diff --git a/litellm/router_strategy/complexity_router/complexity_router.py b/litellm/router_strategy/complexity_router/complexity_router.py
index 2a1bcad68e7..736bb6cc59c 100644
--- a/litellm/router_strategy/complexity_router/complexity_router.py
+++ b/litellm/router_strategy/complexity_router/complexity_router.py
@@ -3453,11 +3453,17 @@ class ComplexityRouter(CustomLogger):
             return default_model, self._tier_for_model(default_model)
         return None
 
+    @staticmethod
+    def _deployment_model_id(deployment: Mapping[str, object]) -> str | None:
+        model_info: Final = deployment.get("model_info")
+        deployment_id: Final = cast(  # cast-ok: the isinstance check in the condition narrows model_info to a Mapping with str keys
+            "Mapping[str, object]", model_info
+        ).get("id") if isinstance(model_info, Mapping) else None
+        return None if deployment_id is None else str(deployment_id)
+
     def _group_has_deployment_id(self, group: str, model_id: str) -> bool:
         for deployment in self._group_deployments(group):
-            model_info: Final = deployment.get("model_info")
-            deployment_id: Final = model_info.get("id") if isinstance(model_info, Mapping) else None
-            if deployment_id is not None and str(deployment_id) == model_id:
+            if self._deployment_model_id(deployment) == model_id:
                 return True
         return False
 
@@ -4419,7 +4425,12 @@ class ComplexityRouter(CustomLogger):
         marker_pairs: Final = self._reminder_markers_for_request(request_kwargs)
         conversation_continuing: Final = (
             _conversation_is_continuing(resolved_messages)
-            or _previous_response_id_from_request_kwargs(request_kwargs) is not None
+            or _previous_response_id_from_request_kwargs(
+                cast(  # cast-ok: the hook's bare kwargs dict is the str-keyed request mapping
+                    Mapping[str, object], request_kwargs
+                )
+            )
+            is not None
         )
 
         use_session_affinity: Final = self._uses_tier_pin
@@ -4732,7 +4743,11 @@ class ComplexityRouter(CustomLogger):
         classifier_images: Final = self._classifier_image_parts(resolved_messages)
 
         if user_message is None and not classifier_images:
-            previous_response: Final = self._previous_response_model(request_kwargs)
+            previous_response: Final = self._previous_response_model(
+                cast(  # cast-ok: the hook's bare kwargs dict is the str-keyed request mapping
+                    Mapping[str, object], request_kwargs
+                )
+            )
             if previous_response is not None:
                 reused_model, reused_tier = previous_response
                 reused_params: Final = self._litellm_params_for_model(reused_tier, reused_model)
diff --git a/tests/unit/router_strategy/test_complexity_router.py b/tests/unit/router_strategy/test_complexity_router.py
index 11ac3148557..a5c39e616b5 100644
--- a/tests/unit/router_strategy/test_complexity_router.py
+++ b/tests/unit/router_strategy/test_complexity_router.py
@@ -17404,6 +17404,28 @@ class TestPreviousResponseReuse:
         assert decision["tier"] == "COMPLEX", decision
         assert decision["conversation_continuing"] is True, decision
 
+    @pytest.mark.asyncio
+    async def test_tool_continuation_reuses_default_model_when_only_it_served_the_previous_response(
+        self, mock_router_instance, basic_config
+    ):
+        router = self._router(
+            mock_router_instance,
+            basic_config,
+            tiers={"SIMPLE": "gpt-4o-mini"},
+            default_model="gpt-4o",
+        )
+        result = await router.async_pre_routing_hook(
+            model="test-complexity-router",
+            request_kwargs={"previous_response_id": self._encoded_response_id("deployment-4o")},
+            messages=[{"role": "tool", "content": "search results", "tool_call_id": "call_1"}],
+        )
+        assert result is not None
+        assert result.model == "gpt-4o", result.routing_decision
+        decision = result.routing_decision
+        assert decision is not None
+        assert decision["cause"] == "previous_response_reuse", decision
+        assert decision["conversation_continuing"] is True, decision
+
     @pytest.mark.asyncio
     async def test_unencoded_previous_response_id_still_falls_back_to_default(
         self, mock_router_instance, basic_config