fix: complexity_router crashes on list-format message content (OpenAI multi-part messages) (#22761)

* fix: complexity_router fails on list-format message content (OpenAI multi-part messages)

When a client sends messages with list-format content
(e.g. [{"type": "text", "text": "..."}] as used by the OpenAI JS SDK
and other clients), the complexity_router's async_pre_routing_hook
skipped those messages because it only handled str content. This caused
user_message to be None, the hook returned None, and the router fell
through to selecting the complexity_router deployment itself
(model="auto_router/complexity_router") which litellm cannot dispatch,
resulting in LiteLLMUnknownProvider.

Fixes:
- Extract text from list-format content parts (type=text) before
  classifying
- Return default_model instead of None when no user message can be
  extracted, preventing the crash fallthrough
- Loosen PreRoutingHookResponse.messages type from Dict[str, str] to
  Dict[str, Any] to accommodate list-format content values

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix: update messages type annotation in async_pre_routing_hook to Dict[str, Any]

Consistent with PreRoutingHookResponse.messages type change and the
list-format content support added in the previous commit.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix: normalize None content to empty string in complexity_router message parsing

msg.get("content", "") returns None when the key exists with value None
(e.g. assistant messages with tool calls). Use `or ""` to normalize
None to an empty string explicitly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix: strip whitespace from joined list content parts in complexity_router

Prevents leading/trailing spaces when some content parts have empty
text values (e.g. " ".join(["", "hello"]) → " hello").

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

litellm/router_strategy/complexity_router/complexity_router.py

@@ -326,7 +326,7 @@ class ComplexityRouter(CustomLogger):
         self,
         model: str,
         request_kwargs: Dict,
-        messages: Optional[List[Dict[str, str]]] = None,
+        messages: Optional[List[Dict[str, Any]]] = None,
         input: Optional[Union[str, List]] = None,
         specific_deployment: Optional[bool] = False,
     ) -> Optional["PreRoutingHookResponse"]:
@@ -359,7 +359,15 @@ class ComplexityRouter(CustomLogger):
         
         for msg in reversed(messages):
             role = msg.get("role", "")
-            content = msg.get("content", "")
+            content = msg.get("content") or ""
+            # content may be a list of content parts (e.g. [{"type": "text", "text": "..."}])
+            if isinstance(content, list):
+                text_parts = [
+                    part.get("text", "")
+                    for part in content
+                    if isinstance(part, dict) and part.get("type") == "text"
+                ]
+                content = " ".join(text_parts).strip()
             if isinstance(content, str) and content:
                 if role == "user" and user_message is None:
                     user_message = content
@@ -368,9 +376,12 @@ class ComplexityRouter(CustomLogger):
         
         if user_message is None:
             verbose_router_logger.debug(
-                "ComplexityRouter: No user message found, skipping routing"
+                "ComplexityRouter: No user message found, routing to default model"
+            )
+            return PreRoutingHookResponse(
+                model=self.config.default_model or self.get_model_for_tier(ComplexityTier.MEDIUM),
+                messages=messages,
             )
-            return None
         
         # Classify the request
         tier, score, signals = self.classify(user_message, system_prompt)

litellm/types/router.py

@@ -866,4 +866,4 @@ class PreRoutingHookResponse(BaseModel):
     """
 
     model: str
-    messages: Optional[List[Dict[str, str]]]
+    messages: Optional[List[Dict[str, Any]]]

tests/test_litellm/router_strategy/test_complexity_router.py

@@ -483,7 +483,7 @@ class TestAsyncPreRoutingHookEdgeCases:
 
     @pytest.mark.asyncio
     async def test_pre_routing_hook_no_user_message(self, complexity_router):
-        """Test pre-routing hook returns None when no user message found."""
+        """Test pre-routing hook falls back to default model when no user message found."""
         messages = [
             {"role": "system", "content": "You are helpful."},
             {"role": "assistant", "content": "Hello!"},
@@ -493,21 +493,45 @@ class TestAsyncPreRoutingHookEdgeCases:
             request_kwargs={},
             messages=messages,
         )
-        assert result is None
+        # Should return default model rather than None (None would cause
+        # the complexity_router deployment itself to be selected, crashing)
+        assert result is not None
+        assert result.model in ["gpt-4o-mini", "gpt-4o", "claude-sonnet-4-20250514", "o1-preview"]
 
     @pytest.mark.asyncio
-    async def test_pre_routing_hook_only_list_content(self, complexity_router):
-        """Test pre-routing hook returns None when all user content is list type."""
+    async def test_pre_routing_hook_list_content(self, complexity_router):
+        """Test pre-routing hook handles list-format message content (OpenAI multi-part format)."""
         messages = [
-            {"role": "user", "content": [{"type": "text", "text": "Hello"}]},
+            {"role": "user", "content": [{"type": "text", "text": "Hello, how are you?"}]},
         ]
         result = await complexity_router.async_pre_routing_hook(
             model="test-model",
             request_kwargs={},
             messages=messages,
         )
-        # Should return None since we can't extract string content
-        assert result is None
+        # Should extract text from list content and classify normally
+        assert result is not None
+        assert result.model == "gpt-4o-mini"  # "Hello, how are you?" is SIMPLE
+
+    @pytest.mark.asyncio
+    async def test_pre_routing_hook_list_content_complex(self, complexity_router):
+        """Test pre-routing hook classifies list-format content by complexity."""
+        messages = [
+            {
+                "role": "user",
+                "content": [
+                    {"type": "text", "text": "Think step by step and reason through this: design a distributed system"},
+                    {"type": "image_url", "image_url": {"url": "data:image/png;base64,abc"}},
+                ],
+            }
+        ]
+        result = await complexity_router.async_pre_routing_hook(
+            model="test-model",
+            request_kwargs={},
+            messages=messages,
+        )
+        assert result is not None
+        assert result.model == "o1-preview"  # REASONING tier
 
     @pytest.mark.asyncio
     async def test_pre_routing_hook_preserves_messages(self, complexity_router):
@@ -526,7 +550,7 @@ class TestAsyncPreRoutingHookEdgeCases:
 
     @pytest.mark.asyncio
     async def test_pre_routing_hook_empty_string_content(self, complexity_router):
-        """Test pre-routing hook returns None for empty string content."""
+        """Test pre-routing hook falls back to default model for empty string content."""
         messages = [
             {"role": "user", "content": ""},
         ]
@@ -535,8 +559,9 @@ class TestAsyncPreRoutingHookEdgeCases:
             request_kwargs={},
             messages=messages,
         )
-        # Empty string content is treated as "no user message found"
-        assert result is None
+        # Empty string content → no extractable user message → routes to default model
+        assert result is not None
+        assert result.model in ["gpt-4o-mini", "gpt-4o", "claude-sonnet-4-20250514", "o1-preview"]
 
 
 class TestSingletonMutation: