diff --git a/src/metrics/hypervolume.rs b/src/metrics/hypervolume.rs
index c6a4449..008eafa 100644
--- a/src/metrics/hypervolume.rs
+++ b/src/metrics/hypervolume.rs
@@ -483,4 +483,107 @@ mod nd_tests {
         let hv_with = hypervolume_nd(&with_dominated, &s, &[2.0, 2.0, 2.0]);
         assert!((hv_base - hv_with).abs() < 1e-12, "{hv_base} vs {hv_with}");
     }
+
+    // ---- Mutation-test pinned helpers --------------------------------------
+
+    /// `dominates(a, b)` is true iff `a` is ≤ `b` on every axis and strictly
+    /// better on at least one. Pin all the boundary cases so the `<` / `>`
+    /// comparison flips are caught.
+    #[test]
+    fn dominates_strict_and_boundary_cases() {
+        // a strictly dominates b on both axes.
+        assert!(dominates(&[1.0, 1.0], &[2.0, 2.0], 2));
+        // b does not dominate a (reverse).
+        assert!(!dominates(&[2.0, 2.0], &[1.0, 1.0], 2));
+        // Equal points: neither dominates (no strict improvement).
+        assert!(!dominates(&[1.0, 1.0], &[1.0, 1.0], 2));
+        // a better on axis 0, equal on axis 1 → a dominates b.
+        assert!(dominates(&[1.0, 2.0], &[2.0, 2.0], 2));
+        // a better on axis 0 but worse on axis 1 → no domination.
+        assert!(!dominates(&[1.0, 3.0], &[2.0, 2.0], 2));
+    }
+
+    /// `non_dominated_projection` drops dominated members and keeps the
+    /// rest. Pin the exact retained set.
+    #[test]
+    fn non_dominated_projection_drops_dominated() {
+        let pts = vec![
+            vec![1.0, 3.0], // non-dominated
+            vec![3.0, 1.0], // non-dominated
+            vec![2.0, 2.0], // non-dominated (trade-off)
+            vec![4.0, 4.0], // dominated by all three
+        ];
+        let nd = non_dominated_projection(&pts);
+        assert_eq!(nd.len(), 3);
+        assert!(!nd.contains(&vec![4.0, 4.0]));
+        assert!(nd.contains(&vec![1.0, 3.0]));
+        assert!(nd.contains(&vec![3.0, 1.0]));
+        assert!(nd.contains(&vec![2.0, 2.0]));
+    }
+
+    #[test]
+    fn non_dominated_projection_empty_input_is_empty() {
+        let pts: Vec<Vec<f64>> = Vec::new();
+        assert!(non_dominated_projection(&pts).is_empty());
+    }
+
+    #[test]
+    fn non_dominated_projection_all_nondominated_keeps_all() {
+        let pts = vec![vec![1.0, 3.0], vec![2.0, 2.0], vec![3.0, 1.0]];
+        let nd = non_dominated_projection(&pts);
+        assert_eq!(nd.len(), 3);
+    }
+
+    /// `hso_recursive` 1-D base case: HV is `reference - min_point`,
+    /// clamped at 0.
+    #[test]
+    fn hso_recursive_1d_base_case() {
+        let pts = vec![vec![0.5], vec![1.5], vec![0.2]];
+        // min is 0.2, reference is 2.0 → HV = 1.8
+        assert!((hso_recursive(&pts, &[2.0]) - 1.8).abs() < 1e-12);
+        // A point past the reference → clamped to 0 contribution; min still 0.2.
+        let pts2 = vec![vec![3.0]];
+        assert_eq!(hso_recursive(&pts2, &[2.0]), 0.0);
+    }
+
+    /// `hso_recursive` 2-D base case: classic staircase area.
+    #[test]
+    fn hso_recursive_2d_staircase() {
+        // Three points (1,3), (2,2), (3,1) against reference (4,4).
+        // Dominated area = 6 (same as the hypervolume_2d doctest).
