feat(webui): show the actual fallback model (#5017)

tests/agent/test_runner_fallback.py

@@ -285,6 +285,30 @@ class TestFallbackOnPrimaryError:
         assert primary.chat_calls[0]["model"] == "primary-model"
         assert fallback.chat_calls[0]["model"] == "fallback-a"
 
+    @pytest.mark.asyncio
+    async def test_reports_the_fallback_model_before_its_request(self) -> None:
+        primary = _FakeProvider("primary", _error_response())
+        fallback = _FakeProvider("fallback", _make_response("fallback ok"))
+        fallback_models: list[str] = []
+
+        async def _observe(model: str) -> None:
+            fallback_models.append(model)
+
+        fb = FallbackProvider(
+            primary=primary,
+            fallback_presets=[_fallback("fallback-a", provider="backup")],
+            provider_factory=MagicMock(return_value=fallback),
+            fallback_model_observer=_observe,
+        )
+
+        result = await fb.chat_with_retry(
+            messages=[{"role": "user", "content": "hi"}],
+            model="primary-model",
+        )
+
+        assert result.content == "fallback ok"
+        assert fallback_models == ["fallback-a"]
+
     @pytest.mark.asyncio
     async def test_logs_primary_error_before_fallback(self) -> None:
         primary = _FakeProvider("primary", _error_response("primary overloaded"))

tests/utils/test_webui_turn_helpers.py

@@ -4,8 +4,9 @@ from unittest.mock import AsyncMock, MagicMock
 
 import pytest
 
+from nanobot.agent.tools.context import RequestContext, request_context
 from nanobot.bus.events import InboundMessage
-from nanobot.bus.outbound_events import GoalStatusEvent
+from nanobot.bus.outbound_events import GoalStatusEvent, TurnModelUpdatedEvent
 from nanobot.session import webui_turns as wth
 
 
@@ -69,3 +70,38 @@ async def test_publish_turn_run_status_non_websocket_noop_registry() -> None:
     await wth.publish_turn_run_status(bus, msg, "running")
 
     assert wth._WEBSOCKET_TURN_WALL_STARTED_AT == {}
+
+
+@pytest.mark.asyncio
+async def test_fallback_model_is_scoped_to_its_websocket_chat() -> None:
+    bus = MagicMock()
+    bus.publish_outbound = AsyncMock()
+    observer = wth.build_webui_fallback_model_observer(bus)
+
+    with request_context(
+        RequestContext(
+            channel="websocket",
+            chat_id="chat-model",
+            metadata={"webui": True},
+        )
+    ):
+        await observer("deepseek/deepseek-chat")
+
+    outbound = bus.publish_outbound.await_args.args[0]
+    assert outbound.channel == "websocket"
+    assert outbound.chat_id == "chat-model"
+    assert outbound.metadata == {"webui": True}
+    assert isinstance(outbound.event, TurnModelUpdatedEvent)
+    assert outbound.event.model == "deepseek/deepseek-chat"
+
+
+@pytest.mark.asyncio
+async def test_fallback_model_ignores_non_websocket_requests() -> None:
+    bus = MagicMock()
+    bus.publish_outbound = AsyncMock()
+    observer = wth.build_webui_fallback_model_observer(bus)
+
+    with request_context(RequestContext(channel="telegram", chat_id="chat-model")):
+        await observer("fallback")
+
+    bus.publish_outbound.assert_not_awaited()