).await;
+        let mut evaluations = 1usize;
+
+        let mut archive = ParetoArchive::new(objectives.clone());
+        archive.insert(Candidate::new(
+            current_decision.clone(),
+            current_eval.clone(),
+        ));
+
+        for _ in 0..self.config.iterations {
+            let parents = vec![current_decision.clone()];
+            let children = self.variation.vary(&parents, &mut rng);
+            assert!(!children.is_empty(), "PAES variation returned no children",);
+            let child_decision = children.into_iter().next().unwrap();
+            let child_eval = problem.evaluate_async(&child_decision).await;
+            evaluations += 1;
+
+            match pareto_compare(&child_eval, &current_eval, &objectives) {
+                Dominance::Dominates => {
+                    current_decision = child_decision.clone();
+                    current_eval = child_eval.clone();
+                }
+                Dominance::DominatedBy => {
+                    // Stay at current.
+                }
+                Dominance::NonDominated | Dominance::Equal => {
+                    current_decision = child_decision.clone();
+                    current_eval = child_eval.clone();
+                }
+            }
+
+            archive.insert(Candidate::new(child_decision, child_eval));
+            archive.insert(Candidate::new(
+                current_decision.clone(),
+                current_eval.clone(),
+            ));
+            archive.truncate(self.config.archive_size);
+        }
+
+        let members = archive.into_vec();
+        let front = pareto_front(&members, &objectives);
+        let best = best_candidate(&members, &objectives);
+        OptimizationResult::new(
+            Population::new(members),
+            front,
+            best,
+            evaluations,
+            self.config.iterations,
+        )
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/parallel_eval_async.rs

@@ -0,0 +1,91 @@
+//! Async population evaluator.
+//!
+//! Available only with the `async` feature. Used by the `run_async`
+//! method on algorithms that support async problems.
+
+use futures::stream::{FuturesOrdered, StreamExt};
+
+use crate::core::async_problem::{AsyncPartialProblem, AsyncProblem};
+use crate::core::candidate::Candidate;
+use crate::core::evaluation::Evaluation;
+
+/// Evaluate every decision concurrently against `problem`, preserving
+/// input order in the returned vector. Concurrency is bounded by
+/// `concurrency` (≥ 1) — too high a value wastes memory and may
+/// overload downstream services; too low forfeits parallelism.
+///
+/// Returns a future that the caller drives via their preferred
+/// runtime (typically tokio).
+pub async fn evaluate_batch_async<P>(
+    problem: &P,
+    decisions: Vec<P::Decision>,
+    concurrency: usize,
+) -> Vec<Candidate<P::Decision>>
+where
+    P: AsyncProblem,
+{
+    assert!(
+        concurrency >= 1,
+        "evaluate_batch_async concurrency must be >= 1"
+    );
+    let mut out: Vec<Candidate<P::Decision>> = Vec::with_capacity(decisions.len());
+
+    // Process in concurrency-bounded chunks to keep peak memory low
+    // and avoid blasting downstream services. Each chunk uses
+    // FuturesOrdered to preserve per-chunk order, and chunks are
+    // emitted in their natural order.
+    let mut iter = decisions.into_iter();
+    loop {
+        let mut futs = FuturesOrdered::new();
+        for _ in 0..concurrency {
+            match iter.next() {
+                Some(d) => {
+                    futs.push_back(async move {
+                        let e = problem.evaluate_async(&d).await;
+                        Candidate::new(d, e)
+                    });
+                }
+                None => break,
+            }
+        }
+        if futs.is_empty() {
+            break;
+        }
+        while let Some(c) = futs.next().await {
+            out.push(c);
+        }
+    }
+    out
+}
+
+/// Evaluate every decision at the given `budget` concurrently against a
+/// multi-fidelity `problem`, preserving input order. Hyperband's async
+/// path uses this for each Successive-Halving rung.
+pub async fn evaluate_batch_at_budget_async<P>(
+    problem: &P,
+    decisions: &[P::Decision],
+    budget: f64,
+    concurrency: usize,
+) -> Vec<Evaluation>
+where
+    P: AsyncPartialProblem,
+{
+    assert!(
+        concurrency >= 1,
+        "evaluate_batch_at_budget_async concurrency must be >= 1"
+    );
+    let mut out: Vec<Evaluation> = Vec::with_capacity(decisions.len());
+    let mut idx = 0usize;
+    while idx < decisions.len() {
+        let mut futs = FuturesOrdered::new();
+        let end = (idx + concurrency).min(decisions.len());
+        for d in &decisions[idx..end] {
+            futs.push_back(async move { problem.evaluate_at_budget_async(d, budget).await });
+        }
+        while let Some(e) = futs.next().await {
+            out.push(e);
+        }
+        idx = end;
+    }
+    out
+}

src/algorithms/particle_swarm.rs

@@ -220,6 +220,129 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl ParticleSwarm {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations per batch (initial
+    /// swarm, per-generation positions, and the final evaluation pass).
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<Vec<f64>>
+    where
+        P: crate::core::async_problem::AsyncProblem<Decision = Vec<f64>>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+
+        assert!(
+            self.config.swarm_size >= 1,
+            "ParticleSwarm swarm_size must be >= 1",
+        );
+        let objectives = problem.objectives();
+        assert!(
+            objectives.is_single_objective(),
+            "ParticleSwarm requires exactly one objective",
+        );
+        let direction = objectives.objectives[0].direction;
+        let dim = self.bounds.bounds.len();
+        let n = self.config.swarm_size;
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let mut positions: Vec<Vec<f64>> = {
+            use crate::traits::Initializer as _;
+            self.bounds.initialize(n, &mut rng)
+        };
+        let mut velocities: Vec<Vec<f64>> = (0..n)
+            .map(|_| {
+                self.bounds
+                    .bounds
+                    .iter()
+                    .map(|&(lo, hi)| 0.1 * (hi - lo) * (rng.random::<f64>() * 2.0 - 1.0))
+                    .collect()
+            })
+            .collect();
+        let v_max: Vec<f64> = self.bounds.bounds.iter().map(|&(lo, hi)| hi - lo).collect();
+
+        let initial_pop = evaluate_batch_async(problem, positions.clone(), concurrency).await;
+        let mut evaluations = initial_pop.len();
+
+        let mut pbest_decisions: Vec<Vec<f64>> = positions.clone();
+        let mut pbest_evals: Vec<f64> = initial_pop
+            .iter()
+            .map(|c| c.evaluation.objectives[0])
+            .collect();
+
+        let mut gbest_idx = best_index(&pbest_evals, direction);
+        let mut gbest_decision = pbest_decisions[gbest_idx].clone();
+        let mut gbest_eval = pbest_evals[gbest_idx];
+
+        for _ in 0..self.config.generations {
+            for i in 0..n {
+                #[allow(clippy::needless_range_loop)]
+                for j in 0..dim {
+                    let r1: f64 = rng.random();
+                    let r2: f64 = rng.random();
+                    let cognitive_term =
+                        self.config.cognitive * r1 * (pbest_decisions[i][j] - positions[i][j]);
+                    let social_term =
+                        self.config.social * r2 * (gbest_decision[j] - positions[i][j]);
+                    let mut v =
+                        self.config.inertia * velocities[i][j] + cognitive_term + social_term;
+                    if v > v_max[j] {
+                        v = v_max[j];
+                    } else if v < -v_max[j] {
+                        v = -v_max[j];
+                    }
+                    velocities[i][j] = v;
+                    let (lo, hi) = self.bounds.bounds[j];
+                    positions[i][j] = (positions[i][j] + v).clamp(lo, hi);
+                }
+            }
+
+            let evaluated = evaluate_batch_async(problem, positions.clone(), concurrency).await;
+            evaluations += evaluated.len();
+
+            for (i, cand) in evaluated.iter().enumerate() {
+                let f = cand.evaluation.objectives[0];
+                let improves = match direction {
+                    Direction::Minimize => f < pbest_evals[i],
+                    Direction::Maximize => f > pbest_evals[i],
+                };
+                if improves {
+                    pbest_decisions[i] = positions[i].clone();
+                    pbest_evals[i] = f;
+                    gbest_idx = i;
+                    let beats_global = match direction {
+                        Direction::Minimize => f < gbest_eval,
+                        Direction::Maximize => f > gbest_eval,
+                    };
+                    if beats_global {
+                        gbest_decision = pbest_decisions[i].clone();
+                        gbest_eval = f;
+                    }
+                }
+            }
+        }
+        let _ = gbest_idx;
+
+        let final_pop = evaluate_batch_async(problem, positions, concurrency).await;
+        evaluations += final_pop.len();
+        let best = best_candidate(&final_pop, &objectives);
+        let front: Vec<Candidate<Vec<f64>>> = best.iter().cloned().collect();
+        OptimizationResult::new(
+            Population::new(final_pop),
+            front,
+            best,
+            evaluations,
+            self.config.generations,
+        )
+    }
+}
+
 fn best_index(values: &[f64], direction: Direction) -> usize {
     let mut idx = 0;
     for i in 1..values.len() {

src/algorithms/pesa2.rs

@@ -204,6 +204,109 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl<I, V> PesaII<I, V> {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations of the initial
+    /// population. Per-step evaluations are sequential to preserve the
+    /// algorithm's exact RNG sequencing.
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<P::Decision>
+    where
+        P: crate::core::async_problem::AsyncProblem,
+        I: Initializer<P::Decision>,
+        V: Variation<P::Decision>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+
+        assert!(
+            self.config.population_size > 0,
+            "PesaII population_size must be > 0"
+        );
+        assert!(
+            self.config.archive_size > 0,
+            "PesaII archive_size must be > 0"
+        );
+        assert!(
+            self.config.grid_divisions >= 1,
+            "PesaII grid_divisions must be >= 1"
+        );
+        let n = self.config.population_size;
+        let objectives = problem.objectives();
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let initial_decisions = self.initializer.initialize(n, &mut rng);
+        let mut internal: Vec<Candidate<P::Decision>> =
+            evaluate_batch_async(problem, initial_decisions, concurrency).await;
+        let mut evaluations = internal.len();
+
+        let mut archive = ParetoArchive::new(objectives.clone());
+        for c in &internal {
+            archive.insert(c.clone());
+        }
+        truncate_by_grid(
+            &mut archive,
+            self.config.archive_size,
+            self.config.grid_divisions,
+        );
+
+        for _ in 0..self.config.generations {
+            let (boxes, counts) = build_grid(&archive, &objectives, self.config.grid_divisions);
+
+            let mut offspring: Vec<Candidate<P::Decision>> = Vec::with_capacity(n);
+            while offspring.len() < n {
+                let p1 = region_tournament(&archive, &boxes, &counts, &mut rng);
+                let p2 = region_tournament(&archive, &boxes, &counts, &mut rng);
+                let parents = vec![
+                    archive.members()[p1].decision.clone(),
+                    archive.members()[p2].decision.clone(),
+                ];
+                let children = self.variation.vary(&parents, &mut rng);
+                assert!(
+                    !children.is_empty(),
+                    "PesaII variation returned no children"
+                );
+                for child in children {
+                    if offspring.len() >= n {
+                        break;
+                    }
+                    let eval = problem.evaluate_async(&child).await;
+                    evaluations += 1;
+                    offspring.push(Candidate::new(child, eval));
+                }
+            }
+
+            for c in &offspring {
+                archive.insert(c.clone());
+            }
+            truncate_by_grid(
+                &mut archive,
+                self.config.archive_size,
+                self.config.grid_divisions,
+            );
+            internal = offspring;
+        }
+
+        let _ = internal;
+        let members = archive.into_vec();
+        let front = pareto_front(&members, &objectives);
+        let best = best_candidate(&members, &objectives);
+        OptimizationResult::new(
+            Population::new(members),
+            front,
+            best,
+            evaluations,
+            self.config.generations,
+        )
+    }
+}
+
 /// Compute per-member box index (M-tuple of grid coordinates) and the
 /// population count of each occupied box.
 fn build_grid<D: Clone>(

src/algorithms/random_search.rs

@@ -88,49 +88,73 @@ where
     I: Initializer<P::Decision>,
 {
     fn run(&mut self, problem: &P) -> OptimizationResult<P::Decision> {
-        self.run_with(problem, &mut ())
-    }
-
-    fn run_with<O>(&mut self, problem: &P, observer: &mut O) -> OptimizationResult<P::Decision>
-    where
-        O: crate::observer::Observer<P::Decision>,
-    {
-        use crate::observer::Snapshot;
-        use std::ops::ControlFlow;
-
         let objectives = problem.objectives();
         let mut rng = rng_from_seed(self.config.seed);
         let mut all: Vec<Candidate<P::Decision>> = Vec::new();
         let mut evaluations = 0usize;
-        let started = std::time::Instant::now();
-        let mut completed: usize = 0;
 