+        let pts = vec![vec![1.0, 3.0], vec![2.0, 2.0], vec![3.0, 1.0]];
+        let hv = hso_recursive(&pts, &[4.0, 4.0]);
+        assert!((hv - 6.0).abs() < 1e-12, "hv = {hv}");
+    }
+
+    /// `hypervolume_nd_from_evaluations` returns 0 for an empty slice and a
+    /// positive value for a dominating point.
+    #[test]
+    fn hypervolume_nd_from_evaluations_empty_and_nonempty() {
+        let s = ObjectiveSpace::new(vec![
+            Objective::minimize("f1"),
+            Objective::minimize("f2"),
+        ]);
+        let empty: Vec<&Evaluation> = Vec::new();
+        assert_eq!(hypervolume_nd_from_evaluations(&empty, &s, &[2.0, 2.0]), 0.0);
+
+        let e = Evaluation::new(vec![1.0, 1.0]);
+        let evals = vec![&e];
+        let hv = hypervolume_nd_from_evaluations(&evals, &s, &[2.0, 2.0]);
+        // Single point (1,1) vs reference (2,2) → 1×1 = 1.
+        assert!((hv - 1.0).abs() < 1e-12, "hv = {hv}");
+    }
+
+    /// A point that does not strictly dominate the reference contributes 0.
+    #[test]
+    fn hypervolume_nd_from_evaluations_skips_non_dominating() {
+        let s = ObjectiveSpace::new(vec![
+            Objective::minimize("f1"),
+            Objective::minimize("f2"),
+        ]);
+        // (2, 1): axis 0 equals the reference → not strictly dominating.
+        let e = Evaluation::new(vec![2.0, 1.0]);
+        let evals = vec![&e];
+        assert_eq!(hypervolume_nd_from_evaluations(&evals, &s, &[2.0, 2.0]), 0.0);
+    }
 }
diff --git a/src/metrics/spacing.rs b/src/metrics/spacing.rs
index add9af0..9b5862a 100644
--- a/src/metrics/spacing.rs
+++ b/src/metrics/spacing.rs
@@ -119,4 +119,36 @@ mod tests {
         let s_val = spacing(&pts, &s);
         assert!(s_val > 0.0);
     }
+
+    /// Pins the exact spacing for a front with *varying* nearest-neighbor
+    /// distances, exercising the `(a-b).abs()` sum, the `d < nearest`
+    /// comparison, and both `/ n` divisions in the mean/variance.
+    #[test]
+    fn varying_nn_distances_pinned() {
+        let s = space_min2();
+        // (0,10), (1,9), (10,0): L1 nearest distances are 2, 2, 18.
+        // mean = 22/3, variance = 1536/27, spacing = sqrt(1536/27).
+        let front = [
+            cand(vec![0.0, 10.0]),
+            cand(vec![1.0, 9.0]),
+            cand(vec![10.0, 0.0]),
+        ];
+        let got = spacing(&front, &s);
+        let expected = (1536.0_f64 / 27.0).sqrt();
+        assert!((got - expected).abs() < 1e-9, "got {got}, expected {expected}");
+    }
+
+    /// A perfectly even front has zero spacing — the variance term is 0.
+    /// Distinct from the doctest case in that it uses three points whose
+    /// nearest-neighbor L1 distances are all equal to 4.
+    #[test]
+    fn evenly_spaced_front_is_zero_spacing() {
+        let s = space_min2();
+        let front = [
+            cand(vec![0.0, 4.0]),
+            cand(vec![2.0, 2.0]),
+            cand(vec![4.0, 0.0]),
+        ];
+        assert!(spacing(&front, &s) < 1e-12);
+    }
 }
diff --git a/src/pareto/archive.rs b/src/pareto/archive.rs
index 8c90b12..c8c6fc6 100644
--- a/src/pareto/archive.rs
+++ b/src/pareto/archive.rs
@@ -265,4 +265,62 @@ mod tests {
         a.extend(vec![cand(1, vec![1.0, 4.0]), cand(2, vec![3.0, 2.0])]);
         assert_eq!(a.members().len(), 2);
     }
+
+    /// `truncate` keeps the archive untouched when it is already at or
+    /// below `max_size`, and trims it when over. Pins the `>` boundary.
