diff --git a/scripts/player-api-server.sh b/scripts/player-api-server.sh
index 36d5c8aa..17c0984b 100755
--- a/scripts/player-api-server.sh
+++ b/scripts/player-api-server.sh
@@ -42,7 +42,8 @@ PROJECT_DIR="$(dirname "$SCRIPT_DIR")"
 # flatpak runtime just for this harness is silly when native Godot 4
 # works directly. Env-var passthrough is automatic for the native path.
 export CP_SEED CP_PLAYERS CP_PLAYER_SLOT CP_MAP_SIZE CP_MAP_TYPE \
-    CP_OMNISCIENT CP_TIMEOUT_SEC CP_LOG_FILE CP_VICTORY_MODE
+    CP_OMNISCIENT CP_TIMEOUT_SEC CP_LOG_FILE CP_VICTORY_MODE \
+    CP_PLAYER_CONTROLLERS CP_PLAYER_SLOTS
 
 case "$(uname -s)" in
     Darwin)
@@ -68,6 +69,8 @@ case "$(uname -s)" in
             --env=CP_TIMEOUT_SEC="$CP_TIMEOUT_SEC" \
             --env=CP_LOG_FILE="$CP_LOG_FILE" \
             --env=CP_VICTORY_MODE="${CP_VICTORY_MODE:-}" \
+            --env=CP_PLAYER_CONTROLLERS="${CP_PLAYER_CONTROLLERS:-}" \
+            --env=CP_PLAYER_SLOTS="${CP_PLAYER_SLOTS:-}" \
             org.godotengine.Godot \
             --path "$PROJECT_DIR/src/game" \
             --headless \
diff --git a/tooling/rl_self_play/harness_client.py b/tooling/rl_self_play/harness_client.py
index c12fcfa2..8bf3146b 100644
--- a/tooling/rl_self_play/harness_client.py
+++ b/tooling/rl_self_play/harness_client.py
@@ -42,6 +42,14 @@ class HarnessConfig:
     seed: int = 42
     players: int = 2
     player_slot: int = 0
+    # Stage 4 (multi-slot adapter) — externally-driven slots when this
+    # process is driving more than one slot (e.g. 5 learned slots in a
+    # 5v5 FFA). When this tuple has >1 entry, every wire `view` / `act`
+    # call MUST include a `slot` field naming which slot it targets;
+    # `HarnessClient.view`/`act`/`end_turn` accept an optional `slot`
+    # kwarg for this. Defaults to `(player_slot,)` so single-slot
+    # callers keep the existing wire shape unchanged.
+    player_slots: tuple[int, ...] = ()
     map_size: str = "duel"
     map_type: str = "continents"
     omniscient: bool = False
@@ -53,18 +61,34 @@ class HarnessConfig:
     # simulator falls back to a city-count check that almost never fires
     # in 1v1 duel play.
     victory_mode: str = "domination"
+    # Stage 4 — per-AI-slot controller registry id, comma-joined in slot
+    # order over AI slots (i.e. excluding `player_slot`). Empty = every
+    # AI slot defaults to `"scripted:default"` (the MCTS+heuristic).
+    # Set this to mix learned + scripted opponents in one game, e.g.
+    # `("learned:duel-v1b", "", "")` puts learned on the first AI slot.
+    player_controllers: tuple[str, ...] = ()
+
+    @property
+    def effective_player_slots(self) -> tuple[int, ...]:
+        """Resolve the back-compat fallback: empty tuple → `(player_slot,)`."""
+        return self.player_slots if self.player_slots else (self.player_slot,)
 
     def to_env(self) -> dict[str, str]:
-        return {
+        slots = self.effective_player_slots
+        env: dict[str, str] = {
             "CP_SEED": str(self.seed),
             "CP_PLAYERS": str(self.players),
-            "CP_PLAYER_SLOT": str(self.player_slot),
+            "CP_PLAYER_SLOT": str(slots[0]),
+            "CP_PLAYER_SLOTS": ",".join(str(s) for s in slots),
             "CP_MAP_SIZE": self.map_size,
             "CP_MAP_TYPE": self.map_type,
             "CP_OMNISCIENT": "1" if self.omniscient else "0",
             "CP_TIMEOUT_SEC": str(self.timeout_sec),
             "CP_VICTORY_MODE": self.victory_mode,
         }
+        if self.player_controllers:
+            env["CP_PLAYER_CONTROLLERS"] = ",".join(self.player_controllers)
+        return env
 
 
 class HarnessClient:
@@ -128,24 +152,32 @@ class HarnessClient:
             f"no correlated response for id={msg['id']} within {MAX_LINES_PER_RESPONSE} lines"
         )
 
-    def view(self) -> dict[str, Any]:
-        r = self._send({"type": "view"})
+    def view(self, slot: int | None = None) -> dict[str, Any]:
+        msg: dict[str, Any] = {"type": "view"}
+        if slot is not None:
+            msg["slot"] = slot
+        r = self._send(msg)
         if not r.get("ok"):
             raise HarnessError(f"view failed: {r.get('error')}")
         return r["view"]
 
-    def act(self, action: dict[str, Any]) -> dict[str, Any]:
-        r = self._send({"type": "act", "action": action})
+    def act(
+        self, action: dict[str, Any], slot: int | None = None
+    ) -> dict[str, Any]:
+        msg: dict[str, Any] = {"type": "act", "action": action}
+        if slot is not None:
+            msg["slot"] = slot
+        r = self._send(msg)
         if not r.get("ok"):
             err = r.get("error", {})
             raise HarnessError(
-                f"act({action.get('type')!r}) failed: "
+                f"act({action.get('type')!r}, slot={slot}) failed: "
                 f"{err.get('code')}: {err.get('message')}"
             )
         return r
 
-    def end_turn(self) -> dict[str, Any]:
-        return self.act({"type": "end_turn"})
+    def end_turn(self, slot: int | None = None) -> dict[str, Any]:
+        return self.act({"type": "end_turn"}, slot=slot)
 
     def drain_notifications(self) -> list[dict[str, Any]]:
         """Pop and return all async notifications that arrived since the