-        for iteration in 1..=self.config.iterations {
+        for _ in 0..self.config.iterations {
             let decisions = self
                 .initializer
                 .initialize(self.config.batch_size, &mut rng);
             evaluations += decisions.len();
             all.extend(evaluate_batch(problem, decisions));
-            completed = iteration;
-
-            let best = best_candidate(&all, &objectives);
-            let snap = Snapshot {
-                iteration,
-                evaluations,
-                elapsed: started.elapsed(),
-                population: &all,
-                pareto_front: None,
-                best: best.as_ref(),
-                objectives: &objectives,
-            };
-            if let ControlFlow::Break(()) = observer.observe(&snap) {
-                break;
-            }
         }
 
         let front = pareto_front(&all, &objectives);
         let best = best_candidate(&all, &objectives);
-        OptimizationResult::new(Population::new(all), front, best, evaluations, completed)
+        OptimizationResult::new(
+            Population::new(all),
+            front,
+            best,
+            evaluations,
+            self.config.iterations,
+        )
+    }
+}
+
+#[cfg(feature = "async")]
+impl<I> RandomSearch<I> {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime (typically tokio). Useful when
+    /// `evaluate` is IO-bound (HTTP, RPC, subprocess).
+    ///
+    /// `concurrency` bounds how many evaluations are in-flight at once;
+    /// `1` is sequential, larger values push more load to the
+    /// downstream service.
+    ///
+    /// Available only with the `async` feature.
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<P::Decision>
+    where
+        P: crate::core::async_problem::AsyncProblem,
+        I: Initializer<P::Decision>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+        let objectives = problem.objectives();
+        let mut rng = rng_from_seed(self.config.seed);
+        let mut all: Vec<Candidate<P::Decision>> = Vec::new();
+        let mut evaluations = 0usize;
+        for _ in 0..self.config.iterations {
+            let decisions = self
+                .initializer
+                .initialize(self.config.batch_size, &mut rng);
+            evaluations += decisions.len();
+            let cands = evaluate_batch_async(problem, decisions, concurrency).await;
+            all.extend(cands);
+        }
+        let front = pareto_front(&all, &objectives);
+        let best = best_candidate(&all, &objectives);
+        OptimizationResult::new(
+            Population::new(all),
+            front,
+            best,
+            evaluations,
+            self.config.iterations,
+        )
     }
 }
 

src/algorithms/rvea.rs

@@ -261,6 +261,163 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl<I, V> Rvea<I, V> {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations per batch.
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<P::Decision>
+    where
+        P: crate::core::async_problem::AsyncProblem,
+        I: Initializer<P::Decision>,
+        V: Variation<P::Decision>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+
+        assert!(
+            self.config.population_size > 0,
+            "Rvea population_size must be > 0"
+        );
+        let n = self.config.population_size;
+        let objectives = problem.objectives();
+        let m = objectives.len();
+        let raw_refs = das_dennis(m, self.config.reference_divisions);
+        let references: Vec<Vec<f64>> = raw_refs.into_iter().map(unit_normalize).collect();
+        assert!(
+            !references.is_empty(),
+            "Rvea: no reference vectors generated"
+        );
+
+        let theta_max = smallest_neighbor_angle(&references);
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let initial_decisions = self.initializer.initialize(n, &mut rng);
+        let mut population: Vec<Candidate<P::Decision>> =
+            evaluate_batch_async(problem, initial_decisions, concurrency).await;
+        let mut evaluations = population.len();
+
+        for gen_idx in 0..self.config.generations {
+            let mut offspring_decisions: Vec<P::Decision> = Vec::with_capacity(n);
+            while offspring_decisions.len() < n {
+                let p1 = rng.random_range(0..population.len());
+                let p2 = rng.random_range(0..population.len());
+                let parents = vec![
+                    population[p1].decision.clone(),
+                    population[p2].decision.clone(),
+                ];
+                let children = self.variation.vary(&parents, &mut rng);
+                assert!(!children.is_empty(), "Rvea variation returned no children");
+                for child in children {
+                    if offspring_decisions.len() >= n {
+                        break;
+                    }
+                    offspring_decisions.push(child);
+                }
+            }
+            let offspring = evaluate_batch_async(problem, offspring_decisions, concurrency).await;
+            evaluations += offspring.len();
+
+            let mut combined: Vec<Candidate<P::Decision>> = Vec::with_capacity(2 * n);
+            combined.extend(population);
+            combined.extend(offspring);
+
+            let m_dim = m;
+            let mut ideal = vec![f64::INFINITY; m_dim];
+            for c in &combined {
+                let oriented = objectives.as_minimization(&c.evaluation.objectives);
+                for (k, v) in oriented.iter().enumerate() {
+                    if *v < ideal[k] {
+                        ideal[k] = *v;
+                    }
+                }
+            }
+            let translated: Vec<Vec<f64>> = combined
+                .iter()
+                .map(|c| {
+                    let oriented = objectives.as_minimization(&c.evaluation.objectives);
+                    oriented
+                        .iter()
+                        .enumerate()
+                        .map(|(k, v)| v - ideal[k])
+                        .collect()
+                })
+                .collect();
+
+            let mut assoc: Vec<usize> = vec![0; combined.len()];
+            let mut angles: Vec<f64> = vec![0.0; combined.len()];
+            for (i, t) in translated.iter().enumerate() {
+                let (best_ref, best_angle) = closest_reference(t, &references);
+                assoc[i] = best_ref;
+                angles[i] = best_angle;
+            }
+
+            let alpha_t = (gen_idx as f64 / (self.config.generations as f64).max(1.0))
+                .powf(self.config.alpha);
+            let mut keep: Vec<Option<(usize, f64)>> = vec![None; references.len()];
+            for i in 0..combined.len() {
+                let r = assoc[i];
+                let length: f64 = translated[i].iter().map(|v| v * v).sum::<f64>().sqrt();
+                let theta_max_safe = theta_max.max(1e-12);
+                let penalty = 1.0 + (m_dim as f64) * alpha_t * (angles[i] / theta_max_safe);
+                let apd = penalty * length;
+                match keep[r] {
+                    None => keep[r] = Some((i, apd)),
+                    Some((_, current)) if apd < current => keep[r] = Some((i, apd)),
+                    _ => {}
+                }
+            }
+
+            let mut next: Vec<Candidate<P::Decision>> = keep
+                .into_iter()
+                .flatten()
+                .map(|(i, _)| combined[i].clone())
+                .collect();
+            if next.len() < n {
+                let mut all_apds: Vec<(usize, f64)> = (0..combined.len())
+                    .map(|i| {
+                        let length: f64 = translated[i].iter().map(|v| v * v).sum::<f64>().sqrt();
+                        let theta_max_safe = theta_max.max(1e-12);
+                        let penalty = 1.0 + (m_dim as f64) * alpha_t * (angles[i] / theta_max_safe);
+                        (i, penalty * length)
+                    })
+                    .collect();
+                all_apds.sort_by(|a, b| a.1.partial_cmp(&b.1).unwrap_or(std::cmp::Ordering::Equal));
+                for (i, _) in all_apds {
+                    if next.len() >= n {
+                        break;
+                    }
+                    if !next
+                        .iter()
+                        .any(|c| std::ptr::eq(c as *const _, &combined[i] as *const _))
+                    {
+                        next.push(combined[i].clone());
+                    }
+                }
+            }
+            if next.len() > n {
+                next.truncate(n);
+            }
+            population = next;
+        }
+
+        let front = pareto_front(&population, &objectives);
+        let best = best_candidate(&population, &objectives);
+        OptimizationResult::new(
+            Population::new(population),
+            front,
+            best,
+            evaluations,
+            self.config.generations,
+        )
+    }
+}
+
 fn unit_normalize(mut v: Vec<f64>) -> Vec<f64> {
     let n: f64 = v.iter().map(|x| x * x).sum::<f64>().sqrt();
     if n > 1e-12 {

src/algorithms/simulated_annealing.rs

@@ -215,6 +215,124 @@ fn better_than(
     }
 }
 
+#[cfg(feature = "async")]
+impl<I, V> SimulatedAnnealing<I, V> {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` is mostly inert here because SA evaluates one
+    /// child per iteration; it's accepted for API parity with other
+    /// algorithms.
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<P::Decision>
+    where
+        P: crate::core::async_problem::AsyncProblem,
+        I: Initializer<P::Decision>,
+        V: Variation<P::Decision>,
+    {
+        let _ = concurrency;
+        let objectives = problem.objectives();
+        assert!(
+            objectives.is_single_objective(),
+            "SimulatedAnnealing requires exactly one objective",
+        );
+        assert!(
+            self.config.initial_temperature > 0.0,
+            "SimulatedAnnealing initial_temperature must be positive",
+        );
+        assert!(
+            self.config.final_temperature > 0.0,
+            "SimulatedAnnealing final_temperature must be positive",
+        );
+        assert!(
+            self.config.final_temperature <= self.config.initial_temperature,
+            "SimulatedAnnealing final_temperature must be <= initial_temperature",
+        );
+        let direction = objectives.objectives[0].direction;
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let mut initial = self.initializer.initialize(1, &mut rng);
+        assert!(
+            !initial.is_empty(),
+            "SimulatedAnnealing initializer returned no decisions",
+        );
+        let mut current_decision = initial.remove(0);
+        let mut current_eval = problem.evaluate_async(&current_decision).await;
+        let mut best_decision = current_decision.clone();
+        let mut best_eval = current_eval.clone();
+        let mut evaluations = 1usize;
+
+        let cooling = if self.config.iterations <= 1 {
+            1.0
+        } else {
+            (self.config.final_temperature / self.config.initial_temperature)
+                .powf(1.0 / (self.config.iterations as f64 - 1.0))
+        };
+        let mut temperature = self.config.initial_temperature;
+
+        for _ in 0..self.config.iterations {
+            let parents = vec![current_decision.clone()];
+            let children = self.variation.vary(&parents, &mut rng);
+            assert!(
+                !children.is_empty(),
+                "SimulatedAnnealing variation returned no children"
+            );
+            let child_decision = children.into_iter().next().unwrap();
+            let child_eval = problem.evaluate_async(&child_decision).await;
+            evaluations += 1;
+
+            let accept = match (child_eval.is_feasible(), current_eval.is_feasible()) {
+                (true, false) => true,
+                (false, true) => false,
+                (false, false) => {
+                    child_eval.constraint_violation <= current_eval.constraint_violation
+                }
+                (true, true) => {
+                    let delta = match direction {
+                        Direction::Minimize => {
+                            child_eval.objectives[0] - current_eval.objectives[0]
+                        }
+                        Direction::Maximize => {
+                            current_eval.objectives[0] - child_eval.objectives[0]
+                        }
+                    };
+                    if delta <= 0.0 {
+                        true
+                    } else {
+                        let prob = (-delta / temperature).exp();
+                        rng.random::<f64>() < prob
+                    }
+                }
+            };
+
+            if accept {
+                current_decision = child_decision;
+                current_eval = child_eval;
+                if better_than(&current_eval, &best_eval, direction) {
+                    best_decision = current_decision.clone();
+                    best_eval = current_eval.clone();
+                }
+            }
+            temperature *= cooling;
+        }
+
+        let best = Candidate::new(best_decision, best_eval);
+        let population = Population::new(vec![best.clone()]);
+        let front = vec![best.clone()];
+        OptimizationResult::new(
+            population,
+            front,
+            Some(best),
+            evaluations,
+            self.config.iterations,
+        )
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/sms_emoa.rs

@@ -173,6 +173,80 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl<I, V> SmsEmoa<I, V> {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations of the initial
+    /// population. Per-generation evaluations are sequential because
+    /// SMS-EMOA is a steady-state algorithm (one child per generation).
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<P::Decision>
+    where
+        P: crate::core::async_problem::AsyncProblem,
+        I: Initializer<P::Decision>,
+        V: Variation<P::Decision>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+
+        assert!(
+            self.config.population_size > 0,
+            "SmsEmoa population_size must be > 0"
+        );
+        let n = self.config.population_size;
+        let objectives = problem.objectives();
+        assert_eq!(
+            self.config.reference_point.len(),
+            objectives.len(),
+            "SmsEmoa reference_point.len() must equal number of objectives",
+        );
+        let reference = self.config.reference_point.clone();
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let initial_decisions = self.initializer.initialize(n, &mut rng);
+        let mut population: Vec<Candidate<P::Decision>> =
+            evaluate_batch_async(problem, initial_decisions, concurrency).await;
+        let mut evaluations = population.len();
+
+        for _ in 0..self.config.generations {
+            let p1 = rng.random_range(0..population.len());
+            let p2 = rng.random_range(0..population.len());
+            let parents = vec![
+                population[p1].decision.clone(),
+                population[p2].decision.clone(),
+            ];
+            let children = self.variation.vary(&parents, &mut rng);
+            assert!(
+                !children.is_empty(),
+                "SmsEmoa variation returned no children"
+            );
+            let child_decision = children.into_iter().next().unwrap();
+            let child_eval = problem.evaluate_async(&child_decision).await;
+            evaluations += 1;
+            let child = Candidate::new(child_decision, child_eval);
+
+            population.push(child);
+            let drop_idx = pick_drop_index(&population, &objectives, &reference);
+            population.swap_remove(drop_idx);
+        }
+
+        let front = pareto_front(&population, &objectives);
+        let best = best_candidate(&population, &objectives);
+        OptimizationResult::new(
+            Population::new(population),
+            front,
+            best,
+            evaluations,
+            self.config.generations,
+        )
+    }
+}
+
 /// Choose the index in `pool` whose removal is preferred per SMS-EMOA's
 /// rules: drop from the worst non-dominated front; within that front,
 /// drop the member whose removal increases hypervolume the most (= the