+    #[test]
+    fn truncate_boundary_behavior() {
+        let mut a = ParetoArchive::<u32>::new(space_min2());
+        // Three mutually non-dominated members.
+        a.insert(cand(1, vec![1.0, 3.0]));
+        a.insert(cand(2, vec![2.0, 2.0]));
+        a.insert(cand(3, vec![3.0, 1.0]));
+        assert_eq!(a.members().len(), 3);
+        // max_size == len → no-op (kills `>` → `>=`).
+        a.truncate(3);
+        assert_eq!(a.members().len(), 3);
+        // max_size > len → no-op.
+        a.truncate(10);
+        assert_eq!(a.members().len(), 3);
+        // max_size < len → trims.
+        a.truncate(2);
+        assert_eq!(a.members().len(), 2);
+    }
+
+    /// A trade-off candidate (better on one axis, worse on the other) is
+    /// neither dominated nor dominating — it must be *added* alongside the
+    /// existing member. Pins the per-axis `<` / `>` scan in both
+    /// `member_dominates_or_equals` and `candidate_dominates_member`.
+    #[test]
+    fn trade_off_candidate_is_kept_alongside() {
+        let mut a = ParetoArchive::<u32>::new(space_min2());
+        a.insert(cand(1, vec![1.0, 5.0]));
+        a.insert(cand(2, vec![5.0, 1.0])); // trade-off — must be kept
+        assert_eq!(a.members().len(), 2);
+    }
+
+    /// An equal-objectives candidate is rejected (a member dominates-or-
+    /// equals it). Pins the Equal branch — distinguishes `<=` from `<` in
+    /// `candidate_dominates_member` and the `<=` in
+    /// `member_dominates_or_equals`'s infeasible branch.
+    #[test]
+    fn equal_candidate_is_rejected() {
+        let mut a = ParetoArchive::<u32>::new(space_min2());
+        a.insert(cand(1, vec![2.0, 2.0]));
+        a.insert(cand(2, vec![2.0, 2.0])); // identical objectives → rejected
+        assert_eq!(a.members().len(), 1);
+        assert_eq!(a.members()[0].decision, 1);
+    }
+
+    /// Two infeasible candidates: the one with smaller constraint violation
+    /// wins. Pins the `<` / `<=` in the infeasible branches.
+    #[test]
+    fn infeasible_candidate_with_smaller_violation_evicts_larger() {
+        let mut a = ParetoArchive::<u32>::new(space_min2());
+        a.insert(Candidate::new(1u32, Evaluation::constrained(vec![0.0, 0.0], 1.0)));
+        // Smaller violation → dominates the existing infeasible member.
+        a.insert(Candidate::new(2u32, Evaluation::constrained(vec![9.0, 9.0], 0.5)));
+        assert_eq!(a.members().len(), 1);
+        assert_eq!(a.members()[0].decision, 2);
+    }
 }
diff --git a/src/pareto/crowding.rs b/src/pareto/crowding.rs
index db23ee5..aa06bce 100644
--- a/src/pareto/crowding.rs
+++ b/src/pareto/crowding.rs
@@ -160,4 +160,33 @@ mod tests {
         assert!(d[2].is_infinite());
         assert!(d[1].is_finite());
     }
+
+    /// Crowding distance pins the exact interior contribution: for a 3-point
+    /// 2-objective front, the middle point's distance is the sum over both
+    /// objectives of (next - prev) / span. With evenly-spaced points the
+    /// value is exactly 2.0 (1.0 per objective).
+    #[test]
+    fn interior_point_distance_is_pinned() {
+        let s = space_min2();
+        // Front along the line f1 + f2 = 4: (0,4), (2,2), (4,0).
