fix(auto_router): handle Anthropic-style list content blocks in routing

Previously, AutoRouter assumed message content was always a plain string,
which caused it to pass a raw list representation to the semantic router
when Anthropic-style content blocks were used (e.g. multimodal messages
with [{"type": "text", "text": "..."}]).

This change detects when content is a list, extracts the "text" value
from each dict block, and joins them into a single string before routing.
Non-dict entries in the list are silently skipped.

Also adds a unit test covering the Anthropic content block path, including
mixed input (multiple text blocks, a block without a "text" key, and a
bare string entry).

litellm/router_strategy/auto_router/auto_router.py

@@ -119,11 +119,14 @@ class AutoRouter(CustomLogger):
                 auto_sync=self.auto_sync_value,
             )
 
-        user_message: Dict[str, str] = messages[-1]
-        message_content: str = user_message.get("content", "")
-        route_choice: Optional[Union[RouteChoice, List[RouteChoice]]] = self.routelayer(
-            text=message_content
-        )
+        user_message: Dict[str, Any] = messages[-1]
+        raw_content = user_message.get("content", "")
+        # Handle Anthropic-style content blocks: [{"type": "text", "text": "..."}]
+        if isinstance(raw_content, list):
+            message_content = " ".join(block.get("text", "") for block in raw_content if isinstance(block, dict))
+        else:
+            message_content = str(raw_content)
+        route_choice: Optional[Union[RouteChoice, List[RouteChoice]]] = self.routelayer(text=message_content)
         verbose_router_logger.debug(f"route_choice: {route_choice}")
         if isinstance(route_choice, RouteChoice):
             model = route_choice.name or self.default_model

tests/test_litellm/router_strategy/test_auto_router.py

@@ -181,3 +181,57 @@ class TestAutoRouter:
         # Assert
         assert result is None
 
+    @pytest.mark.asyncio
+    @patch("semantic_router.routers.SemanticRouter")
+    @patch("litellm.router_strategy.auto_router.litellm_encoder.LiteLLMRouterEncoder")
+    async def test_async_pre_routing_hook_with_anthropic_content_blocks(
+        self,
+        mock_encoder_class,
+        mock_semantic_router_class,
+        mock_router_instance,
+        mock_route_choice,
+    ):
+        """Test that Anthropic-style list content blocks are joined into a plain string (line 127).
+
+        When a message's content is a list of typed dicts (e.g. Anthropic format),
+        only dict blocks are considered and their "text" values are joined with a space.
+        Non-dict entries in the list must be skipped.
+        """
+        # Arrange
+        mock_loaded_router = MagicMock()
+        mock_loaded_router.routes = ["route1", "route2"]
+        mock_semantic_router_class.from_json.return_value = mock_loaded_router
+
+        mock_routelayer = MagicMock()
+        mock_routelayer.return_value = mock_route_choice
+        mock_semantic_router_class.return_value = mock_routelayer
+
+        auto_router = AutoRouter(
+            model_name="test-auto-router",
+            auto_router_config_path="test/path/router.json",
+            default_model="gpt-4o-mini",
+            embedding_model="text-embedding-model",
+            litellm_router_instance=mock_router_instance,
+        )
+
+        # Anthropic-style content blocks: list of dicts with a "type" and "text" key.
+        # Also includes a non-dict entry to verify it is skipped by isinstance(block, dict).
+        anthropic_content = [
+            {"type": "text", "text": "What is the weather today?"},
+            {"type": "text", "text": "In Paris."},
+            {"type": "image_url", "image_url": {"url": "http://example.com/img.png"}},  # no "text" key
+            "unexpected string block",  # non-dict — must be ignored
+        ]
+        messages = [{"role": "user", "content": anthropic_content}]
+
+        # Act
+        result = await auto_router.async_pre_routing_hook(
+            model="test-model",
+            request_kwargs={},
+            messages=messages,
+        )
+        
+        # Assert: only dict blocks contribute; "text" defaults to "" when missing
+        mock_routelayer.assert_called_once_with(text="What is the weather today? In Paris. ")
+        assert result is not None
+        assert result.messages == messages