diff --git a/src/game/engine/src/autoloads/event_bus.gd b/src/game/engine/src/autoloads/event_bus.gd
index c598d454..b7c33205 100644
--- a/src/game/engine/src/autoloads/event_bus.gd
+++ b/src/game/engine/src/autoloads/event_bus.gd
@@ -226,6 +226,10 @@ signal marine_creature_spawned(tile_pos: Vector2i, creature_type: String)
 signal marine_creature_cleared(tile_pos: Vector2i, creature_type: String)
 
 # -- Ecology signals --
+## Ambient encounter rolled during unit movement through a tile with
+## non-zero fauna_density. Fired by the GDExtension bridge after
+## AmbientEncounterFired appears in TurnResult.events_emitted. (p2-58b)
+signal ambient_encounter_fired(unit_id: int, tile_pos: Vector2i, species_id: String)
 ## A lair's habitat degraded below threshold for too long — converted to ruin.
 signal lair_abandoned(pos: Vector2i)
 ## A lair's habitat is thriving — creatures quality-up faster.
diff --git a/src/simulator/api-gdext/src/lib.rs b/src/simulator/api-gdext/src/lib.rs
index f9f25f58..9c53793c 100644
--- a/src/simulator/api-gdext/src/lib.rs
+++ b/src/simulator/api-gdext/src/lib.rs
@@ -915,6 +915,15 @@ fn dict_to_tile(dict: &Dictionary, tile: &mut mc_core::grid::TileState) {
     if let Some(v) = dict.get("habitat_suitability") { tile.habitat_suitability = v.to::<f64>() as f32; }
     if let Some(v) = dict.get("aerosol_mitigation") { tile.aerosol_mitigation = v.to::<f64>() as f32; }
     if let Some(v) = dict.get("resource_id") { tile.resource_id = v.to::<GString>().to_string(); }
+    // p2-58b: ambient encounter fields — settable from GDScript for test scenarios.
+    if let Some(v) = dict.get("fauna_density") { tile.fauna_density = v.to::<f64>() as f32; }
+    if let Some(v) = dict.get("ecosystem_tier") { tile.ecosystem_tier = v.to::<i64>() as i32; }
+    if let Some(v) = dict.get("fauna_index") {
+        let arr = v.to::<Array<GString>>();
+        tile.fauna_index = arr.iter_shared()
+            .map(|s| mc_core::ids::SpeciesId::new(s.to_string()))
+            .collect();
+    }
 }
 
 /// Create a TileState from a Dictionary (for spec eval functions that need a full tile).
@@ -3861,6 +3870,32 @@ fn turn_result_to_dict(result: &mc_turn::TurnResult, post_turn: u32) -> Dictiona
     d.set("t7_t10_deaths", t7_t10_deaths);
     d.set("fauna_combat_log", log);
 
+    // p2-58b: ambient encounter surface. Count AmbientEncounterFired events
+    // from events_emitted and expose as `ambient_encounter_count` plus a
+    // lightweight `ambient_encounters` array for GDScript dispatch.
+    let mut ambient_enc: Array<Dictionary> = Array::new();
+    for ev in &result.events_emitted {
+        if let mc_turn::TurnEvent::AmbientEncounterFired {
+            turn: ev_turn,
+            clan,
+            hex,
+            species,
+            group_size,
+        } = ev
+        {
+            let mut e = Dictionary::new();
+            e.set("turn", *ev_turn as i64);
+            e.set("clan", clan.0 as i64);
+            e.set("col", hex.q as i64);
+            e.set("row", hex.r as i64);
+            e.set("species_id", species.as_str());
+            e.set("group_size", *group_size as i64);
+            ambient_enc.push(&e);
+        }
+    }
+    d.set("ambient_encounter_count", ambient_enc.len() as i64);
+    d.set("ambient_encounters", ambient_enc);
+
     // p2-55 Wave 2: civilian-capture event surface. Each array is empty when
     // no events fired this turn (the GDScript chronicle wiring should be a
     // no-op in that case).