+        let pop = [cand(vec![0.0, 4.0]), cand(vec![2.0, 2.0]), cand(vec![4.0, 0.0])];
+        let d = crowding_distance(&pop, &[0, 1, 2], &s);
+        // Boundary points are infinite; the middle point gets
+        // (4-0)/4 + (4-0)/4 = 2.0 (objective 0 span 4, objective 1 span 4).
+        assert!(d[0].is_infinite());
+        assert!(d[2].is_infinite());
+        assert!((d[1] - 2.0).abs() < 1e-12, "interior distance = {}", d[1]);
+    }
+
+    /// An asymmetric front pins the per-objective `(next - prev) / span`
+    /// arithmetic: catches the `-` ↔ `+`/`/` and `/` ↔ `*` mutants.
+    #[test]
+    fn asymmetric_interior_distance_is_pinned() {
+        let s = space_min2();
+        // (0,10), (1,2), (10,0): objective-0 span = 10, objective-1 span = 10.
+        let pop = [cand(vec![0.0, 10.0]), cand(vec![1.0, 2.0]), cand(vec![10.0, 0.0])];
+        let d = crowding_distance(&pop, &[0, 1, 2], &s);
+        // middle point: obj0 (10-0)/10 = 1.0; obj1 (10-0)/10 = 1.0 → 2.0.
+        assert!((d[1] - 2.0).abs() < 1e-12, "got {}", d[1]);
+    }
 }
diff --git a/src/pareto/dominance.rs b/src/pareto/dominance.rs
index 119470e..ba8aa6a 100644
--- a/src/pareto/dominance.rs
+++ b/src/pareto/dominance.rs
@@ -152,4 +152,35 @@ mod tests {
         let b = Evaluation::new(vec![2.0, 0.8]);
         assert_eq!(pareto_compare(&a, &b, &s), Dominance::Dominates);
     }
+
+    /// `a` better on one axis, worse on the other → NonDominated. Pins the
+    /// `av < bv` / `av > bv` comparisons in the per-objective scan.
+    #[test]
+    fn trade_off_is_non_dominated() {
+        let s = space_min2();
+        let a = Evaluation::new(vec![1.0, 5.0]);
+        let b = Evaluation::new(vec![5.0, 1.0]);
+        assert_eq!(pareto_compare(&a, &b, &s), Dominance::NonDominated);
+        assert_eq!(pareto_compare(&b, &a, &s), Dominance::NonDominated);
+    }
+
+    /// `a` better on one axis, equal on the other → Dominates. This is the
+    /// boundary case that distinguishes `<` from `<=` in the scan.
+    #[test]
+    fn better_on_one_equal_on_other_dominates() {
+        let s = space_min2();
+        let a = Evaluation::new(vec![1.0, 2.0]);
+        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);
+    }
+
+    /// Identical objectives → Equal (neither `<` nor `>` ever fires).
+    #[test]
+    fn identical_objectives_are_equal() {
+        let s = space_min2();
+        let a = Evaluation::new(vec![3.0, 3.0]);
+        let b = Evaluation::new(vec![3.0, 3.0]);
+        assert_eq!(pareto_compare(&a, &b, &s), Dominance::Equal);
+    }
 }
diff --git a/src/pareto/front.rs b/src/pareto/front.rs
index 9f5f7d4..1148076 100644
--- a/src/pareto/front.rs
+++ b/src/pareto/front.rs
@@ -180,4 +180,18 @@ mod tests {
         ];
         assert!(best_candidate(&pop, &s).is_none());
     }
+
+    /// `best_candidate` keeps the *first* minimum on a tie — pins the strict
+    /// `v < best_min` (a `<=` mutant would keep the last tied candidate).
