fix(responses): filter out non-litellm kwargs in Responses API completion bridge

When the Responses API bridge forwards requests to litellm.completion(),
it merges all **kwargs into the completion args. Client-specific params
like `client_metadata` (sent by the OpenAI SDK) are not valid
litellm.completion() arguments and cause 'unexpected keyword argument'
errors at the provider SDK level.

Filter kwargs to only pass litellm-internal params (prefixed with
`litellm_`) to the completion/acompletion calls.

Fixes #28539

litellm/responses/litellm_completion_transformation/handler.py

@@ -20,6 +20,18 @@ from litellm.types.llms.openai import (
 from litellm.types.utils import ModelResponse
 
 
+def _filter_kwargs_for_completion(kwargs: dict) -> dict:
+    """
+    Filter kwargs to only include litellm-internal params that are safe to pass
+    to litellm.completion() / litellm.acompletion().
+
+    Responses API requests may contain client-specific params (e.g. client_metadata)
+    that are NOT valid litellm.completion() arguments. Passing them through causes
+    'unexpected keyword argument' errors at the provider SDK level.
+    """
+    return {k: v for k, v in kwargs.items() if k.startswith("litellm_")}
+
+
 class LiteLLMCompletionTransformationHandler:
     def response_api_handler(
         self,
@@ -59,7 +71,7 @@ class LiteLLMCompletionTransformationHandler:
             )
 
         completion_args = {}
-        completion_args.update(kwargs)
+        completion_args.update(_filter_kwargs_for_completion(kwargs))
         completion_args.update(litellm_completion_request)
 
         litellm_completion_response: Union[
@@ -109,7 +121,7 @@ class LiteLLMCompletionTransformationHandler:
             )
 
         acompletion_args = {}
-        acompletion_args.update(kwargs)
+        acompletion_args.update(_filter_kwargs_for_completion(kwargs))
         acompletion_args.update(litellm_completion_request)
 
         litellm_completion_response: Union[

tests/test_litellm/test_responses_api_client_metadata.py

@@ -0,0 +1,63 @@
+"""
+Unit tests for fix #28539: client_metadata should NOT be forwarded from
+Responses API bridge to litellm.completion()/acompletion().
+
+The Responses API handler merges **kwargs into the completion args dict.
+When a client sends client_metadata (an OpenAI Responses API param), it flows
+through kwargs and causes 'unexpected keyword argument' errors at the provider
+SDK level. The fix filters kwargs to only pass litellm-internal params
+(prefixed with 'litellm_').
+"""
+
+import pytest
+
+from litellm.responses.litellm_completion_transformation.handler import (
+    _filter_kwargs_for_completion,
+)
+
+
+class TestFilterKwargsForCompletion:
+    """Verify _filter_kwargs_for_completion strips non-litellm kwargs."""
+
+    def test_strips_client_metadata(self):
+        """client_metadata from the Responses API must not pass through."""
+        kwargs = {
+            "litellm_logging_obj": "mock_obj",
+            "litellm_metadata": {"key": "value"},
+            "client_metadata": {"session": "abc123"},
+        }
+        filtered = _filter_kwargs_for_completion(kwargs)
+        assert "client_metadata" not in filtered
+        assert filtered["litellm_logging_obj"] == "mock_obj"
+        assert filtered["litellm_metadata"] == {"key": "value"}
+
+    def test_preserves_all_litellm_prefixed_kwargs(self):
+        """All litellm_* kwargs should be preserved."""
+        kwargs = {
+            "litellm_call_id": "call-123",
+            "litellm_metadata": {},
+            "litellm_logging_obj": None,
+            "litellm_parent_otel_span": "span",
+        }
+        filtered = _filter_kwargs_for_completion(kwargs)
+        assert filtered == kwargs
+
+    def test_strips_all_non_litellm_kwargs(self):
+        """Any non-litellm_* kwarg should be stripped."""
+        kwargs = {
+            "litellm_metadata": {},
+            "client_metadata": {"x": 1},
+            "some_other_param": True,
+            "prompt_id": "p-1",
+        }
+        filtered = _filter_kwargs_for_completion(kwargs)
+        assert filtered == {"litellm_metadata": {}}
+
+    def test_empty_kwargs(self):
+        """Empty kwargs should return empty dict."""
+        assert _filter_kwargs_for_completion({}) == {}
+
+    def test_no_litellm_kwargs(self):
+        """If only non-litellm kwargs, return empty dict."""
+        kwargs = {"client_metadata": {}, "extra_param": "value"}
+        assert _filter_kwargs_for_completion(kwargs) == {}