fix(responses): preserve forced-function tool_choice name in Responses to Chat transform (#29812)

The Responses API forces a specific function with a top-level name
({"type": "function", "name": "X"}), but _transform_tool_choice only handled the
nested Chat Completions shape and fell through to returning "required" for the flat
form, silently dropping the function name and degrading a forced function call to
force-any-tool. Map the flat Responses shape to the nested Chat shape, keeping the
"required" fallback when no name is present.

litellm/responses/litellm_completion_transformation/transformation.py

@@ -148,7 +148,9 @@ class LiteLLMCompletionResponsesConfig:
                 # which is equivalent to "required" in OpenAI format
                 return "required"
             elif tool_choice_type == "function":
-                # function type without name - fall back to required
+                function_name = tool_choice.get("name")
+                if function_name:
+                    return {"type": "function", "function": {"name": function_name}}
                 return "required"
 
         # Return as-is for unknown formats

tests/test_litellm/responses/litellm_completion_transformation/test_litellm_completion_responses.py

@@ -949,6 +949,28 @@ class TestToolChoiceTransformation:
         result = LiteLLMCompletionResponsesConfig._transform_tool_choice(tool_choice)
         assert result == tool_choice
 
+    def test_transform_tool_choice_responses_flat_function_name(self):
+        """Responses-API forced-function with a top-level name maps to the nested Chat
+        Completions shape instead of degrading to required and dropping the name"""
+        result = LiteLLMCompletionResponsesConfig._transform_tool_choice(
+            {"type": "function", "name": "get_weather"}
+        )
+        assert result == {"type": "function", "function": {"name": "get_weather"}}
+
+    def test_transform_tool_choice_function_without_name_falls_back_to_required(self):
+        """A function-type dict with no name still falls back to required"""
+        result = LiteLLMCompletionResponsesConfig._transform_tool_choice(
+            {"type": "function"}
+        )
+        assert result == "required"
+
+    def test_transform_tool_choice_function_empty_name_falls_back_to_required(self):
+        """An empty top-level name is falsy and must not produce an empty function name"""
+        result = LiteLLMCompletionResponsesConfig._transform_tool_choice(
+            {"type": "function", "name": ""}
+        )
+        assert result == "required"
+
 
 class TestContentTypeTransformation:
     """Test content type transformation from Responses API to Chat Completion format"""