+    #[test]
+    fn best_candidate_keeps_first_on_tie() {
+        use crate::core::objective::Objective;
+        let s = ObjectiveSpace::new(vec![Objective::minimize("f")]);
+        let pop = [
+            Candidate::new(1u32, Evaluation::new(vec![1.0])),
+            Candidate::new(2u32, Evaluation::new(vec![1.0])),
+        ];
+        let best = best_candidate(&pop, &s).unwrap();
+        assert_eq!(best.decision, 1, "should keep the first of two tied minima");
+    }
 }
diff --git a/src/pareto/sort.rs b/src/pareto/sort.rs
index e1c7f32..703438a 100644
--- a/src/pareto/sort.rs
+++ b/src/pareto/sort.rs
@@ -227,4 +227,39 @@ mod tests {
         assert_eq!(f1, vec![3]);
         assert_eq!(f2, vec![4]);
     }
+
+    /// Three mutually non-dominated points all land in front 0; a fourth
+    /// point dominated by all three lands in front 1. Pins the `<` / `>`
+    /// comparisons in the inline dominance check.
+    #[test]
+    fn three_nondominated_then_one_dominated() {
+        let s = space_min2();
+        let pop = [
+            cand(vec![1.0, 3.0]),
+            cand(vec![2.0, 2.0]),
+            cand(vec![3.0, 1.0]),
+            cand(vec![5.0, 5.0]), // dominated by all three
+        ];
+        let fronts = non_dominated_sort(&pop, &s);
+        assert_eq!(fronts.len(), 2);
+        assert_eq!(fronts[0].len(), 3);
+        assert_eq!(fronts[1], vec![3]);
+    }
+
+    /// A strict chain a ▷ b ▷ c produces three singleton fronts. Pins the
+    /// front-peeling `while` loop and the `&&` guard at line 127.
+    #[test]
+    fn strict_chain_produces_three_singleton_fronts() {
+        let s = space_min2();
+        let pop = [
+            cand(vec![1.0, 1.0]), // dominates everything
+            cand(vec![2.0, 2.0]),
+            cand(vec![3.0, 3.0]),
+        ];
+        let fronts = non_dominated_sort(&pop, &s);
+        assert_eq!(fronts.len(), 3);
+        assert_eq!(fronts[0], vec![0]);
+        assert_eq!(fronts[1], vec![1]);
+        assert_eq!(fronts[2], vec![2]);
+    }
 }
diff --git a/src/selection/tournament.rs b/src/selection/tournament.rs
index 53975c7..ab9ef9c 100644
--- a/src/selection/tournament.rs
+++ b/src/selection/tournament.rs
@@ -269,4 +269,105 @@ mod tests {
         let mut rng = rng_from_seed(0);
         let _ = stochastic_ranking_select(&pop, &s, 1.5, 1, &mut rng);
     }
+
+    // ---- Mutation-test pinned helpers --------------------------------------
+
+    fn constrained(d: u32, obj: f64, cv: f64) -> Candidate<u32> {
+        Candidate::new(d, Evaluation::constrained(vec![obj], cv))
+    }
+
+    #[test]
+    fn challenger_wins_feasibility_first() {
+        // Feasible challenger beats infeasible best, regardless of objective.
+        let feasible = cand_min(1, 100.0);
+        let infeasible = constrained(2, 0.0, 1.0);
+        assert!(challenger_wins(&feasible, &infeasible, Direction::Minimize));
+        assert!(!challenger_wins(&infeasible, &feasible, Direction::Minimize));
+    }
+
+    #[test]
+    fn challenger_wins_two_infeasible_compares_violation() {
+        let less_violating = constrained(1, 0.0, 0.5);
+        let more_violating = constrained(2, 0.0, 1.0);
+        assert!(challenger_wins(&less_violating, &more_violating, Direction::Minimize));
+        assert!(!challenger_wins(&more_violating, &less_violating, Direction::Minimize));
+    }
+
+    #[test]
+    fn challenger_wins_two_feasible_under_min_and_max() {
+        let lower = cand_min(1, 1.0);
+        let higher = cand_min(2, 2.0);
+        assert!(challenger_wins(&lower, &higher, Direction::Minimize));
+        assert!(!challenger_wins(&higher, &lower, Direction::Minimize));
+        assert!(challenger_wins(&higher, &lower, Direction::Maximize));
+        assert!(!challenger_wins(&lower, &higher, Direction::Maximize));
+    }
+
+    #[test]
+    fn challenger_wins_equal_objectives_does_not_win() {
+        // Strict comparison: equal objectives → challenger does NOT win.
+        let a = cand_min(1, 1.0);
+        let b = cand_min(2, 1.0);
+        assert!(!challenger_wins(&a, &b, Direction::Minimize));
+        assert!(!challenger_wins(&a, &b, Direction::Maximize));
+    }
+
+    #[test]
+    fn better_by_objective_min_and_max() {
+        let a = Evaluation::new(vec![1.0]);
+        let b = Evaluation::new(vec![2.0]);
+        assert!(better_by_objective(&a, &b, Direction::Minimize));
+        assert!(!better_by_objective(&b, &a, Direction::Minimize));
+        assert!(better_by_objective(&b, &a, Direction::Maximize));
+        assert!(!better_by_objective(&a, &b, Direction::Maximize));
+        // Equal → not strictly better.
+        let c = Evaluation::new(vec![1.0]);
+        assert!(!better_by_objective(&a, &c, Direction::Minimize));
+    }
+
+    #[test]
+    fn better_by_feasibility_all_four_branches() {
+        let feasible_a = Evaluation::new(vec![10.0]);
+        let infeasible_b = Evaluation::constrained(vec![0.0], 1.0);
+        // feasible vs infeasible
+        assert!(better_by_feasibility(&feasible_a, &infeasible_b, Direction::Minimize));
+        assert!(!better_by_feasibility(&infeasible_b, &feasible_a, Direction::Minimize));
+        // two infeasible: smaller violation wins
+        let low_cv = Evaluation::constrained(vec![0.0], 0.3);
+        let high_cv = Evaluation::constrained(vec![0.0], 0.9);
+        assert!(better_by_feasibility(&low_cv, &high_cv, Direction::Minimize));
+        assert!(!better_by_feasibility(&high_cv, &low_cv, Direction::Minimize));
+        // two feasible: delegates to better_by_objective
+        let feasible_lower = Evaluation::new(vec![1.0]);
+        let feasible_higher = Evaluation::new(vec![2.0]);
+        assert!(better_by_feasibility(&feasible_lower, &feasible_higher, Direction::Minimize));
+    }
+
+    #[test]
+    fn stochastic_ranking_select_pf_zero_is_pure_feasibility_order() {
+        // pf = 0 → always compare by feasibility. The feasible candidate
+        // must rank first regardless of objective value.
+        let s = ObjectiveSpace::new(vec![Objective::minimize("f")]);
+        let pop = [
+            constrained(1, 0.0, 2.0),  // infeasible, great objective
+            cand_min(2, 100.0),         // feasible, terrible objective
+        ];
+        let mut rng = rng_from_seed(7);
+        let picks = stochastic_ranking_select(&pop, &s, 0.0, 1, &mut rng);
+        // With pf=0, feasibility dominates → candidate 2 ranked first.
+        assert_eq!(picks, vec![2]);
+    }
+
+    #[test]
+    fn stochastic_ranking_select_count_wraps_modulo_population() {
+        // count > population size wraps around via `order[k % n]`.
+        let s = ObjectiveSpace::new(vec![Objective::minimize("f")]);
+        let pop = [cand_min(1, 1.0), cand_min(2, 2.0)];
+        let mut rng = rng_from_seed(0);
+        let picks = stochastic_ranking_select(&pop, &s, 0.0, 5, &mut rng);
+        assert_eq!(picks.len(), 5);
+        // Best (candidate 1) is at index 0; index 2 wraps to it again.
+        assert_eq!(picks[0], 1);
+        assert_eq!(picks[2], 1);
+    }
 }