fix(router): track routed model in fallback attempts

litellm/router_utils/fallback_event_handlers.py

@@ -470,10 +470,11 @@ async def run_async_fallback(
     attempted: Final = (
         carried_targets if isinstance(carried_targets, AttemptedFallbackTargets) else AttemptedFallbackTargets()
     )
-    attempted.record(original_model_group)
+    failed_model_group: Final = get_pre_routing_selection(kwargs) or original_model_group
+    attempted.record(failed_model_group)
 
     for mg in fallback_model_group:
-        if mg == original_model_group:
+        if mg == failed_model_group:
             continue
         if same_model_group_only and _get_fallback_target_model_group(mg) != original_model_group:
             verbose_router_logger.info(

tests/test_litellm/router_utils/test_fallback_event_handlers.py

@@ -614,6 +614,27 @@ async def test_run_async_fallback_forwards_attempted_model_groups_to_nested_call
     )
 
 
+@pytest.mark.asyncio
+async def test_run_async_fallback_can_target_the_requested_group_when_a_pre_router_replaced_it():
+    """The requested group was never called when a pre-router selected a tier, so a
+    tier fallback may legitimately target that originally requested group."""
+    router = RecordingRouter()
+
+    await run_async_fallback(
+        litellm_router=router,
+        fallback_model_group=["requested-model"],
+        original_model_group="requested-model",
+        original_exception=RuntimeError("selected tier failed"),
+        max_fallbacks=3,
+        fallback_depth=0,
+        model="requested-model",
+        metadata={"pre_routing_selected_model": "selected-tier"},
+    )
+
+    assert router.received_kwargs["model"] == "requested-model"
+    assert router.received_kwargs["attempted_targets"].keys == frozenset({"selected-tier", "requested-model"})
+
+
 @pytest.mark.asyncio
 @pytest.mark.parametrize(
     "entry",

tests/test_litellm/test_router.py

@@ -8335,20 +8335,26 @@ class TestClaudeCodeSubagentSessionRouterBinding:
             )
 
     @classmethod
-    def _router(cls) -> "litellm.Router":
+    def _router(
+        cls,
+        cheap_response: str = "cheap response",
+        fallbacks: list[dict[str, list[str]]] | None = None,
+    ) -> "litellm.Router":
         from litellm.types.router import TaggedPreRoutingStrategy
 
         router = litellm.Router(
             model_list=[
                 {
                     "model_name": "cheap-model",
-                    "litellm_params": {"model": "openai/gpt-4o-mini", "mock_response": "cheap response"},
+                    "litellm_params": {"model": "openai/gpt-4o-mini", "mock_response": cheap_response},
                 },
                 {
                     "model_name": "expensive-model",
                     "litellm_params": {"model": "openai/gpt-4o", "mock_response": "expensive response"},
                 },
-            ]
+            ],
+            fallbacks=fallbacks,
+            num_retries=0,
         )
         router.complexity_routers = {
             "smart-router": [TaggedPreRoutingStrategy(tags=(), strategy=cls._RewriteStrategy())]
@@ -8477,6 +8483,29 @@ class TestClaudeCodeSubagentSessionRouterBinding:
 
         assert response is None
 
+    @pytest.mark.asyncio
+    async def test_subagent_can_fallback_to_its_original_requested_model(self):
+        router = self._router(
+            cheap_response="litellm.RateLimitError",
+            fallbacks=[{"cheap-model": ["expensive-model"]}],
+        )
+
+        await router.acompletion(
+            model="smart-router",
+            messages=[{"role": "user", "content": "main turn"}],
+            **self._request_kwargs(),
+        )
+        subagent_kwargs = self._request_kwargs(agent_id="agent-1234")
+
+        response = await router.acompletion(
+            model="expensive-model",
+            messages=[{"role": "user", "content": "subagent turn"}],
+            **subagent_kwargs,
+        )
+
+        assert response.choices[0].message.content == "expensive response"
+        assert subagent_kwargs["metadata"]["routing_decision"]["routed_model"] == "cheap-model"
+
     @pytest.mark.asyncio
     async def test_session_router_binding_is_scoped_to_the_authenticated_key(self):
         router = self._router()