diff --git a/api/routes.py b/api/routes.py
index ee14690c..235e857d 100644
--- a/api/routes.py
+++ b/api/routes.py
@@ -19367,11 +19367,15 @@ def _handle_chat_start(handler, body, diag=None):
             if "model_provider" in body
             else getattr(s, "model_provider", None)
         )
+<<<<<<< ours
         _pp_provider, _pp_default, _pp_cfg = _read_profile_model_config(s, requested_provider)
+=======
+>>>>>>> theirs
         explicit_model_pick = bool(body.get("explicit_model_pick"))
         moa_config = None
+        gateway_chat_enabled = webui_gateway_chat_enabled(get_config())
         if body.get("moa_config"):
-            if webui_gateway_chat_enabled(get_config()):
+            if gateway_chat_enabled:
                 return bad(handler, "MoA override is unavailable on gateway-backed sessions", 409)
             from api.commands import resolve_moa_config
 
@@ -19379,6 +19383,7 @@ def _handle_chat_start(handler, body, diag=None):
                 moa_config = resolve_moa_config()
             except RuntimeError as e:
                 return bad(handler, str(e), 503)
+        _pp_provider, _pp_default = _read_profile_model_config(s, requested_provider)
         diag.stage("resolve_model_provider") if diag else None
         model, model_provider, normalized_model = _resolve_compatible_session_model_state(
             requested_model,
@@ -19401,18 +19406,22 @@ def _handle_chat_start(handler, body, diag=None):
         # with start_session_turn so both entry points behave identically
         # under runtime_adapter_enabled() / runtime_adapter_runner_enabled()
         # — Q-2979-A2 / Copilot discussion_r3305864087/r3305864173).
+        start_run_kwargs = {
+            "msg": msg,
+            "attachments": attachments,
+            "workspace": workspace,
+            "model": model,
+            "model_provider": model_provider,
+            "normalized_model": normalized_model,
+            "source": "webui",
+            "route": "/api/chat/start",
+            "diag": diag,
+        }
+        if not gateway_chat_enabled and moa_config is not None:
+            start_run_kwargs["moa_config"] = moa_config
         response = _start_run(
             s,
-            msg=msg,
-            attachments=attachments,
-            workspace=workspace,
-            model=model,
-            model_provider=model_provider,
-            normalized_model=normalized_model,
-            source="webui",
-            route="/api/chat/start",
-            diag=diag,
-            moa_config=moa_config,
+            **start_run_kwargs,
         )
         # Map adapter-selection NotImplementedError (501) onto the legacy
         # bad-request response shape that this route exposed historically
diff --git a/tests/test_issue5057_moa_webui_route.py b/tests/test_issue5057_moa_webui_route.py
index d516de8d..391852bb 100644
--- a/tests/test_issue5057_moa_webui_route.py
+++ b/tests/test_issue5057_moa_webui_route.py
@@ -127,3 +127,64 @@ def test_moa_config_is_per_turn_not_persisted():
     js_source = js_path.read_text(encoding="utf-8")
     assert "moa_config:_pendingMoaConfig?true:undefined" in js_source
     assert "_pendingMoaConfig=null" in js_source
+
+
+def test_moa_gateway_chat_start_fails_closed(monkeypatch, tmp_path):
+    """Gateway-backed WebUI sessions must reject /moa until the gateway consumes runtime overrides."""
+    import api.commands as commands
+    import api.routes as routes
+
+    class _Handler:
+        def __init__(self):
+            import io
+
+            self.status = None
+            self.response_headers = []
+            self.wfile = io.BytesIO()
+
+        def send_response(self, status):
+            self.status = status
+
+        def send_header(self, key, value):
+            self.response_headers.append((key, value))
+
+        def end_headers(self):
+            self.response_headers.append(("__end__", ""))
+
+    class _Session:
+        session_id = "sess-moa-gateway"
+        workspace = str(tmp_path)
+        model = "gpt-5.5"
+        model_provider = "openai-codex"
+        profile = "default"
+        messages = []
+        context_messages = []
+        pending_user_message = None
+
+    def start_run(*_args, **_kwargs):  # pragma: no cover - should fail before run start
+        raise AssertionError("gateway-backed /moa must fail closed before starting a run")
+
+    def resolve_moa_config():  # pragma: no cover - should fail before resolving MoA
+        raise AssertionError("gateway-backed /moa must fail before resolving MoA config")
+
+    monkeypatch.setattr(routes, "get_session", lambda _sid: _Session())
+    monkeypatch.setattr(routes, "_resolve_chat_workspace_with_recovery", lambda _s, _w: str(tmp_path))
+    monkeypatch.setattr(routes, "_start_run", start_run)
+    monkeypatch.setattr(routes, "get_config", lambda: {"chat_backend": "gateway"})
+    monkeypatch.setattr(routes, "webui_gateway_chat_enabled", lambda _cfg: True)
+    monkeypatch.setattr(commands, "resolve_moa_config", resolve_moa_config)
+
+    handler = _Handler()
+    routes._handle_chat_start(
+        handler,
+        {
+            "session_id": "sess-moa-gateway",
+            "message": "diagnose issue",
+            "workspace": str(tmp_path),
+            "moa_config": True,
+        },
+    )
+
+    body = json.loads(handler.wfile.getvalue().decode("utf-8"))
+    assert handler.status == 409
+    assert body["error"] == "MoA override is unavailable on gateway-backed sessions"