fix(responses): normalize chat tool_choice for completions→responses bridge

litellm/completion_extras/litellm_responses_transformation/transformation.py

@@ -119,6 +119,20 @@ class LiteLLMResponsesTransformationHandler(CompletionTransformationBridge):
     def __init__(self):
         pass
 
+    def _normalize_tool_choice_for_responses_api(self, tool_choice: Any) -> Any:
+        """Chat tool_choice uses function.name; Responses API expects top-level name."""
+        if not isinstance(tool_choice, dict) or tool_choice.get("type") != "function":
+            return tool_choice
+        if isinstance(tool_choice.get("name"), str) and tool_choice.get("name"):
+            # Return only Responses shape so stray chat ``function`` key is not sent upstream.
+            return {"type": "function", "name": tool_choice["name"]}
+        fn = tool_choice.get("function")
+        if isinstance(fn, dict):
+            fn_name = fn.get("name")
+            if isinstance(fn_name, str) and fn_name:
+                return {"type": "function", "name": fn_name}
+        return tool_choice
+
     def _handle_raw_dict_response_item(
         self, item: Dict[str, Any], index: int
     ) -> Tuple[Optional[Any], int]:
@@ -309,6 +323,10 @@ def _map_optional_params_to_responses_api_request(
                 text_format = self._transform_response_format_to_text_format(value)
                 if text_format:
                     responses_api_request["text"] = text_format  # type: ignore
+            elif key == "tool_choice":
+                responses_api_request["tool_choice"] = (  # type: ignore[assignment]
+                    self._normalize_tool_choice_for_responses_api(value)
+                )
             elif key in ResponsesAPIOptionalRequestParams.__annotations__.keys():
                 responses_api_request[key] = value  # type: ignore
             elif key == "previous_response_id":

tests/test_litellm/completion_extras/litellm_responses_transformation/test_completion_extras_litellm_responses_transformation_transformation.py

@@ -2098,6 +2098,56 @@ def test_map_optional_params_preserves_reasoning_summary():
     assert responses_api_request["reasoning"]["summary"] == "detailed"
 
 
+def test_map_optional_params_tool_choice_chat_nested_to_responses_api():
+    """Chat tool_choice must become Responses ToolChoiceFunction (top-level name)."""
+    from litellm.completion_extras.litellm_responses_transformation.transformation import (
+        LiteLLMResponsesTransformationHandler,
+    )
+    from litellm.types.llms.openai import ResponsesAPIOptionalRequestParams
+
+    handler = LiteLLMResponsesTransformationHandler()
+    responses_api_request = ResponsesAPIOptionalRequestParams()
+    handler._map_optional_params_to_responses_api_request(
+        {
+            "stream": False,
+            "tool_choice": {
+                "type": "function",
+                "function": {"name": "Echo"},
+            },
+        },
+        responses_api_request,
+    )
+    assert responses_api_request["tool_choice"] == {
+        "type": "function",
+        "name": "Echo",
+    }
+
+
+@pytest.mark.parametrize(
+    ("tool_choice", "expected"),
+    [
+        ("auto", "auto"),
+        ("none", "none"),
+        (
+            {"type": "function", "name": "Echo"},
+            {"type": "function", "name": "Echo"},
+        ),
+        (
+            {"type": "function", "name": "foo", "function": {"name": "bar"}},
+            {"type": "function", "name": "foo"},
+        ),
+        ({"type": "required"}, {"type": "required"}),
+    ],
+)
+def test_normalize_tool_choice_for_responses_api(tool_choice, expected):
+    from litellm.completion_extras.litellm_responses_transformation.transformation import (
+        LiteLLMResponsesTransformationHandler,
+    )
+
+    handler = LiteLLMResponsesTransformationHandler()
+    assert handler._normalize_tool_choice_for_responses_api(tool_choice) == expected
+
+
 def test_convert_chat_completion_file_type_to_input_file():
     """
     Test that Chat Completion content with type 'file' is correctly mapped