src/algorithms/snes.rs

@@ -222,6 +222,137 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl SeparableNes {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations per generation.
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<Vec<f64>>
+    where
+        P: crate::core::async_problem::AsyncProblem<Decision = Vec<f64>>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+
+        assert!(
+            self.config.population_size >= 2,
+            "SeparableNes population_size must be >= 2",
+        );
+        assert!(
+            self.config.initial_sigma > 0.0,
+            "SeparableNes initial_sigma must be > 0"
+        );
+        let objectives = problem.objectives();
+        assert!(
+            objectives.is_single_objective(),
+            "SeparableNes requires exactly one objective",
+        );
+        let direction = objectives.objectives[0].direction;
+        let n = self.bounds.bounds.len();
+        let lambda = self.config.population_size;
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let mut mean: Vec<f64> = self
+            .bounds
+            .bounds
+            .iter()
+            .map(|&(lo, hi)| 0.5 * (lo + hi))
+            .collect();
+        let mut sigma = vec![self.config.initial_sigma; n];
+
+        let eta_sigma = self
+            .config
+            .sigma_learning_rate
+            .unwrap_or_else(|| (3.0 + (n as f64).ln()) / (5.0 * (n as f64).sqrt()));
+        let eta_mean = self.config.mean_learning_rate;
+
+        let utilities = nes_utilities(lambda);
+
+        let mut best_seen: Option<Candidate<Vec<f64>>> = None;
+        let mut total_evaluations = 0usize;
+
+        for _ in 0..self.config.generations {
+            // Sample λ offspring; matches the sync RNG draw order so seeded
+            // runs reproduce exactly.
+            let mut z_samples: Vec<Vec<f64>> = Vec::with_capacity(lambda);
+            let mut x_samples: Vec<Vec<f64>> = Vec::with_capacity(lambda);
+            for _ in 0..lambda {
+                let z: Vec<f64> = (0..n)
+                    .map(|_| Normal::new(0.0, 1.0).unwrap().sample(&mut rng))
+                    .collect();
+                let x: Vec<f64> = (0..n)
+                    .map(|j| {
+                        let v = mean[j] + sigma[j] * z[j];
+                        let (lo, hi) = self.bounds.bounds[j];
+                        v.clamp(lo, hi)
+                    })
+                    .collect();
+                z_samples.push(z);
+                x_samples.push(x);
+            }
+
+            let cands = evaluate_batch_async(problem, x_samples.clone(), concurrency).await;
+            total_evaluations += cands.len();
+            let evals: Vec<Evaluation> = cands.iter().map(|c| c.evaluation.clone()).collect();
+            for c in &cands {
+                let beats_best = match &best_seen {
+                    None => true,
+                    Some(b) => better(&c.evaluation, &b.evaluation, direction),
+                };
+                if beats_best {
+                    best_seen = Some(c.clone());
+                }
+            }
+
+            let mut order: Vec<usize> = (0..lambda).collect();
+            order.sort_by(|&a, &b| compare(&evals[a], &evals[b], direction));
+
+            let mut grad_mean = vec![0.0_f64; n];
+            for k in 0..lambda {
+                let u = utilities[k];
+                let z = &z_samples[order[k]];
+                for j in 0..n {
+                    grad_mean[j] += u * z[j];
+                }
+            }
+            for j in 0..n {
+                mean[j] += eta_mean * sigma[j] * grad_mean[j];
+                let (lo, hi) = self.bounds.bounds[j];
+                mean[j] = mean[j].clamp(lo, hi);
+            }
+
+            for j in 0..n {
+                let mut grad_sigma_j = 0.0;
+                for k in 0..lambda {
+                    let u = utilities[k];
+                    let z = &z_samples[order[k]];
+                    grad_sigma_j += u * (z[j] * z[j] - 1.0);
+                }
+                sigma[j] *= (0.5 * eta_sigma * grad_sigma_j).exp();
+                if !sigma[j].is_finite() || sigma[j] < 1e-30 {
+                    sigma[j] = 1e-30;
+                }
+            }
+        }
+
+        let best = best_seen.expect("at least one generation evaluated");
+        let population = Population::new(vec![best.clone()]);
+        let front = vec![best.clone()];
+        OptimizationResult::new(
+            population,
+            front,
+            Some(best),
+            total_evaluations,
+            self.config.generations,
+        )
+    }
+}
+
 fn nes_utilities(lambda: usize) -> Vec<f64> {
     let half = lambda as f64 / 2.0 + 1.0;
     let raw: Vec<f64> = (0..lambda)

src/algorithms/spea2.rs

@@ -169,6 +169,91 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl<I, V> Spea2<I, V> {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations per batch.
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<P::Decision>
+    where
+        P: crate::core::async_problem::AsyncProblem,
+        I: Initializer<P::Decision>,
+        V: Variation<P::Decision>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+
+        assert!(
+            self.config.population_size > 0,
+            "Spea2 population_size must be greater than 0",
+        );
+        assert!(
+            self.config.archive_size > 0,
+            "Spea2 archive_size must be greater than 0",
+        );
+        let n_pop = self.config.population_size;
+        let n_arc = self.config.archive_size;
+        let objectives = problem.objectives();
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let initial_decisions = self.initializer.initialize(n_pop, &mut rng);
+        assert_eq!(
+            initial_decisions.len(),
+            n_pop,
+            "SPEA2 initializer must return exactly population_size decisions",
+        );
+        let mut population: Vec<Candidate<P::Decision>> =
+            evaluate_batch_async(problem, initial_decisions, concurrency).await;
+        let mut evaluations = population.len();
+        let mut archive: Vec<Candidate<P::Decision>> = Vec::new();
+
+        for _ in 0..self.config.generations {
+            let mut pool: Vec<Candidate<P::Decision>> =
+                Vec::with_capacity(population.len() + archive.len());
+            pool.append(&mut population);
+            pool.append(&mut archive);
+            let fitness = compute_fitness(&pool, &objectives);
+
+            archive = build_archive(&pool, &fitness, &objectives, n_arc);
+
+            let archive_fitness = compute_fitness(&archive, &objectives);
+            let mut offspring_decisions: Vec<P::Decision> = Vec::with_capacity(n_pop);
+            while offspring_decisions.len() < n_pop {
+                let p1 = binary_tournament(&archive_fitness, &mut rng);
+                let p2 = binary_tournament(&archive_fitness, &mut rng);
+                let parents = vec![archive[p1].decision.clone(), archive[p2].decision.clone()];
+                let children = self.variation.vary(&parents, &mut rng);
+                assert!(!children.is_empty(), "SPEA2 variation returned no children");
+                for child_decision in children {
+                    if offspring_decisions.len() >= n_pop {
+                        break;
+                    }
+                    offspring_decisions.push(child_decision);
+                }
+            }
+            let new_population =
+                evaluate_batch_async(problem, offspring_decisions, concurrency).await;
+            evaluations += new_population.len();
+            population = new_population;
+        }
+
+        let front = pareto_front(&archive, &objectives);
+        let best = best_candidate(&archive, &objectives);
+        OptimizationResult::new(
+            Population::new(archive),
+            front,
+            best,
+            evaluations,
+            self.config.generations,
+        )
+    }
+}
+
 /// SPEA2 fitness: `R(i) + D(i)`, where lower is better.
 ///
 /// `R(i)` is the sum of `S(j)` over all `j` that dominate `i`. `S(j)` is the

src/algorithms/tabu_search.rs

@@ -209,6 +209,128 @@ fn better_than(
     }
 }
 
+#[cfg(feature = "async")]
+impl<D, I, N> TabuSearch<D, I, N>
+where
+    D: Clone + Hash + Eq,
+    I: Initializer<D>,
+    N: FnMut(&D, &mut Rng) -> Vec<D>,
+{
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// Each iteration evaluates the K neighbors of the current
+    /// incumbent concurrently (bounded by `concurrency`), then picks
+    /// the best non-tabu (or aspiration-passing) move.
+    pub async fn run_async<P>(&mut self, problem: &P, concurrency: usize) -> OptimizationResult<D>
+    where
+        P: crate::core::async_problem::AsyncProblem<Decision = D>,
+        D: Send + Sync,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+
+        let objectives = problem.objectives();
+        assert!(
+            objectives.is_single_objective(),
+            "TabuSearch requires exactly one objective",
+        );
+        assert!(
+            self.config.tabu_tenure >= 1,
+            "TabuSearch tabu_tenure must be >= 1",
+        );
+        let direction = objectives.objectives[0].direction;
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let mut initial = self.initializer.initialize(1, &mut rng);
+        assert!(
+            !initial.is_empty(),
+            "TabuSearch initializer returned no decisions"
+        );
+        let mut current_decision = initial.remove(0);
+        let mut current_eval = problem.evaluate_async(&current_decision).await;
+        let mut best_decision = current_decision.clone();
+        let mut best_eval = current_eval.clone();
+        let mut evaluations = 1usize;
+
+        let mut tabu_queue: VecDeque<D> = VecDeque::with_capacity(self.config.tabu_tenure);
+        let mut tabu_set: HashSet<D> = HashSet::new();
+
+        for _ in 0..self.config.iterations {
+            let candidates = (self.neighbors)(&current_decision, &mut rng);
+            if candidates.is_empty() {
+                break;
+            }
+
+            let cand_results = evaluate_batch_async(problem, candidates.clone(), concurrency).await;
+            let mut cand_evals: Vec<crate::core::evaluation::Evaluation> =
+                cand_results.into_iter().map(|c| c.evaluation).collect();
+            evaluations += candidates.len();
+
+            let mut best_idx: Option<usize> = None;
+            let mut best_cand_eval: Option<crate::core::evaluation::Evaluation> = None;
+
+            for (i, c) in candidates.iter().enumerate() {
+                let is_tabu = tabu_set.contains(c);
+                let aspires = is_tabu && better_than(&cand_evals[i], &best_eval, direction);
+                if is_tabu && !aspires {
+                    continue;
+                }
+                let eligible = match &best_cand_eval {
+                    None => true,
+                    Some(b) => better_than(&cand_evals[i], b, direction),
+                };
+                if eligible {
+                    best_idx = Some(i);
+                    best_cand_eval = Some(cand_evals[i].clone());
+                }
+            }
+
+            if best_idx.is_none() {
+                for (i, _) in candidates.iter().enumerate() {
+                    let eligible = match &best_cand_eval {
+                        None => true,
+                        Some(b) => better_than(&cand_evals[i], b, direction),
+                    };
+                    if eligible {
+                        best_idx = Some(i);
+                        best_cand_eval = Some(cand_evals[i].clone());
+                    }
+                }
+            }
+
+            let chosen_idx = best_idx.expect("non-empty candidate list");
+            let chosen_decision = candidates[chosen_idx].clone();
+            current_eval = cand_evals.remove(chosen_idx);
+            current_decision = chosen_decision.clone();
+
+            if better_than(&current_eval, &best_eval, direction) {
+                best_decision = current_decision.clone();
+                best_eval = current_eval.clone();
+            }
+
+            tabu_queue.push_back(chosen_decision.clone());
+            tabu_set.insert(chosen_decision);
+            if tabu_queue.len() > self.config.tabu_tenure {
+                if let Some(old) = tabu_queue.pop_front() {
+                    tabu_set.remove(&old);
+                }
+            }
+        }
+
+        let best = Candidate::new(best_decision, best_eval);
+        let population = Population::new(vec![best.clone()]);
+        let front = vec![best.clone()];
+        OptimizationResult::new(
+            population,
+            front,
+            Some(best),
+            evaluations,
+            self.config.iterations,
+        )
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/tlbo.rs

@@ -187,6 +187,122 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl Tlbo {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations within batched phases
+    /// (only the initial population uses a batch; the teacher and learner
+    /// phases evaluate sequentially because each accept/reject step
+    /// depends on the previous one).
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<Vec<f64>>
+    where
+        P: crate::core::async_problem::AsyncProblem<Decision = Vec<f64>>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+
+        assert!(
+            self.config.population_size >= 2,
+            "Tlbo population_size must be >= 2"
+        );
+        let objectives = problem.objectives();
+        assert!(
+            objectives.is_single_objective(),
+            "Tlbo requires exactly one objective",
+        );
+        let direction = objectives.objectives[0].direction;
+        let dim = self.bounds.bounds.len();
+        let n = self.config.population_size;
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let mut decisions: Vec<Vec<f64>> = {
+            use crate::traits::Initializer as _;
+            self.bounds.initialize(n, &mut rng)
+        };
+        let initial = evaluate_batch_async(problem, decisions.clone(), concurrency).await;
+        let mut evals: Vec<Evaluation> = initial.iter().map(|c| c.evaluation.clone()).collect();
+        let mut evaluations = initial.len();
+
+        for _ in 0..self.config.generations {
+            let teacher_idx = best_index(&evals, direction);
+            let teacher = decisions[teacher_idx].clone();
+            let mut mean = vec![0.0_f64; dim];
+            for d in &decisions {
+                for j in 0..dim {
+                    mean[j] += d[j];
+                }
+            }
+            for v in mean.iter_mut() {
+                *v /= n as f64;
+            }
+            let tf = if rng.random_bool(0.5) { 1.0 } else { 2.0 };
+
+            for i in 0..n {
+                let mut candidate = decisions[i].clone();
+                for j in 0..dim {
+                    let r: f64 = rng.random();
+                    candidate[j] += r * (teacher[j] - tf * mean[j]);
+                    let (lo, hi) = self.bounds.bounds[j];
+                    candidate[j] = candidate[j].clamp(lo, hi);
+                }
+                let cand_eval = problem.evaluate_async(&candidate).await;
+                evaluations += 1;
+                if better(&cand_eval, &evals[i], direction) {
+                    decisions[i] = candidate;
+                    evals[i] = cand_eval;
+                }
+            }
+
+            for i in 0..n {
+                let mut k = rng.random_range(0..n);
+                while k == i && n > 1 {
+                    k = rng.random_range(0..n);
+                }
+                let partner_better = better(&evals[k], &evals[i], direction);
+                let mut candidate = decisions[i].clone();
+                for j in 0..dim {
+                    let r: f64 = rng.random();
+                    let delta = if partner_better {
+                        r * (decisions[k][j] - decisions[i][j])
+                    } else {
+                        r * (decisions[i][j] - decisions[k][j])
+                    };
+                    candidate[j] += delta;
+                    let (lo, hi) = self.bounds.bounds[j];
+                    candidate[j] = candidate[j].clamp(lo, hi);
+                }
+                let cand_eval = problem.evaluate_async(&candidate).await;
+                evaluations += 1;
+                if better(&cand_eval, &evals[i], direction) {
+                    decisions[i] = candidate;
+                    evals[i] = cand_eval;
+                }
+            }
+        }
+
+        let final_pop: Vec<Candidate<Vec<f64>>> = decisions
+            .into_iter()
+            .zip(evals)
+            .map(|(d, e)| Candidate::new(d, e))
+            .collect();
+        let best = best_candidate(&final_pop, &objectives);
+        let front: Vec<Candidate<Vec<f64>>> = best.iter().cloned().collect();
+        OptimizationResult::new(
+            Population::new(final_pop),
+            front,
+            best,
+            evaluations,
+            self.config.generations,
+        )
+    }
+}
+
 fn best_index(evals: &[Evaluation], direction: Direction) -> usize {
     let mut idx = 0;
     for i in 1..evals.len() {

src/algorithms/tpe.rs

@@ -356,6 +356,129 @@ fn scott_bandwidths(decisions: &[Vec<f64>], support: &[usize], factor: f64) -> V
         .collect()
 }
 
