diff --git a/src/simulator/crates/mc-turn/src/processor.rs b/src/simulator/crates/mc-turn/src/processor.rs
index f1cbf763..b705a698 100644
--- a/src/simulator/crates/mc-turn/src/processor.rs
+++ b/src/simulator/crates/mc-turn/src/processor.rs
@@ -5727,6 +5727,75 @@ mod tests {
         assert!(el.is_empty() && es.is_empty(), "no categories → no tradeables");
     }
 
+    #[test]
+    fn process_trade_phase_forms_and_persists_strategic_swap() {
+        // p3-25 end-to-end: complementary owned-tile strategics → evaluate_trades
+        // forms a StrategicSwap → persisted to state.trade_ledger → fanned onto
+        // both players' traded_strategics. Biomes guarantee complementarity:
+        // rainforest yields only hardwood (categorized strategic) for p0; mountains
+        // yield only iron_ore (categorized) for p1 — neither has the other's.
+        use mc_core::grid::GridState;
+        let mut state = GameState::default();
+        state.turn = 5;
+        state.map_seed = 0x1234_5678;
+        let mut grid = GridState::new(40, 40);
+        for c in 0..10 {
+            if let Some(t) = grid.tile_mut(c, 0) {
+                t.biome_label_id = "tropical_rainforest".into();
+                t.quality = 6;
+            }
+        }
+        for c in 0..12 {
+            if let Some(t) = grid.tile_mut(c, 10) {
+                t.biome_label_id = "mountains".into();
+                t.quality = 6;
+            }
+        }
+        state.grid = Some(grid);
+        state
+            .resource_categories
+            .insert("hardwood".into(), "strategic".into());
+        state
+            .resource_categories
+            .insert("iron_ore".into(), "strategic".into());
+
+        let mut p0 = crate::game_state::PlayerState::default();
+        p0.player_index = 0;
+        p0.cities = vec![CityState::starter()];
+        p0.cities[0].owned_tiles = (0..10).map(|c| (c, 0)).collect();
+        p0.city_positions = vec![(0, 0)];
+        p0.strategic_axes.insert("trade_willingness".into(), 9);
+        let mut p1 = crate::game_state::PlayerState::default();
+        p1.player_index = 1;
+        p1.cities = vec![CityState::starter()];
+        p1.cities[0].owned_tiles = (0..12).map(|c| (c, 10)).collect();
+        p1.city_positions = vec![(0, 10)];
+        p1.strategic_axes.insert("trade_willingness".into(), 9);
+        state.players = vec![p0, p1];
+
+        let processor = TurnProcessor::new(100);
+        processor.process_trade_phase(&mut state);
+
+        let has_swap = state.trade_ledger.agreements.iter().any(|ag| {
+            matches!(ag, mc_trade::DiplomaticAgreement::StrategicSwap(ta) if ta.partners == (0, 1))
+        });
+        assert!(
+            has_swap,
+            "strategic swap should form + persist: {:?}",
+            state.trade_ledger.agreements
+        );
+        assert!(
+            state.players[0].traded_strategics.contains("iron_ore"),
+            "p0 gains iron_ore via the swap: {:?}",
+            state.players[0].traded_strategics
+        );
+        assert!(
+            state.players[1].traded_strategics.contains("hardwood"),
+            "p1 gains hardwood via the swap: {:?}",
+            state.players[1].traded_strategics
+        );
+    }
+
     #[test]
     fn processor_is_deterministic() {
         use mc_core::grid::GridState;