fix(responses): merge sync completion kwargs to avoid duplicate keys

Double-splatting litellm_completion_request and kwargs raised TypeError
when metadata or service_tier were set. Match the async merge pattern.

Co-authored-by: Cursor <cursoragent@cursor.com>

litellm/responses/litellm_completion_transformation/handler.py

@@ -58,11 +58,14 @@ class LiteLLMCompletionTransformationHandler:
                 **kwargs,
             )
 
+        completion_args = {}
+        completion_args.update(kwargs)
+        completion_args.update(litellm_completion_request)
+
         litellm_completion_response: Union[
             ModelResponse, litellm.CustomStreamWrapper
         ] = litellm.completion(
-            **litellm_completion_request,
-            **kwargs,
+            **completion_args,
         )
 
         if isinstance(litellm_completion_response, ModelResponse):

tests/llm_responses_api_testing/test_anthropic_responses_api.py

@@ -3,7 +3,7 @@ import sys
 import pytest
 import asyncio
 from typing import Optional
-from unittest.mock import patch, AsyncMock
+from unittest.mock import patch, AsyncMock, MagicMock
 from litellm.responses.litellm_completion_transformation.handler import (
     LiteLLMCompletionTransformationHandler,
 )
@@ -130,6 +130,26 @@ def test_multiturn_tool_calls():
     print("follow_up_response=", follow_up_response)
 
 
+def test_response_api_handler_merges_metadata_and_service_tier_without_error():
+    """Sync path must merge kwargs like async; double-splat raises TypeError."""
+    handler = LiteLLMCompletionTransformationHandler()
+
+    with patch("litellm.completion", new_callable=MagicMock) as mock_completion:
+        mock_completion.return_value = ModelResponse(
+            id="id", created=0, model="test", object="chat.completion", choices=[]
+        )
+        handler.response_api_handler(
+            model="test",
+            input="hi",
+            responses_api_request={},
+            metadata={"trace": "abc"},
+            service_tier="auto",
+        )
+        assert mock_completion.call_count == 1
+        assert mock_completion.call_args.kwargs["metadata"] == {"trace": "abc"}
+        assert mock_completion.call_args.kwargs["service_tier"] == "auto"
+
+
 @pytest.mark.asyncio
 async def test_async_response_api_handler_merges_trace_id_without_error():
     handler = LiteLLMCompletionTransformationHandler()