+#[cfg(feature = "async")]
+impl Tpe {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations during the initial
+    /// uniform-sample design; the sequential TPE loop runs one
+    /// evaluation per iteration regardless.
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<Vec<f64>>
+    where
+        P: crate::core::async_problem::AsyncProblem<Decision = Vec<f64>>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+
+        assert!(
+            self.config.initial_samples >= 2,
+            "Tpe initial_samples must be >= 2"
+        );
+        assert!(
+            self.config.good_fraction > 0.0 && self.config.good_fraction < 1.0,
+            "Tpe good_fraction must be in (0, 1)",
+        );
+        assert!(
+            self.config.candidate_samples >= 1,
+            "Tpe candidate_samples must be >= 1",
+        );
+        assert!(
+            self.config.bandwidth_factor > 0.0,
+            "Tpe bandwidth_factor must be > 0"
+        );
+        let objectives = problem.objectives();
+        assert!(
+            objectives.is_single_objective(),
+            "Tpe requires exactly one objective",
+        );
+        let direction = objectives.objectives[0].direction;
+        let dim = self.bounds.bounds.len();
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let mut decisions: Vec<Vec<f64>> = Vec::new();
+        let mut targets: Vec<f64> = Vec::new();
+        let mut evals: Vec<Evaluation> = Vec::new();
+
+        let initial_decisions: Vec<Vec<f64>> = (0..self.config.initial_samples)
+            .map(|_| sample_uniform_in_bounds(&self.bounds, &mut rng))
+            .collect();
+        let initial_cands = evaluate_batch_async(problem, initial_decisions, concurrency).await;
+        for c in initial_cands {
+            targets.push(oriented_target(&c.evaluation, direction));
+            decisions.push(c.decision);
+            evals.push(c.evaluation);
+        }
+
+        for _ in 0..self.config.iterations {
+            let (good_idx, bad_idx) = split_good_bad(&targets, self.config.good_fraction);
+
+            let mut best_x: Option<Vec<f64>> = None;
+            let mut best_ratio = f64::NEG_INFINITY;
+            for _ in 0..self.config.candidate_samples {
+                let cand = sample_from_kde(
+                    &decisions,
+                    &good_idx,
+                    &self.bounds,
+                    self.config.bandwidth_factor,
+                    &mut rng,
+                );
+                let l = log_kde_density(
+                    &cand,
+                    &decisions,
+                    &good_idx,
+                    &self.bounds,
+                    self.config.bandwidth_factor,
+                );
+                let g = log_kde_density(
+                    &cand,
+                    &decisions,
+                    &bad_idx,
+                    &self.bounds,
+                    self.config.bandwidth_factor,
+                );
+                let ratio = l - g;
+                if ratio > best_ratio {
+                    best_ratio = ratio;
+                    best_x = Some(cand);
+                }
+            }
+            let x = best_x.expect("at least one candidate sampled");
+            let _ = dim;
+            let e = problem.evaluate_async(&x).await;
+            targets.push(oriented_target(&e, direction));
+            decisions.push(x);
+            evals.push(e);
+        }
+
+        let mut best_idx = 0;
+        for i in 1..evals.len() {
+            if better(&evals[i], &evals[best_idx], direction) {
+                best_idx = i;
+            }
+        }
+        let total_evals = evals.len();
+        let final_pop: Vec<Candidate<Vec<f64>>> = decisions
+            .into_iter()
+            .zip(evals)
+            .map(|(d, e)| Candidate::new(d, e))
+            .collect();
+        let best = final_pop[best_idx].clone();
+        let front = vec![best.clone()];
+        OptimizationResult::new(
+            Population::new(final_pop),
+            front,
+            Some(best),
+            total_evals,
+            self.config.iterations + self.config.initial_samples,
+        )
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/umda.rs

@@ -202,6 +202,125 @@ where
     }
 }
 
+#[cfg(feature = "async")]
+impl Umda {
+    /// Async version of [`Optimizer::run`] — drives evaluations through
+    /// the user-chosen async runtime. Available only with the `async`
+    /// feature.
+    ///
+    /// `concurrency` bounds in-flight evaluations per batch (initial
+    /// population and per-generation samples).
+    pub async fn run_async<P>(
+        &mut self,
+        problem: &P,
+        concurrency: usize,
+    ) -> OptimizationResult<Vec<bool>>
+    where
+        P: crate::core::async_problem::AsyncProblem<Decision = Vec<bool>>,
+    {
+        use crate::algorithms::parallel_eval_async::evaluate_batch_async;
+
+        assert!(
+            self.config.population_size >= 2,
+            "Umda population_size must be >= 2"
+        );
+        assert!(
+            self.config.selected_size >= 1,
+            "Umda selected_size must be >= 1",
+        );
+        assert!(
+            self.config.selected_size <= self.config.population_size,
+            "Umda selected_size must be <= population_size",
+        );
+        assert!(self.config.bits >= 1, "Umda bits must be >= 1");
+        let objectives = problem.objectives();
+        assert!(
+            objectives.is_single_objective(),
+            "Umda requires exactly one objective",
+        );
+        let direction = objectives.objectives[0].direction;
+        let n = self.config.population_size;
+        let bits = self.config.bits;
+        let mu = self.config.selected_size;
+        let mut rng = rng_from_seed(self.config.seed);
+
+        let mut decisions: Vec<Vec<bool>> = (0..n)
+            .map(|_| (0..bits).map(|_| rng.random_bool(0.5)).collect())
+            .collect();
+        let mut population = evaluate_batch_async(problem, decisions.clone(), concurrency).await;
+        let mut evaluations = population.len();
+
+        let smoothing = 1.0 / (2.0 * mu as f64);
+        let prob_min = smoothing;
+        let prob_max = 1.0 - smoothing;
+
+        let mut best_seen: Option<Candidate<Vec<bool>>> = None;
+        for c in &population {
+            let beats = match &best_seen {
+                None => true,
+                Some(b) => better_than_so(&c.evaluation, &b.evaluation, direction),
+            };
+            if beats {
+                best_seen = Some(c.clone());
+            }
+        }
+
+        for _ in 0..self.config.generations {
+            let mut order: Vec<usize> = (0..population.len()).collect();
+            order.sort_by(|&a, &b| {
+                compare_so(
+                    &population[a].evaluation,
+                    &population[b].evaluation,
+                    direction,
+                )
+            });
+            let selected: Vec<&Candidate<Vec<bool>>> =
+                order.iter().take(mu).map(|&i| &population[i]).collect();
+
+            let mut probs = vec![0.0_f64; bits];
+            for c in &selected {
+                for (i, b) in c.decision.iter().enumerate() {
+                    if *b {
+                        probs[i] += 1.0;
+                    }
+                }
+            }
+            for p in probs.iter_mut() {
+                *p = (*p / mu as f64).clamp(prob_min, prob_max);
+            }
+
+            decisions = (0..n)
+                .map(|_| probs.iter().map(|&p| rng.random_bool(p)).collect())
+                .collect();
+
+            population = evaluate_batch_async(problem, decisions.clone(), concurrency).await;
+            evaluations += population.len();
+
+            for c in &population {
+                let beats = match &best_seen {
+                    None => true,
+                    Some(b) => better_than_so(&c.evaluation, &b.evaluation, direction),
+                };
+                if beats {
+                    best_seen = Some(c.clone());
+                }
+            }
+        }
+
+        let best = best_seen.expect("at least one generation evaluated");
+        let final_pop = vec![best.clone()];
+        let front = vec![best.clone()];
+        let best_opt = best_candidate(&final_pop, &objectives);
+        OptimizationResult::new(
+            Population::new(final_pop),
+            front,
+            best_opt,
+            evaluations,
+            self.config.generations,
+        )
+    }
+}
+
 fn compare_so(
     a: &crate::core::evaluation::Evaluation,
     b: &crate::core::evaluation::Evaluation,

src/core/async_problem.rs

@@ -0,0 +1,81 @@
+//! Async-evaluable problems for IO-bound workloads.
+//!
+//! Most heuropt algorithms operate synchronously: their `Problem::evaluate`
+//! returns immediately. For workloads where evaluation is *IO-bound* — calling
+//! an HTTP service, querying a remote model, spawning a subprocess —
+//! awaiting an async fn is much more efficient than blocking a worker
+//! thread.
+//!
+//! [`AsyncProblem`] mirrors [`Problem`](crate::core::Problem) but its
+//! `evaluate_async` returns a future. Every algorithm in heuropt exposes
+//! a `run_async` method that drives evaluations through a user-chosen
+//! async runtime (typically tokio). Hyperband uses
+//! [`AsyncPartialProblem`] instead, which mirrors
+//! [`PartialProblem`](crate::core::partial_problem::PartialProblem) for
+//! multi-fidelity workloads.
+//!
+//! Available only with the `async` feature.
+
+use std::future::Future;
+
+use crate::core::evaluation::Evaluation;
+use crate::core::objective::ObjectiveSpace;
+
+/// A problem whose evaluation is async — useful when `evaluate` does
+/// IO (HTTP, RPC, subprocess) rather than pure CPU work.
+///
+/// Mirrors [`Problem`](crate::core::Problem) one-for-one except that
+/// `evaluate_async` returns a future. The returned future must be
+/// `Send` so the algorithm can run many evaluations concurrently
+/// across a runtime's worker pool.
+///
+/// Implementors who already have a synchronous `Problem` can adapt
+/// to `AsyncProblem` with a one-line wrapper:
+///
+/// ```ignore
+/// impl AsyncProblem for MyProblem {
+///     type Decision = <Self as Problem>::Decision;
+///     fn objectives(&self) -> ObjectiveSpace { Problem::objectives(self) }
+///     async fn evaluate_async(&self, x: &Self::Decision) -> Evaluation {
+///         Problem::evaluate(self, x)
+///     }
+/// }
+/// ```
+pub trait AsyncProblem: Sync {
+    /// The thing the optimizer changes. Same constraints as
+    /// [`Problem::Decision`](crate::core::Problem::Decision).
+    type Decision: Clone + Send + Sync;
+
+    /// Return the objectives for this problem.
+    fn objectives(&self) -> ObjectiveSpace;
+
+    /// Evaluate `decision` asynchronously. The returned future is
+    /// driven by whichever runtime the algorithm's `run_async` is
+    /// invoked from.
+    fn evaluate_async(&self, decision: &Self::Decision) -> impl Future<Output = Evaluation> + Send;
+}
+
+/// Async equivalent of [`PartialProblem`](crate::core::partial_problem::PartialProblem)
+/// for multi-fidelity workloads — used by Hyperband's `run_async`.
+///
+/// Like [`AsyncProblem`], `evaluate_at_budget_async` returns a future
+/// so callers can fan out budgeted evaluations across an async runtime.
+pub trait AsyncPartialProblem: Sync {
+    /// The thing the optimizer changes. Same constraints as
+    /// [`PartialProblem::Decision`](crate::core::partial_problem::PartialProblem::Decision).
+    type Decision: Clone + Send + Sync;
+
+    /// Return the objectives for this problem.
+    fn objectives(&self) -> ObjectiveSpace;
+
+    /// Evaluate `decision` at the given fidelity `budget` asynchronously.
+    ///
+    /// Same monotonicity contract as
+    /// [`PartialProblem::evaluate_at_budget`](crate::core::partial_problem::PartialProblem::evaluate_at_budget):
+    /// higher budget should give a more accurate estimate.
+    fn evaluate_at_budget_async(
+        &self,
+        decision: &Self::Decision,
+        budget: f64,
+    ) -> impl Future<Output = Evaluation> + Send;
+}

src/core/mod.rs

@@ -1,5 +1,7 @@
 //! Concrete data types and the `Problem` trait that the rest of the crate is built on.
 
+#[cfg(feature = "async")]
+pub mod async_problem;
 pub mod candidate;
 pub mod evaluation;
 pub mod objective;
@@ -9,6 +11,8 @@ pub mod problem;
 pub mod result;
 pub mod rng;
 
+#[cfg(feature = "async")]
+pub use async_problem::AsyncProblem;
 pub use candidate::*;
 pub use evaluation::*;
 pub use objective::*;

src/core/population.rs

@@ -34,11 +34,6 @@ impl<D> Population<D> {
         self.candidates.iter()
     }
 
-    /// View the candidates as a slice.
-    pub fn as_slice(&self) -> &[Candidate<D>] {
-        &self.candidates
-    }
-
     /// Unwrap into the inner `Vec<Candidate<D>>`.
     pub fn into_vec(self) -> Vec<Candidate<D>> {
         self.candidates

src/lib.rs

@@ -4,7 +4,7 @@
 //! The crate aims to make three things obvious:
 //!
 //! 1. **Define a problem** by implementing [`Problem`](crate::core::Problem).
-//! 2. **Run a built-in optimizer** — pick from 35 algorithms in
+//! 2. **Run a built-in optimizer** — pick from 33 algorithms in
 //!    [`algorithms`] covering single-objective continuous (CMA-ES,
 //!    Differential Evolution, Nelder-Mead, …), multi-objective
 //!    (NSGA-II, MOPSO, IBEA, MOEA/D, …), many-objective (NSGA-III,
@@ -32,6 +32,12 @@
 //! - `parallel` — rayon-backed parallel population evaluation in
 //!   every population-based algorithm. Seeded runs stay bit-
 //!   identical to serial mode.
+//! - `async` — adds the
+//!   [`AsyncProblem`](crate::core::async_problem::AsyncProblem) and
+//!   [`AsyncPartialProblem`](crate::core::async_problem::AsyncPartialProblem)
+//!   traits and a `run_async(&problem, concurrency).await` method on
+//!   every algorithm. Use this when your `evaluate` does IO (HTTP,
+//!   RPC, subprocess) — see the [Async evaluation cookbook recipe](https://swaits.github.io/heuropt/cookbook/async.html).
 //!
 //! # Quick example
 //!
@@ -68,7 +74,6 @@ pub mod algorithms;
 pub mod core;
 pub(crate) mod internal;
 pub mod metrics;
-pub mod observer;
 pub mod operators;
 pub mod pareto;
 pub mod prelude;

