diff --git a/.cargo/mutants.toml b/.cargo/mutants.toml
index 03c3591..981cbee 100644
--- a/.cargo/mutants.toml
+++ b/.cargo/mutants.toml
@@ -7,6 +7,12 @@ test_tool = "nextest"
 # Async tests + crypto work takes longer under mutation load.
 timeout_multiplier = 3.0
 
+# Enable `test-support` so cfg-gated setters (e.g. Builder::dht_backend) and
+# the integration tests that exercise them are part of the mutation surface.
+# Without this, dht_backend mutations always "survive" trivially because the
+# method isn't compiled in.
+additional_cargo_args = ["--features", "test-support"]
+
 # Skip paths that either can't be meaningfully mutated or are covered by
 # other tools:
 #   - sim_impl.rs: simulator is exercised via the simulator example, not
diff --git a/src/builder.rs b/src/builder.rs
index b369e09..0dd5589 100644
--- a/src/builder.rs
+++ b/src/builder.rs
@@ -470,6 +470,28 @@ mod tests {
         assert_eq!(Builder::default().heal_period(d).heal_period, d);
     }
 
+    #[test]
+    fn failure_retry_period_setter_stores_exact_value() {
+        let d = Duration::from_millis(42);
+        assert_eq!(
+            Builder::default().failure_retry_period(d).failure_retry_period,
+            d
+        );
+    }
+
+    #[test]
+    #[cfg(feature = "test-support")]
+    fn dht_backend_setter_stores_value() {
+        use std::sync::Arc;
+
+        use crate::sim::InMemoryDht;
+        let dht = Arc::new(InMemoryDht::new());
+        let before = Builder::default();
+        let after = Builder::default().dht_backend(dht);
+        assert!(before.dht_backend.is_none());
+        assert!(after.dht_backend.is_some());
+    }
+
     #[test]
     fn jitter_setter_stores_exact_value() {
         let b = Builder::default().jitter(0.25);