diff --git a/tooling/rl_self_play/smoke_model_opponent.py b/tooling/rl_self_play/smoke_model_opponent.py
new file mode 100644
index 00000000..6542e5cc
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+++ b/tooling/rl_self_play/smoke_model_opponent.py
@@ -0,0 +1,120 @@
+"""Smoke test for the model-backed (self-play) opponent path.
+
+Unlike `smoke.py` (stdlib-only, MCTS opponent) this necessarily imports
+`sb3_contrib` + `torch` because it loads a frozen MaskablePPO snapshot
+into the opponent slot. It exercises the full integration: a real
+`MagicCivEnv` configured with a `ModelOpponent`, driven by a *random*
+masked policy on the learner slot for a bounded number of steps.
+
+Verifies, against a live harness:
+  * the multi-slot wire is honoured (both slots externally driven, no
+    harness errors),
+  * the frozen opponent takes real turns (the turn counter advances),
+  * opponent-driven events bubble into the env (no silent captures),
+  * the episode can run without wedging and reports a terminal/truncation
+    reason.
+
+Usage:
+    python3 -m tooling.rl_self_play.smoke_model_opponent \
+        --opponent-model tooling/rl_self_play/models/duel-v4-encfix-s7/best_model.zip \
+        --steps 400
+
+Prints a one-line JSON verdict; exit 0 on `passed: true`.
+"""
+from __future__ import annotations
+
+import argparse
+import json
+import sys
+from pathlib import Path
+from typing import Any
+
+import numpy as np
+
+THIS_DIR = Path(__file__).resolve().parent
+PROJECT_ROOT = THIS_DIR.parents[1]
+if __package__ is None:
+    sys.path.insert(0, str(PROJECT_ROOT))
+
+
+def main() -> int:
+    p = argparse.ArgumentParser(description="Model-opponent self-play smoke")
+    p.add_argument("--opponent-model", required=True, type=Path)
+    p.add_argument("--steps", type=int, default=400)
+    p.add_argument("--seed", type=int, default=42)
+    p.add_argument("--max-turns", type=int, default=200)
+    p.add_argument("--opponent-device", default="cpu")
+    args = p.parse_args()
+
+    from tooling.rl_self_play.harness_client import HarnessConfig
+    from tooling.rl_self_play.magic_civ_env import MagicCivEnv
+    from tooling.rl_self_play.opponent import ModelOpponent
+
+    reasons: list[str] = []
+    details: dict[str, Any] = {
+        "steps": 0,
+        "max_turn_seen": 0,
+        "mask_violations": 0,
+        "opp_turns_implied": 0,
+        "terminal_reason": None,
+    }
+
+    if not args.opponent_model.is_file():
+        print(json.dumps({"passed": False,
+                          "reasons": [f"opponent model not found: {args.opponent_model}"],
+                          "details": details}))
+        return 1
+
+    rng = np.random.default_rng(args.seed)
+    opponent = ModelOpponent(
+        model_path=str(args.opponent_model),
+        slots=(1,),
+        device=args.opponent_device,
+        deterministic=False,
+    )
+    cfg = HarnessConfig(seed=args.seed, players=2, player_slot=0, map_size="duel")
+    env = MagicCivEnv(harness_config=cfg, max_turns=args.max_turns, opponent=opponent)
+
+    try:
+        obs, info = env.reset()
+        mask = info["action_mask"]
+        prev_turn = 0
+        for step_idx in range(args.steps):
+            legal = np.flatnonzero(mask)
+            if legal.size == 0:
+                reasons.append(f"empty action mask at step {step_idx}")
+                break
+            action = int(rng.choice(legal))
+            if not mask[action]:
+                details["mask_violations"] += 1
+            obs, reward, terminated, truncated, info = env.step(action)
+            mask = info.get("action_mask", np.zeros_like(mask))
+            turn = int(info.get("turn", 0))
+            if turn > details["max_turn_seen"]:
+                details["max_turn_seen"] = turn
+            # Each turn advance past prev implies the opponent took a turn
+            # (in all-external 2-slot play the processor steps per end_turn).
+            if turn > prev_turn:
+                details["opp_turns_implied"] += 1
+                prev_turn = turn
+            details["steps"] = step_idx + 1
+            if terminated or truncated:
+                details["terminal_reason"] = info.get("reason")
+                break
+    except Exception as e:  # noqa: BLE001 — smoke wants the failure surfaced
+        reasons.append(f"exception: {type(e).__name__}: {e}")
+    finally:
+        env.close()
+
+    if details["max_turn_seen"] < 1:
+        reasons.append("turn counter never advanced — opponent/turn loop stuck")
+    if details["mask_violations"] > 0:
+        reasons.append(f"{details['mask_violations']} mask violations")
+
+    passed = not reasons
+    print(json.dumps({"passed": passed, "reasons": reasons, "details": details}))
+    return 0 if passed else 1
+
+
+if __name__ == "__main__":
+    sys.exit(main())