src/metrics/hypervolume.rs

@@ -14,6 +14,26 @@ use crate::core::objective::ObjectiveSpace;
 ///
 /// # Panics
 /// If `objectives` does not have exactly two objectives.
+///
+/// # Example
+///
+/// ```
+/// use heuropt::prelude::*;
+/// use heuropt::metrics::hypervolume_2d;
+///
+/// let space = ObjectiveSpace::new(vec![
+///     Objective::minimize("f1"),
+///     Objective::minimize("f2"),
+/// ]);
+/// // Reference (4, 4); front at (1,3), (2,2), (3,1) → dominated area = 6.
+/// let front = [
+///     Candidate::new((), Evaluation::new(vec![1.0, 3.0])),
+///     Candidate::new((), Evaluation::new(vec![2.0, 2.0])),
+///     Candidate::new((), Evaluation::new(vec![3.0, 1.0])),
+/// ];
+/// let hv = hypervolume_2d(&front, &space, [4.0, 4.0]);
+/// assert!((hv - 6.0).abs() < 1e-12);
+/// ```
 pub fn hypervolume_2d<D>(
     front: &[Candidate<D>],
     objectives: &ObjectiveSpace,
@@ -147,6 +167,24 @@ mod tests {
 ///
 /// # Panics
 /// If `objectives.len() != reference_point.len()`, or if either is zero.
+///
+/// # Example
+///
+/// ```
+/// use heuropt::prelude::*;
+/// use heuropt::metrics::hypervolume_nd;
+///
+/// let space = ObjectiveSpace::new(vec![
+///     Objective::minimize("f1"),
+///     Objective::minimize("f2"),
+///     Objective::minimize("f3"),
+/// ]);
+/// // Single corner point at the origin against a unit-cube reference:
+/// // dominated volume = 1.
+/// let front = [Candidate::new((), Evaluation::new(vec![0.0, 0.0, 0.0]))];
+/// let hv = hypervolume_nd(&front, &space, &[1.0, 1.0, 1.0]);
+/// assert!((hv - 1.0).abs() < 1e-12);
+/// ```
 pub fn hypervolume_nd<D>(
     front: &[Candidate<D>],
     objectives: &ObjectiveSpace,

src/metrics/igd.rs

@@ -1,207 +0,0 @@
-//! Inverted Generational Distance (IGD) and IGD+ performance indicators.
-//!
-//! Both quantify how well an approximation set covers a reference set
-//! (typically the true Pareto front). Smaller values are better.
-
-use crate::core::candidate::Candidate;
-use crate::core::evaluation::Evaluation;
-use crate::core::objective::ObjectiveSpace;
-
-/// Inverted Generational Distance.
-///
-/// For each point in the `reference` set, compute the Euclidean distance
-/// to its nearest neighbor in the `approximation` set (in minimization-
-/// oriented objective space), then average:
-///
-/// ```text
-/// IGD(A) = (1 / |R|) · Σ_{r ∈ R} min_{a ∈ A} ‖a − r‖₂
-/// ```
-///
-/// Lower is better. IGD captures both convergence (close to the front)
-/// and spread (the approximation must cover the reference).
-///
-/// # Panics
-///
-/// If `reference` is empty.
-///
-/// # Example
-///
-/// ```
-/// use heuropt::prelude::*;
-/// use heuropt::metrics::igd::igd;
-///
-/// let space = ObjectiveSpace::new(vec![
-///     Objective::minimize("f1"),
-///     Objective::minimize("f2"),
-/// ]);
-/// // Approximation: a sparse 2-point front.
-/// let approx = [
-///     Candidate::new((), Evaluation::new(vec![0.0, 1.0])),
-///     Candidate::new((), Evaluation::new(vec![1.0, 0.0])),
-/// ];
-/// // Reference: a dense 3-point sample of the true front.
-/// let reference = [
-///     Evaluation::new(vec![0.0, 1.0]),
-///     Evaluation::new(vec![0.5, 0.5]),
-///     Evaluation::new(vec![1.0, 0.0]),
-/// ];
-/// let v = igd(&approx, &reference, &space);
-/// // The middle reference point is unfortunately distance √(0.5²+0.5²) = 0.707
-/// // from each approximation point; the boundary points are 0 away.
-/// // IGD = (0 + 0.707 + 0) / 3 ≈ 0.236.
-/// assert!((v - 0.2357).abs() < 1e-3);
-/// ```
-pub fn igd<D>(
-    approximation: &[Candidate<D>],
-    reference: &[Evaluation],
-    objectives: &ObjectiveSpace,
-) -> f64 {
-    assert!(
-        !reference.is_empty(),
-        "igd: reference set must not be empty"
-    );
-    let approx_oriented: Vec<Vec<f64>> = approximation
-        .iter()
-        .map(|c| objectives.as_minimization(&c.evaluation.objectives))
-        .collect();
-    if approx_oriented.is_empty() {
-        return f64::INFINITY;
-    }
-    let mut total = 0.0_f64;
-    for r in reference {
-        let r_oriented = objectives.as_minimization(&r.objectives);
-        let mut min_d = f64::INFINITY;
-        for a in &approx_oriented {
-            let d: f64 = a
-                .iter()
-                .zip(r_oriented.iter())
-                .map(|(x, y)| (x - y).powi(2))
-                .sum::<f64>()
-                .sqrt();
-            if d < min_d {
-                min_d = d;
-            }
-        }
-        total += min_d;
-    }
-    total / reference.len() as f64
-}
-
-/// IGD+ — a dominance-respecting variant of IGD.
-///
-/// For each reference point `r`, the distance to an approximation
-/// point `a` is computed only on objectives where `a` is *worse than*
-/// `r` — i.e. on the "violation" component of the gap. This makes
-/// IGD+ a Pareto-compliant indicator: adding a dominated point to the
-/// approximation never improves the score.
-///
-/// ```text
-/// IGD+(A) = (1 / |R|) · Σ_{r ∈ R} min_{a ∈ A} ‖max(a − r, 0)‖₂
-/// ```
-///
-/// Lower is better.
-///
-/// # Panics
-///
-/// If `reference` is empty.
-pub fn igd_plus<D>(
-    approximation: &[Candidate<D>],
-    reference: &[Evaluation],
-    objectives: &ObjectiveSpace,
-) -> f64 {
-    assert!(
-        !reference.is_empty(),
-        "igd_plus: reference set must not be empty"
-    );
-    let approx_oriented: Vec<Vec<f64>> = approximation
-        .iter()
-        .map(|c| objectives.as_minimization(&c.evaluation.objectives))
-        .collect();
-    if approx_oriented.is_empty() {
-        return f64::INFINITY;
-    }
-    let mut total = 0.0_f64;
-    for r in reference {
-        let r_oriented = objectives.as_minimization(&r.objectives);
-        let mut min_d = f64::INFINITY;
-        for a in &approx_oriented {
-            let d: f64 = a
-                .iter()
-                .zip(r_oriented.iter())
-                .map(|(x, y)| (x - y).max(0.0).powi(2))
-                .sum::<f64>()
-                .sqrt();
-            if d < min_d {
-                min_d = d;
-            }
-        }
-        total += min_d;
-    }
-    total / reference.len() as f64
-}
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-    use crate::core::objective::Objective;
-
-    fn space_min2() -> ObjectiveSpace {
-        ObjectiveSpace::new(vec![Objective::minimize("f1"), Objective::minimize("f2")])
-    }
-
-    fn cand(obj: Vec<f64>) -> Candidate<()> {
-        Candidate::new((), Evaluation::new(obj))
-    }
-
-    #[test]
-    fn igd_perfect_match_is_zero() {
-        let s = space_min2();
-        let approx = [cand(vec![0.0, 1.0]), cand(vec![1.0, 0.0])];
-        let reference = [
-            Evaluation::new(vec![0.0, 1.0]),
-            Evaluation::new(vec![1.0, 0.0]),
-        ];
-        let v = igd(&approx, &reference, &s);
-        assert!(v < 1e-12);
-    }
-
-    #[test]
-    fn igd_known_value() {
-        let s = space_min2();
-        let approx = [cand(vec![0.0, 0.0])];
-        let reference = [Evaluation::new(vec![1.0, 1.0])];
-        let v = igd(&approx, &reference, &s);
-        assert!((v - 2.0_f64.sqrt()).abs() < 1e-12);
-    }
-
-    #[test]
-    fn igd_plus_dominated_point_does_not_improve() {
-        let s = space_min2();
-        let reference = [
-            Evaluation::new(vec![0.0, 1.0]),
-            Evaluation::new(vec![1.0, 0.0]),
-        ];
-        let base = vec![cand(vec![0.5, 0.5])];
-        let with_dominated = vec![cand(vec![0.5, 0.5]), cand(vec![1.0, 1.0])];
-        let v_base = igd_plus(&base, &reference, &s);
-        let v_with = igd_plus(&with_dominated, &reference, &s);
-        // Adding a dominated point should not improve the score.
-        assert!(v_with >= v_base - 1e-12);
-    }
-
-    #[test]
-    fn igd_empty_approximation_is_infinity() {
-        let s = space_min2();
-        let approx: [Candidate<()>; 0] = [];
-        let reference = [Evaluation::new(vec![0.0, 1.0])];
-        assert!(igd(&approx, &reference, &s).is_infinite());
-    }
-
-    #[test]
-    #[should_panic(expected = "reference set must not be empty")]
-    fn igd_empty_reference_panics() {
-        let s = space_min2();
-        let approx = [cand(vec![0.0, 1.0])];
-        let _ = igd::<()>(&approx, &[], &s);
-    }
-}

src/metrics/mod.rs

@@ -1,11 +1,7 @@
 //! Quality metrics for Pareto fronts.
 
 pub mod hypervolume;
-pub mod igd;
-pub mod r2;
 pub mod spacing;
 
 pub use hypervolume::*;
-pub use igd::{igd, igd_plus};
-pub use r2::r2;
 pub use spacing::*;

src/metrics/r2.rs

@@ -1,173 +0,0 @@
-//! R2 indicator — a unary quality measure for Pareto fronts.
-//!
-//! For each weight vector `λ` in a user-supplied set, find the
-//! best (smallest) weighted Tchebycheff value across the front;
-//! average over all weight vectors. Lower is better.
-
-use crate::core::candidate::Candidate;
-use crate::core::objective::ObjectiveSpace;
-
-/// R2 indicator using the weighted Tchebycheff utility.
-///
-/// ```text
-/// R2(A) = (1 / |Λ|) · Σ_{λ ∈ Λ} min_{a ∈ A} max_i { λ_i · |a_i − z*_i| }
-/// ```
-///
-/// where `z*` is the ideal point (per-axis minimum across the
-/// approximation, in minimization-oriented coordinates) and `Λ` is
-/// a set of unit-simplex weight vectors. Lower is better.
-///
-/// Use [`das_dennis`](crate::pareto::das_dennis) to generate the
-/// canonical structured weight set.
-///
-/// # Panics
-///
-/// If the approximation is empty, or any weight vector has wrong
-/// length / negative entries / zero sum.
-///
-/// # Example
-///
-/// ```
-/// use heuropt::prelude::*;
-/// use heuropt::metrics::r2::r2;
-///
-/// let space = ObjectiveSpace::new(vec![
-///     Objective::minimize("f1"),
-///     Objective::minimize("f2"),
-/// ]);
-/// let approx = [
-///     Candidate::new((), Evaluation::new(vec![0.0, 1.0])),
-///     Candidate::new((), Evaluation::new(vec![1.0, 0.0])),
-/// ];
-/// // Two weight vectors: (1, 0) and (0, 1) — extreme directions.
-/// let weights = [vec![1.0, 0.0], vec![0.0, 1.0]];
-/// let v = r2(&approx, &weights, &space);
-/// // For each direction, the best front member matches that axis exactly.
-/// // R2 = 0 since the ideal point is achieved on each direction.
-/// assert!(v < 1e-12);
-/// ```
-pub fn r2<D>(
-    approximation: &[Candidate<D>],
-    weights: &[Vec<f64>],
-    objectives: &ObjectiveSpace,
-) -> f64 {
-    assert!(
-        !approximation.is_empty(),
-        "r2: approximation must not be empty"
-    );
-    assert!(!weights.is_empty(), "r2: weight set must not be empty");
-    let m = objectives.len();
-    for (i, w) in weights.iter().enumerate() {
-        assert_eq!(
-            w.len(),
-            m,
-            "r2: weight {i} has wrong length ({} vs {m})",
-            w.len()
-        );
-        assert!(
-            w.iter().all(|&v| v >= 0.0),
-            "r2: weight {i} has a negative entry"
-        );
-        assert!(w.iter().sum::<f64>() > 0.0, "r2: weight {i} has zero sum");
-    }
-
-    // Convert all approximation members to minimization orientation once.
-    let oriented: Vec<Vec<f64>> = approximation
-        .iter()
-        .map(|c| objectives.as_minimization(&c.evaluation.objectives))
-        .collect();
-
-    // Ideal point z* (per-axis minimum).
-    let mut z_star = vec![f64::INFINITY; m];
-    for o in &oriented {
-        for k in 0..m {
-            if o[k] < z_star[k] {
-                z_star[k] = o[k];
-            }
-        }
-    }
-
-    let mut total = 0.0_f64;
-    for w in weights {
-        let mut best = f64::INFINITY;
-        for o in &oriented {
-            // Weighted Tchebycheff: max_i { w_i · |o_i − z*_i| }
-            let mut t = 0.0_f64;
-            for k in 0..m {
-                let dk = (o[k] - z_star[k]).abs() * w[k];
-                if dk > t {
-                    t = dk;
-                }
-            }
-            if t < best {
-                best = t;
-            }
-        }
-        total += best;
-    }
-    total / weights.len() as f64
-}
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-    use crate::core::evaluation::Evaluation;
-    use crate::core::objective::Objective;
-    use crate::pareto::das_dennis;
-
-    fn space_min2() -> ObjectiveSpace {
-        ObjectiveSpace::new(vec![Objective::minimize("f1"), Objective::minimize("f2")])
-    }
-
-    fn cand(obj: Vec<f64>) -> Candidate<()> {
-        Candidate::new((), Evaluation::new(obj))
-    }
-
-    #[test]
-    fn r2_extremes_are_perfect_at_endpoints() {
-        let s = space_min2();
-        let front = [cand(vec![0.0, 1.0]), cand(vec![1.0, 0.0])];
-        let weights = [vec![1.0, 0.0], vec![0.0, 1.0]];
-        assert!(r2(&front, &weights, &s) < 1e-12);
-    }
-
-    #[test]
-    fn r2_dense_dasdennis_finite_for_uniform_front() {
-        let s = space_min2();
-        let weights = das_dennis(2, 5);
-        let front: Vec<Candidate<()>> = (0..=10)
-            .map(|i| {
-                let t = i as f64 / 10.0;
-                cand(vec![t, 1.0 - t])
-            })
-            .collect();
-        let v = r2(&front, &weights, &s);
-        assert!(v.is_finite());
-        assert!(v >= 0.0);
-    }
-
-    #[test]
-    #[should_panic(expected = "approximation must not be empty")]
-    fn r2_empty_approximation_panics() {
-        let s = space_min2();
-        let weights = vec![vec![1.0, 0.0]];
-        let _: f64 = r2::<()>(&[], &weights, &s);
-    }
-
-    #[test]
-    #[should_panic(expected = "weight set must not be empty")]
-    fn r2_empty_weights_panics() {
-        let s = space_min2();
-        let front = [cand(vec![0.0, 1.0])];
-        let _ = r2(&front, &[], &s);
-    }
-
-    #[test]
-    #[should_panic(expected = "wrong length")]
-    fn r2_wrong_dim_weight_panics() {
-        let s = space_min2();
-        let front = [cand(vec![0.0, 1.0])];
-        let weights = vec![vec![1.0, 0.0, 0.0]];
-        let _ = r2(&front, &weights, &s);
-    }
-}

