diff --git a/src/simulator/crates/mc-turn/src/game_state.rs b/src/simulator/crates/mc-turn/src/game_state.rs
index 4862dc44..6c01567a 100644
--- a/src/simulator/crates/mc-turn/src/game_state.rs
+++ b/src/simulator/crates/mc-turn/src/game_state.rs
@@ -715,6 +715,15 @@ pub struct PlayerState {
     /// keep their current behaviour without any changes.
     #[serde(default)]
     pub building_priors: mc_ai::tactical::state::BuildingPriors,
+    /// Stage 3 (mod system) — controller registry id used by
+    /// `mc_player_api::dispatch::drive_ai_slot` to look up which
+    /// `AiController` impl decides this slot's turn. Defaults to
+    /// `"scripted:default"` (the in-box MCTS+heuristic). Mod-supplied
+    /// controllers register their own id (`"learned:duel-v1b"`,
+    /// `"community:foo"`, ...) and the game-setup UI picks per slot.
+    /// Empty = fall-through to default at dispatch time.
+    #[serde(default)]
+    pub controller_id: String,
     /// Accumulated expansion capacity (earned from the `expansion` axis).
     pub expansion_points: u32,
     /// Per-city list of constructed building IDs. Aligned with `cities`.