src/metrics/spacing.rs

@@ -11,6 +11,26 @@ use crate::core::objective::ObjectiveSpace;
 /// uniform front has spacing 0.
 ///
 /// Returns `0.0` for empty or single-point fronts (spec §14.1).
+///
+/// # Example
+///
+/// ```
+/// use heuropt::prelude::*;
+/// use heuropt::metrics::spacing;
+///
+/// let space = ObjectiveSpace::new(vec![
+///     Objective::minimize("f1"),
+///     Objective::minimize("f2"),
+/// ]);
+/// // Five points evenly spaced on a line — spacing should be 0.
+/// let front: Vec<Candidate<()>> = (0..5)
+///     .map(|i| {
+///         let t = i as f64;
+///         Candidate::new((), Evaluation::new(vec![t, 4.0 - t]))
+///     })
+///     .collect();
+/// assert!(spacing(&front, &space) < 1e-12);
+/// ```
 pub fn spacing<D>(front: &[Candidate<D>], objectives: &ObjectiveSpace) -> f64 {
     let n = front.len();
     if n < 2 {

src/observer/builtin.rs

@@ -1,418 +0,0 @@
-//! Built-in observers covering the common stop conditions.
-
-use std::ops::ControlFlow;
-use std::time::Duration;
-
-use super::{Observer, Snapshot};
-use crate::core::objective::Direction;
-
-/// Halt after a fixed wall-clock duration since `run_with` started.
-///
-/// # Example
-///
-/// ```
-/// use heuropt::prelude::*;
-/// use std::time::Duration;
-///
-/// let stop = MaxTime::new(Duration::from_millis(50));
-/// // pass `&mut stop` to `Optimizer::run_with`.
-/// # let _ = stop;
-/// ```
-#[derive(Debug, Clone, Copy)]
-pub struct MaxTime {
-    pub limit: Duration,
-}
-
-impl MaxTime {
-    pub fn new(limit: Duration) -> Self {
-        Self { limit }
-    }
-}
-
-impl<D> Observer<D> for MaxTime {
-    #[inline]
-    fn observe(&mut self, snap: &Snapshot<'_, D>) -> ControlFlow<()> {
-        if snap.elapsed >= self.limit {
-            ControlFlow::Break(())
-        } else {
-            ControlFlow::Continue(())
-        }
-    }
-}
-
-/// Halt after a target number of generations.
-///
-/// Most algorithms already take a `generations` count in their config,
-/// so this is mostly useful for capping algorithms whose configured
-/// loop is open-ended (or for testing).
-#[derive(Debug, Clone, Copy)]
-pub struct MaxIterations {
-    pub limit: usize,
-}
-
-impl MaxIterations {
-    pub fn new(limit: usize) -> Self {
-        Self { limit }
-    }
-}
-
-impl<D> Observer<D> for MaxIterations {
-    #[inline]
-    fn observe(&mut self, snap: &Snapshot<'_, D>) -> ControlFlow<()> {
-        if snap.iteration >= self.limit {
-            ControlFlow::Break(())
-        } else {
-            ControlFlow::Continue(())
-        }
-    }
-}
-
-/// Halt as soon as the best single-objective fitness reaches `target`.
-///
-/// Direction-aware: for `Minimize` axes the target is reached when
-/// `best ≤ target`; for `Maximize`, when `best ≥ target`.
-///
-/// Multi-objective snapshots (where `Snapshot::best` is `None` or the
-/// problem has more than one objective) are silently ignored — this
-/// observer never breaks them.
-#[derive(Debug, Clone, Copy)]
-pub struct TargetFitness {
-    pub target: f64,
-}
-
-impl TargetFitness {
-    pub fn new(target: f64) -> Self {
-        Self { target }
-    }
-}
-
-impl<D> Observer<D> for TargetFitness {
-    fn observe(&mut self, snap: &Snapshot<'_, D>) -> ControlFlow<()> {
-        if !snap.objectives.is_single_objective() {
-            return ControlFlow::Continue(());
-        }
-        let direction = snap.objectives.objectives[0].direction;
-        if let Some(best) = snap.best
-            && let Some(&v) = best.evaluation.objectives.first()
-        {
-            let hit = match direction {
-                Direction::Minimize => v <= self.target,
-                Direction::Maximize => v >= self.target,
-            };
-            if hit {
-                return ControlFlow::Break(());
-            }
-        }
-        ControlFlow::Continue(())
-    }
-}
-
-/// Halt when the best single-objective fitness has not improved by
-/// more than `tolerance` over the last `window` generations.
-///
-/// Multi-objective snapshots are silently ignored.
-#[derive(Debug, Clone)]
-pub struct Stagnation {
-    pub window: usize,
-    pub tolerance: f64,
-    history: std::collections::VecDeque<f64>,
-}
-
-impl Stagnation {
-    pub fn new(window: usize, tolerance: f64) -> Self {
-        assert!(window > 0, "Stagnation window must be > 0");
-        assert!(
-            tolerance >= 0.0,
-            "Stagnation tolerance must be non-negative"
-        );
-        Self {
-            window,
-            tolerance,
-            history: std::collections::VecDeque::with_capacity(window + 1),
-        }
-    }
-}
-
-impl<D> Observer<D> for Stagnation {
-    fn observe(&mut self, snap: &Snapshot<'_, D>) -> ControlFlow<()> {
-        if !snap.objectives.is_single_objective() {
-            return ControlFlow::Continue(());
-        }
-        let direction = snap.objectives.objectives[0].direction;
-        let v = match snap
-            .best
-            .and_then(|c| c.evaluation.objectives.first().copied())
-        {
-            Some(v) => v,
-            None => return ControlFlow::Continue(()),
-        };
-        // Push to history; cap at window+1 so we always have 1 + window samples.
-        self.history.push_back(v);
-        while self.history.len() > self.window + 1 {
-            self.history.pop_front();
-        }
-        if self.history.len() <= self.window {
-            return ControlFlow::Continue(());
-        }
-        let oldest = self.history.front().copied().unwrap();
-        let newest = self.history.back().copied().unwrap();
-        let improvement = match direction {
-            Direction::Minimize => oldest - newest,
-            Direction::Maximize => newest - oldest,
-        };
-        if improvement <= self.tolerance {
-            ControlFlow::Break(())
-        } else {
-            ControlFlow::Continue(())
-        }
-    }
-}
-
-/// Compose two observers — break if **either** signals a break.
-#[derive(Debug, Clone, Copy)]
-pub struct AnyOf<A, B> {
-    pub a: A,
-    pub b: B,
-}
-
-impl<D, A, B> Observer<D> for AnyOf<A, B>
-where
-    A: Observer<D>,
-    B: Observer<D>,
-{
-    fn observe(&mut self, snap: &Snapshot<'_, D>) -> ControlFlow<()> {
-        // Always poll both so stateful observers (Stagnation) update
-        // their history, then OR the results.
-        let ra = self.a.observe(snap);
-        let rb = self.b.observe(snap);
-        if ra.is_break() || rb.is_break() {
-            ControlFlow::Break(())
-        } else {
-            ControlFlow::Continue(())
-        }
-    }
-}
-
-/// Compose two observers — break only if **both** signal a break in
-/// the same call.
-#[derive(Debug, Clone, Copy)]
-pub struct AllOf<A, B> {
-    pub a: A,
-    pub b: B,
-}
-
-impl<D, A, B> Observer<D> for AllOf<A, B>
-where
-    A: Observer<D>,
-    B: Observer<D>,
-{
-    fn observe(&mut self, snap: &Snapshot<'_, D>) -> ControlFlow<()> {
-        let ra = self.a.observe(snap);
-        let rb = self.b.observe(snap);
-        if ra.is_break() && rb.is_break() {
-            ControlFlow::Break(())
-        } else {
-            ControlFlow::Continue(())
-        }
-    }
-}
-
-/// Call a user closure every `every` generations (default 1 = every
-/// generation). Useful for periodic logging without bloating callback
-/// frequency.
-pub struct Periodic<F> {
-    pub every: usize,
-    counter: usize,
-    pub callback: F,
-}
-
-impl<F> Periodic<F> {
-    pub fn new(every: usize, callback: F) -> Self {
-        assert!(every >= 1, "Periodic every must be >= 1");
-        Self {
-            every,
-            counter: 0,
-            callback,
-        }
-    }
-}
-
-impl<D, F> Observer<D> for Periodic<F>
-where
-    F: FnMut(&Snapshot<'_, D>),
-{
-    fn observe(&mut self, snap: &Snapshot<'_, D>) -> ControlFlow<()> {
-        self.counter += 1;
-        if self.counter >= self.every {
-            self.counter = 0;
-            (self.callback)(snap);
-        }
-        ControlFlow::Continue(())
-    }
-}
-
-/// Tracing-backed observer — emits a structured `debug!` event per
-/// generation with iteration / evaluations / elapsed / best fitness.
-///
-/// Available only with the `tracing` feature.
-#[cfg(feature = "tracing")]
-#[derive(Debug, Default, Clone, Copy)]
-pub struct TracingObserver;
-
-#[cfg(feature = "tracing")]
-impl<D> Observer<D> for TracingObserver {
-    fn observe(&mut self, snap: &Snapshot<'_, D>) -> ControlFlow<()> {
-        let best = snap
-            .best
-            .and_then(|c| c.evaluation.objectives.first().copied());
-        tracing::debug!(
-            iteration = snap.iteration,
-            evaluations = snap.evaluations,
-            elapsed_ms = snap.elapsed.as_millis() as u64,
-            best = ?best,
-            front_size = snap.pareto_front.map(|f| f.len()),
-            "heuropt generation",
-        );
-        ControlFlow::Continue(())
-    }
-}
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-    use crate::core::candidate::Candidate;
-    use crate::core::evaluation::Evaluation;
-    use crate::core::objective::{Objective, ObjectiveSpace};
-
-    fn snap_with_best<'a>(
-        iteration: usize,
-        elapsed_ms: u64,
-        best: Option<&'a Candidate<()>>,
-        objectives: &'a ObjectiveSpace,
-        empty_pop: &'a [Candidate<()>],
-    ) -> Snapshot<'a, ()> {
-        Snapshot {
-            iteration,
-            evaluations: 0,
-            elapsed: Duration::from_millis(elapsed_ms),
-            population: empty_pop,
-            pareto_front: None,
-            best,
-            objectives,
-        }
-    }
-
-    #[test]
-    fn max_time_breaks_after_limit() {
-        let space = ObjectiveSpace::new(vec![Objective::minimize("f")]);
-        let pop: Vec<Candidate<()>> = vec![];
-        let mut o = MaxTime::new(Duration::from_millis(100));
-        let s = snap_with_best(0, 50, None, &space, &pop);
-        assert!(o.observe(&s).is_continue());
-        let s = snap_with_best(1, 100, None, &space, &pop);
-        assert!(o.observe(&s).is_break());
-    }
-
-    #[test]
-    fn target_fitness_minimize() {
-        let space = ObjectiveSpace::new(vec![Objective::minimize("f")]);
-        let pop: Vec<Candidate<()>> = vec![];
-        let cand = Candidate::new((), Evaluation::new(vec![0.005]));
-        let mut o = TargetFitness::new(0.01);
-        let s = snap_with_best(0, 0, Some(&cand), &space, &pop);
-        assert!(o.observe(&s).is_break());
-    }
-
-    #[test]
-    fn target_fitness_maximize() {
-        let space = ObjectiveSpace::new(vec![Objective::maximize("f")]);
-        let pop: Vec<Candidate<()>> = vec![];
-        let cand_below = Candidate::new((), Evaluation::new(vec![0.5]));
-        let cand_above = Candidate::new((), Evaluation::new(vec![1.5]));
-        let mut o = TargetFitness::new(1.0);
-        let s = snap_with_best(0, 0, Some(&cand_below), &space, &pop);
-        assert!(o.observe(&s).is_continue());
-        let s = snap_with_best(1, 0, Some(&cand_above), &space, &pop);
-        assert!(o.observe(&s).is_break());
-    }
-
-    #[test]
-    fn stagnation_breaks_on_no_improvement() {
-        let space = ObjectiveSpace::new(vec![Objective::minimize("f")]);
-        let pop: Vec<Candidate<()>> = vec![];
-        let mut o = Stagnation::new(3, 1e-6);
-
-        // Five generations of "no improvement" — same value every time.
-        for i in 0..3 {
-            let cand = Candidate::new((), Evaluation::new(vec![1.0]));
-            let s = snap_with_best(i, 0, Some(&cand), &space, &pop);
-            // First `window` calls just fill history; should not break.
-            assert!(o.observe(&s).is_continue());
-        }
-        let cand = Candidate::new((), Evaluation::new(vec![1.0]));
-        let s = snap_with_best(3, 0, Some(&cand), &space, &pop);
-        assert!(o.observe(&s).is_break());
-    }
-
-    #[test]
-    fn stagnation_does_not_break_on_improvement() {
-        let space = ObjectiveSpace::new(vec![Objective::minimize("f")]);
-        let pop: Vec<Candidate<()>> = vec![];
-        let mut o = Stagnation::new(2, 1e-6);
-        let values = [1.0, 0.9, 0.8, 0.7];
-        for (i, &v) in values.iter().enumerate() {
-            let cand = Candidate::new((), Evaluation::new(vec![v]));
-            let s = snap_with_best(i, 0, Some(&cand), &space, &pop);
-            assert!(o.observe(&s).is_continue(), "iter {i}");
-        }
-    }
-
-    #[test]
-    fn anyof_breaks_when_either_breaks() {
-        let space = ObjectiveSpace::new(vec![Objective::minimize("f")]);
-        let pop: Vec<Candidate<()>> = vec![];
-        let cand = Candidate::new((), Evaluation::new(vec![5.0]));
-        let mut o =
-            <MaxIterations as Observer<()>>::or(MaxIterations::new(3), TargetFitness::new(1.0));
-        for i in 0..3 {
-            let s = snap_with_best(i, 0, Some(&cand), &space, &pop);
-            assert!(o.observe(&s).is_continue(), "iter {i}");
-        }
-        // iteration = 3 hits MaxIterations limit → break
-        let s = snap_with_best(3, 0, Some(&cand), &space, &pop);
-        assert!(o.observe(&s).is_break());
-    }
-
-    #[test]
-    fn periodic_calls_callback_every_n() {
-        let space = ObjectiveSpace::new(vec![Objective::minimize("f")]);
-        let pop: Vec<Candidate<()>> = vec![];
-        let mut count = 0_usize;
-        {
-            let mut o = Periodic::new(3, |_: &Snapshot<'_, ()>| count += 1);
-            for i in 0..10 {
-                let s = snap_with_best(i, 0, None, &space, &pop);
-                let _ = o.observe(&s);
-            }
-        }
-        assert_eq!(count, 3); // every 3rd of 10 = generations 2, 5, 8
-    }
-
-    #[test]
-    fn closure_implements_observer() {
-        let space = ObjectiveSpace::new(vec![Objective::minimize("f")]);
-        let pop: Vec<Candidate<()>> = vec![];
-        let mut count = 0_usize;
-        let mut closure = |_: &Snapshot<'_, ()>| -> ControlFlow<()> {
-            count += 1;
-            if count >= 2 {
-                ControlFlow::Break(())
-            } else {
-                ControlFlow::Continue(())
-            }
-        };
-        let s = snap_with_best(0, 0, None, &space, &pop);
-        assert!(<_ as Observer<()>>::observe(&mut closure, &s).is_continue());
-        assert!(<_ as Observer<()>>::observe(&mut closure, &s).is_break());
-    }
-}

src/observer/mod.rs

@@ -1,101 +0,0 @@
-//! Per-generation observation, callbacks, and stop conditions.
-//!
-//! Algorithms accept an [`Observer`] via [`Optimizer::run_with`] and call
-//! it once per generation (where "generation" makes sense for that
-//! algorithm — see each algorithm's docs). Returning
-//! [`std::ops::ControlFlow::Break`] from an observer halts the optimizer
-//! and the partial [`OptimizationResult`] is returned to the caller.
-//!
-//! Observers can be composed with [`builtin::AnyOf`] / [`builtin::AllOf`].
-//!
-//! [`OptimizationResult`]: crate::core::result::OptimizationResult
-//! [`Optimizer::run_with`]: crate::traits::Optimizer::run_with
-
-pub mod builtin;
-mod snapshot;
-
-pub use snapshot::Snapshot;
-
-use std::ops::ControlFlow;
-
-use crate::core::candidate::Candidate;
-use crate::core::objective::ObjectiveSpace;
-
-/// A callback invoked by an [`Optimizer`](crate::traits::Optimizer)
-/// after every generation. Return [`ControlFlow::Break`] to halt
-/// the optimizer; [`ControlFlow::Continue`] to keep going.
-///
-/// Implement directly for stateful observers that need to track
-/// history (e.g. stagnation detection, convergence trace logging).
-/// For simple stop conditions, use the helpers in
-/// [`builtin`](crate::observer::builtin).
-pub trait Observer<D> {
-    /// Inspect the latest snapshot. Return [`ControlFlow::Break`] to
-    /// halt the run; [`ControlFlow::Continue`] to keep going.
-    fn observe(&mut self, snapshot: &Snapshot<'_, D>) -> ControlFlow<()>;
-
-    /// Compose with another observer that fires when *either* of them
-    /// signals a break.
-    fn or<O: Observer<D>>(self, other: O) -> builtin::AnyOf<Self, O>
-    where
-        Self: Sized,
-    {
-        builtin::AnyOf { a: self, b: other }
-    }
-
-    /// Compose with another observer that fires when *both* of them
-    /// signal a break in the same call.
-    fn and<O: Observer<D>>(self, other: O) -> builtin::AllOf<Self, O>
-    where
-        Self: Sized,
-    {
-        builtin::AllOf { a: self, b: other }
-    }
-}
-
-/// `()` is the no-op observer. Used as the default when callers don't
-/// want any callbacks (it's what `run` uses internally).
-impl<D> Observer<D> for () {
-    #[inline]
-    fn observe(&mut self, _: &Snapshot<'_, D>) -> ControlFlow<()> {
-        ControlFlow::Continue(())
-    }
-}
-
-/// Closures of the right shape implement Observer too — short-form
-/// for one-liner callbacks.
-impl<D, F> Observer<D> for F
-where
-    F: FnMut(&Snapshot<'_, D>) -> ControlFlow<()>,
-{
-    #[inline]
-    fn observe(&mut self, snap: &Snapshot<'_, D>) -> ControlFlow<()> {
-        self(snap)
-    }
-}
-
-/// Build a snapshot for the "final notification" path of the default
-/// `run_with` impl on [`Optimizer`](crate::traits::Optimizer).
-///
-/// Algorithm impls that override `run_with` to call the observer per
-/// generation should construct their own snapshots inline rather than
-/// using this helper, because they have richer per-generation state.
-pub fn finalize_snapshot<'a, D>(
-    iteration: usize,
-    evaluations: usize,
-    elapsed: std::time::Duration,
-    population: &'a [Candidate<D>],
-    pareto_front: Option<&'a [Candidate<D>]>,
-    best: Option<&'a Candidate<D>>,
-    objectives: &'a ObjectiveSpace,
-) -> Snapshot<'a, D> {
-    Snapshot {
-        iteration,
-        evaluations,
-        elapsed,
-        population,
-        pareto_front,
-        best,
-        objectives,
-    }
-}

src/observer/snapshot.rs

@@ -1,45 +0,0 @@
-//! Per-generation observation payload passed to [`Observer`](super::Observer).
-
-use std::time::Duration;
-
-use crate::core::candidate::Candidate;
-use crate::core::objective::ObjectiveSpace;
-
-/// A view of an optimizer's state at one generation boundary.
-///
-/// Borrowed (`&'a ...`) rather than owned so the algorithm doesn't
-/// have to clone the whole population on every call. Observers that
-/// need to retain values across calls should clone what they need
-/// out of the snapshot.
-#[derive(Debug)]
-pub struct Snapshot<'a, D> {
-    /// Zero-indexed generation count. The first call is `iteration = 0`
-    /// for "after the initial population was built and evaluated";
-    /// subsequent calls are after generation 1, 2, …
-    pub iteration: usize,
-
-    /// Total `Problem::evaluate` calls so far, including the initial
-    /// population.
-    pub evaluations: usize,
-
-    /// Wall-clock time since `run_with` started.
-    pub elapsed: Duration,
-
-    /// The current population (whatever the algorithm considers the
-    /// "live" set this generation). For steady-state algorithms this
-    /// is the post-replacement population.
-    pub population: &'a [Candidate<D>],
-
-    /// The current Pareto front, if the algorithm tracks one. `None`
-    /// for single-objective algorithms.
-    pub pareto_front: Option<&'a [Candidate<D>]>,
-
-    /// The current best candidate. `Some` for single-objective
-    /// algorithms; `None` for multi-objective unless the algorithm
-    /// tracks a notion of best (some don't).
-    pub best: Option<&'a Candidate<D>>,
-
-    /// The objective space, useful for observers that need to convert
-    /// raw objective values to minimization-oriented form.
-    pub objectives: &'a ObjectiveSpace,
-}

src/operators/binary.rs

@@ -9,6 +9,19 @@ use crate::traits::Variation;
 ///
 /// Always returns exactly one child (spec §11.3). Panics if `probability` is
 /// outside `[0.0, 1.0]` or if no parents are provided.
+///
+/// # Example
+///
+/// ```
+/// use heuropt::prelude::*;
+///
+/// let mut rng = rng_from_seed(42);
+/// let mut m = BitFlipMutation { probability: 0.5 };
+/// let parent = vec![true, false, true, false];
+/// let children = m.vary(std::slice::from_ref(&parent), &mut rng);
+/// assert_eq!(children.len(), 1);
+/// assert_eq!(children[0].len(), parent.len());
+/// ```
 #[derive(Debug, Clone)]
 pub struct BitFlipMutation {
     /// Per-bit flip probability. Must lie in `[0.0, 1.0]`.

src/operators/permutation.rs

@@ -8,6 +8,22 @@ use crate::traits::Variation;
 /// Swap two distinct random indices in the first parent (spec §11.4).
 ///
 /// If the parent has length `< 2` the child is returned unchanged.
+///
+/// # Example
+///
+/// ```
+/// use heuropt::prelude::*;
+///
+/// let mut rng = rng_from_seed(42);
+/// let mut m = SwapMutation;
+/// let parent: Vec<usize> = (0..6).collect();
+/// let children = m.vary(std::slice::from_ref(&parent), &mut rng);
+/// assert_eq!(children.len(), 1);
+/// // Still a permutation of [0, 1, 2, 3, 4, 5]:
+/// let mut sorted = children[0].clone();
+/// sorted.sort();
+/// assert_eq!(sorted, vec![0, 1, 2, 3, 4, 5]);
+/// ```
 #[derive(Debug, Clone, Copy, Default)]
 pub struct SwapMutation;
 

src/operators/real.rs

@@ -10,6 +10,23 @@ use crate::traits::{Initializer, Variation};
 ///
 /// Bounds are inclusive `(lo, hi)` ranges per dimension. Panics if any bound
 /// has `lo > hi` (spec §11.1).
+///
+/// # Example
+///
+/// ```
+/// use heuropt::prelude::*;
+///
+/// let mut rng = rng_from_seed(42);
+/// let mut init = RealBounds::new(vec![(-1.0, 1.0); 3]);
+/// let decisions = init.initialize(5, &mut rng);
+/// assert_eq!(decisions.len(), 5);
+/// for d in &decisions {
+///     assert_eq!(d.len(), 3);
+///     for &v in d {
+///         assert!(v >= -1.0 && v <= 1.0);
+///     }
+/// }
+/// ```
 #[derive(Debug, Clone)]
 pub struct RealBounds {
     /// Per-variable inclusive bounds in decision order.
@@ -54,6 +71,19 @@ impl Initializer<Vec<f64>> for RealBounds {
 /// Add `Normal(0, sigma)` noise to every variable of the first parent.
 ///
 /// Always returns exactly one child. Does not enforce bounds in v1 (spec §11.2).
+///
+/// # Example
+///
+/// ```
+/// use heuropt::prelude::*;
+///
+/// let mut rng = rng_from_seed(42);
+/// let mut m = GaussianMutation { sigma: 0.1 };
+/// let parent = vec![0.0; 4];
+/// let children = m.vary(std::slice::from_ref(&parent), &mut rng);
+/// assert_eq!(children.len(), 1);
+/// assert_eq!(children[0].len(), parent.len());
+/// ```
 #[derive(Debug, Clone)]
 pub struct GaussianMutation {
     /// Standard deviation of the Gaussian noise. Must be positive.
@@ -88,6 +118,26 @@ impl Variation<Vec<f64>> for GaussianMutation {
 ///
 /// Panics on construction if any bound has `lo > hi`, or at run time if
 /// `parents.len() < 2` or any parent length differs from `bounds.len()`.
+///
+/// # Example
+///
+/// ```
+/// use heuropt::prelude::*;
+///
+/// let bounds = vec![(-1.0, 1.0); 3];
+/// let mut sbx = SimulatedBinaryCrossover::new(bounds.clone(), 15.0, 0.5);
+/// let mut rng = rng_from_seed(42);
+/// let parents = [vec![-0.5, 0.0, 0.5], vec![0.5, 0.5, -0.5]];
+/// let children = sbx.vary(&parents, &mut rng);
+/// assert_eq!(children.len(), 2);
+/// // Children stay in bounds.
+/// for c in &children {
+///     for (j, &v) in c.iter().enumerate() {
+///         let (lo, hi) = bounds[j];
+///         assert!(v >= lo && v <= hi);
+///     }
+/// }
+/// ```
 #[derive(Debug, Clone)]
 pub struct SimulatedBinaryCrossover {
     /// Per-variable inclusive bounds. Length must match the parent decisions.
@@ -180,6 +230,23 @@ impl Variation<Vec<f64>> for SimulatedBinaryCrossover {
 ///
 /// This is the simple bound-rescale form; the bound-aware `δ_q` variant from
 /// the full paper is left as a future refinement.
+///
+/// # Example
+///
+/// ```
+/// use heuropt::prelude::*;
+///
+/// let bounds = vec![(-1.0, 1.0); 3];
+/// let mut pm = PolynomialMutation::new(bounds.clone(), 20.0, 1.0 / 3.0);
+/// let mut rng = rng_from_seed(42);
+/// let parent = vec![0.0, 0.5, -0.5];
+/// let children = pm.vary(std::slice::from_ref(&parent), &mut rng);
+/// assert_eq!(children.len(), 1);
+/// for (j, &v) in children[0].iter().enumerate() {
+///     let (lo, hi) = bounds[j];
+///     assert!(v >= lo && v <= hi);
+/// }
+/// ```
 #[derive(Debug, Clone)]
 pub struct PolynomialMutation {
     /// Per-variable inclusive bounds. Length must match the parent decision.
@@ -254,6 +321,23 @@ impl Variation<Vec<f64>> for PolynomialMutation {
 /// Always returns exactly one child. Use this when you want feasibility
 /// maintained across generations without leaning on
 /// clamp-inside-`Problem::evaluate`.
+///
+/// # Example
+///
+/// ```
+/// use heuropt::prelude::*;
+///
+/// let bounds = vec![(-1.0, 1.0); 3];
+/// let mut m = BoundedGaussianMutation::new(0.3, bounds.clone());
+/// let mut rng = rng_from_seed(42);
+/// let parent = vec![0.0; 3];
+/// let children = m.vary(std::slice::from_ref(&parent), &mut rng);
+/// assert_eq!(children.len(), 1);
+/// for (j, &v) in children[0].iter().enumerate() {
+///     let (lo, hi) = bounds[j];
+///     assert!(v >= lo && v <= hi);
+/// }
+/// ```
 #[derive(Debug, Clone)]
 pub struct BoundedGaussianMutation {
     /// Standard deviation of the Gaussian noise. Must be positive.
@@ -315,6 +399,23 @@ impl Variation<Vec<f64>> for BoundedGaussianMutation {
 /// produce a Lévy(α) sample. `alpha` is the tail exponent in `(0, 2]`;
 /// typical value is `1.5`. `1.0` gives the Cauchy distribution (very
 /// heavy); `2.0` collapses to the Normal.
+///
+/// # Example
+///
+/// ```
+/// use heuropt::prelude::*;
+///
+/// let bounds = vec![(-1.0, 1.0); 3];
+/// let mut m = LevyMutation::new(1.5, 0.1, bounds.clone());
+/// let mut rng = rng_from_seed(42);
+/// let parent = vec![0.0; 3];
+/// let children = m.vary(std::slice::from_ref(&parent), &mut rng);
+/// assert_eq!(children.len(), 1);
+/// for (j, &v) in children[0].iter().enumerate() {
+///     let (lo, hi) = bounds[j];
+///     assert!(v >= lo && v <= hi);
+/// }
+/// ```
 #[derive(Debug, Clone)]
 pub struct LevyMutation {
     /// Tail exponent `α ∈ (0, 2]`. Smaller = heavier tail.

src/operators/repair.rs

@@ -8,6 +8,17 @@ use crate::traits::Repair;
 /// The simplest possible repair — pair with `GaussianMutation` (which
 /// doesn't enforce bounds in v1) to produce a bounds-respecting variant
 /// without writing a custom Variation impl.
+///
+/// # Example
+///
+/// ```
+/// use heuropt::prelude::*;
+///
+/// let mut r = ClampToBounds::new(vec![(-1.0, 1.0); 3]);
+/// let mut x = vec![-2.0, 0.5, 5.0];
+/// r.repair(&mut x);
+/// assert_eq!(x, vec![-1.0, 0.5, 1.0]);
+/// ```
 #[derive(Debug, Clone)]
 pub struct ClampToBounds {
     /// Per-variable inclusive bounds.
@@ -46,6 +57,19 @@ impl Repair<Vec<f64>> for ClampToBounds {
 /// Perpiñán 2013. Useful for portfolio-style problems where the
 /// decision must sum to a budget, and for normalizing reference
 /// directions onto the unit simplex.
+///
+/// # Example
+///
+/// ```
+/// use heuropt::prelude::*;
+///
+/// let mut r = ProjectToSimplex::new(1.0);
+/// let mut x = vec![0.6, 0.5, -0.1, 0.3];
+/// r.repair(&mut x);
+/// let sum: f64 = x.iter().sum();
+/// assert!((sum - 1.0).abs() < 1e-12);
+/// assert!(x.iter().all(|&v| v >= 0.0));
+/// ```
 #[derive(Debug, Clone)]
 pub struct ProjectToSimplex {
     /// Target sum (the simplex's "size"). Standard probability simplex

src/pareto/archive.rs

@@ -9,6 +9,23 @@ use crate::core::objective::ObjectiveSpace;
 /// archive insert/extend operations maintain the non-domination property among
 /// members; `truncate` enforces a maximum size by simple tail-truncation in
 /// v1.
+///
+/// # Example
+///
+/// ```
+/// use heuropt::prelude::*;
+///
+/// let s = ObjectiveSpace::new(vec![
+///     Objective::minimize("f1"),
+///     Objective::minimize("f2"),
+/// ]);
+/// let mut a: ParetoArchive<u32> = ParetoArchive::new(s);
+/// a.insert(Candidate::new(1, Evaluation::new(vec![1.0, 4.0])));
+/// a.insert(Candidate::new(2, Evaluation::new(vec![3.0, 2.0])));
+/// // Dominated by both — should be discarded:
+/// a.insert(Candidate::new(3, Evaluation::new(vec![5.0, 5.0])));
+/// assert_eq!(a.members().len(), 2);
+/// ```
 #[derive(Debug, Clone)]
 pub struct ParetoArchive<D> {
     /// The current approximate non-dominated set.

src/pareto/crowding.rs

@@ -11,6 +11,28 @@ use crate::core::objective::ObjectiveSpace;
 /// `f64::INFINITY`. If the front has 0 entries an empty vector is returned;
 /// 1 or 2 entries return all `f64::INFINITY`. All comparisons happen on
 /// minimization-oriented objective values (spec §9.6).
+///
+/// # Example
+///
+/// ```
+/// use heuropt::prelude::*;
+///
+/// let s = ObjectiveSpace::new(vec![
+///     Objective::minimize("f1"),
+///     Objective::minimize("f2"),
+/// ]);
+/// // Three points along a Pareto-like trade-off; the interior point gets
+/// // a finite crowding distance, the boundaries get +∞.
+/// let pop = [
+///     Candidate::new((), Evaluation::new(vec![0.0, 4.0])),
+///     Candidate::new((), Evaluation::new(vec![2.0, 2.0])),
+///     Candidate::new((), Evaluation::new(vec![4.0, 0.0])),
+/// ];
+/// let d = crowding_distance(&pop, &[0, 1, 2], &s);
+/// assert!(d[0].is_infinite());
+/// assert!(d[1].is_finite() && d[1] > 0.0);
+/// assert!(d[2].is_infinite());
+/// ```
 pub fn crowding_distance<D>(
     population: &[Candidate<D>],
     front: &[usize],

src/pareto/dominance.rs

@@ -29,6 +29,21 @@ pub enum Dominance {
 ///    `constraint_violation` dominates.
 /// 3. Otherwise compare objective values after converting both to
 ///    minimization orientation via [`ObjectiveSpace::as_minimization`].
+///
+/// # Example
+///
+/// ```
+/// use heuropt::prelude::*;
+///
+/// let s = ObjectiveSpace::new(vec![
+///     Objective::minimize("f1"),
+///     Objective::minimize("f2"),
+/// ]);
+/// let a = Evaluation::new(vec![1.0, 1.0]);
+/// let b = Evaluation::new(vec![2.0, 2.0]);
+/// assert_eq!(pareto_compare(&a, &b, &s), Dominance::Dominates);
+/// assert_eq!(pareto_compare(&b, &a, &s), Dominance::DominatedBy);
+/// ```
 pub fn pareto_compare(a: &Evaluation, b: &Evaluation, objectives: &ObjectiveSpace) -> Dominance {
     let a_feasible = a.is_feasible();
     let b_feasible = b.is_feasible();

src/pareto/front.rs

@@ -8,6 +8,25 @@ use crate::pareto::dominance::{Dominance, pareto_compare};
 ///
 /// O(N²·M) in v1 (spec §9.3). Input order is preserved among returned
 /// candidates.
+///
+/// # Example
+///
+/// ```
+/// use heuropt::prelude::*;
+///
+/// let s = ObjectiveSpace::new(vec![
+///     Objective::minimize("f1"),
+///     Objective::minimize("f2"),
+/// ]);
+/// let pop = [
+///     Candidate::new(1u32, Evaluation::new(vec![1.0, 4.0])), // non-dominated
+///     Candidate::new(2u32, Evaluation::new(vec![3.0, 2.0])), // non-dominated
+///     Candidate::new(3u32, Evaluation::new(vec![5.0, 5.0])), // dominated
+/// ];
+/// let front = pareto_front(&pop, &s);
+/// let kept: Vec<u32> = front.iter().map(|c| c.decision).collect();
+/// assert_eq!(kept, vec![1, 2]);
+/// ```
 pub fn pareto_front<D: Clone>(
     population: &[Candidate<D>],
     objectives: &ObjectiveSpace,
@@ -34,6 +53,21 @@ pub fn pareto_front<D: Clone>(
 ///
 /// Returns `None` if there is not exactly one objective, if the population is
 /// empty, or if every candidate is infeasible (spec §9.4).
+///
+/// # Example
+///
+/// ```
+/// use heuropt::prelude::*;
+///
+/// let s = ObjectiveSpace::new(vec![Objective::minimize("f")]);
+/// let pop = [
+///     Candidate::new(1u32, Evaluation::new(vec![3.0])),
+///     Candidate::new(2u32, Evaluation::new(vec![1.0])),
+///     Candidate::new(3u32, Evaluation::new(vec![2.0])),
+/// ];
+/// let best = best_candidate(&pop, &s).unwrap();
+/// assert_eq!(best.decision, 2);
+/// ```
 pub fn best_candidate<D: Clone>(
     population: &[Candidate<D>],
     objectives: &ObjectiveSpace,

src/pareto/reference_points.rs

@@ -10,6 +10,21 @@
 ///
 /// # Panics
 /// If `num_objectives == 0`.
+///
+/// # Example
+///
+/// ```
+/// use heuropt::prelude::*;
+///
+/// // 3 objectives, 4 divisions → binomial(6, 2) = 15 points.
+/// let pts = das_dennis(3, 4);
+/// assert_eq!(pts.len(), 15);
+/// for w in &pts {
+///     assert_eq!(w.len(), 3);
+///     let sum: f64 = w.iter().sum();
+///     assert!((sum - 1.0).abs() < 1e-12);
+/// }
+/// ```
 pub fn das_dennis(num_objectives: usize, divisions: usize) -> Vec<Vec<f64>> {
     assert!(
         num_objectives > 0,

src/pareto/sort.rs

@@ -9,6 +9,26 @@ use crate::core::objective::ObjectiveSpace;
 /// non-dominated after removing `fronts[0]`, and so on. Each entry is an index
 /// into the input population. Equal-objective candidates land on the same
 /// front. O(N²·M) is acceptable for v1 (spec §9.5).
+///
+/// # Example
+///
+/// ```
+/// use heuropt::prelude::*;
+///
+/// let s = ObjectiveSpace::new(vec![
+///     Objective::minimize("f1"),
+///     Objective::minimize("f2"),
+/// ]);
+/// let pop = [
+///     Candidate::new((), Evaluation::new(vec![1.0, 5.0])), // front 0
+///     Candidate::new((), Evaluation::new(vec![2.0, 3.0])), // front 0
+///     Candidate::new((), Evaluation::new(vec![4.0, 1.0])), // front 0
+///     Candidate::new((), Evaluation::new(vec![3.0, 4.0])), // front 1
+///     Candidate::new((), Evaluation::new(vec![5.0, 6.0])), // front 2
+/// ];
+/// let fronts = non_dominated_sort(&pop, &s);
+/// assert_eq!(fronts.len(), 3);
+/// ```
 pub fn non_dominated_sort<D>(
     population: &[Candidate<D>],
     objectives: &ObjectiveSpace,

src/prelude.rs

@@ -4,6 +4,8 @@
 //! use heuropt::prelude::*;
 //! ```
 
+#[cfg(feature = "async")]
+pub use crate::core::async_problem::AsyncProblem;
 pub use crate::core::{
     Candidate, Direction, Evaluation, Objective, ObjectiveSpace, OptimizationResult,
     PartialProblem, Population, Problem, Rng, rng_from_seed,
@@ -11,14 +13,6 @@ pub use crate::core::{
 
 pub use crate::traits::{Initializer, Optimizer, Repair, Variation};
 
-#[cfg(feature = "tracing")]
-pub use crate::observer::builtin::TracingObserver;
-pub use crate::observer::{
-    Observer, Snapshot,
-    builtin::{AllOf, AnyOf, MaxIterations, MaxTime, Periodic, Stagnation, TargetFitness},
-};
-pub use std::ops::ControlFlow;
-
 pub use crate::pareto::{
     Dominance, ParetoArchive, best_candidate, crowding_distance, das_dennis, non_dominated_sort,
     pareto_compare, pareto_front,

src/traits/optimizer.rs

@@ -1,71 +1,18 @@
 //! The single trait users implement to add a new optimizer.
 
-use std::time::{Duration, Instant};
-
 use crate::core::problem::Problem;
 use crate::core::result::OptimizationResult;
-use crate::observer::{Observer, Snapshot};
 
 /// An optimizer that runs to completion in a single call.
 ///
 /// Implementations own their main loop, manage their own state, and return an
-/// [`OptimizationResult`]. Invalid configuration panics with a clear
+/// [`OptimizationResult`]. v1 deliberately does not expose a step-by-step API
-/// message rather than returning a `Result`.
+/// or an associated error type — invalid configuration may panic with a clear
+/// message.
 pub trait Optimizer<P>
 where
     P: Problem,
 {
     /// Run the optimizer to completion against `problem`.
     fn run(&mut self, problem: &P) -> OptimizationResult<P::Decision>;
-
-    /// Run with an [`Observer`] called after each generation.
-    ///
-    /// The observer can halt the run by returning
-    /// [`std::ops::ControlFlow::Break`]; the partial result is still
-    /// returned. Built-in observers in
-    /// [`heuropt::observer::builtin`](crate::observer::builtin) cover
-    /// the common stop conditions (`MaxTime`, `TargetFitness`,
-    /// `Stagnation`, …).
-    ///
-    /// **Default impl:** falls back to `run` plus a single final
-    /// notification. Algorithms that override this method get true
-    /// per-generation observation; algorithms that don't get a single
-    /// notification at the end. The trait-level docstring on each
-    /// algorithm calls out which behavior it supports.
-    fn run_with<O>(&mut self, problem: &P, observer: &mut O) -> OptimizationResult<P::Decision>
-    where
-        O: Observer<P::Decision>,
-    {
-        let started = Instant::now();
-        let result = self.run(problem);
-        let elapsed = started.elapsed();
-        notify_final(&result, elapsed, problem, observer);
-        result
-    }
-}
-
-/// Helper used by the default `run_with` impl: build a single final-
-/// state snapshot and hand it to the observer once. Algorithms that
-/// override `run_with` for per-generation reporting don't go through
-/// this path — they construct their own per-iteration snapshots.
-fn notify_final<P, O>(
-    result: &OptimizationResult<P::Decision>,
-    elapsed: Duration,
-    problem: &P,
-    observer: &mut O,
-) where
-    P: Problem,
-    O: Observer<P::Decision>,
-{
-    let objectives = problem.objectives();
-    let snap = Snapshot {
-        iteration: result.generations,
-        evaluations: result.evaluations,
-        elapsed,
-        population: result.population.as_slice(),
-        pareto_front: Some(result.pareto_front.as_slice()),
-        best: result.best.as_ref(),
-        objectives: &objectives,
-    };
-    let _ = observer.observe(&snap);
 }