fix: wrap structured union outputs explicitly

Author: he-yufeng

src/mcp/server/mcpserver/utilities/func_metadata.py

@@ -478,7 +478,7 @@ def _create_wrapped_model(func_name: str, annotation: Any) -> type[BaseModel]:
     """
     model_name = f"{func_name}Output"
 
-    return create_model(model_name, result=annotation)
+    return create_model(model_name, result=(annotation, ...))
 
 
 def _create_dict_model(func_name: str, dict_annotation: Any) -> type[BaseModel]:

tests/server/mcpserver/test_func_metadata.py

@@ -680,6 +680,9 @@ def func_union() -> str | int:  # pragma: no cover
     def func_optional() -> str | None:  # pragma: no cover
         return None
 
+    def func_builtin_union() -> dict | list | str:  # pragma: no cover
+        return {"hello": "world"}
+
     # Test list
     meta = func_metadata(func_list_str)
     assert meta.output_schema == {
@@ -715,6 +718,23 @@ def func_optional() -> str | None:  # pragma: no cover
         "title": "func_optionalOutput",
     }
 
+    meta = func_metadata(func_builtin_union)
+    assert meta.output_schema == {
+        "type": "object",
+        "properties": {
+            "result": {
+                "title": "Result",
+                "anyOf": [
+                    {"type": "object", "additionalProperties": True},
+                    {"type": "array", "items": {}},
+                    {"type": "string"},
+                ],
+            }
+        },
+        "required": ["result"],
+        "title": "func_builtin_unionOutput",
+    }
+
 
 def test_structured_output_dataclass():
     """Test structured output with dataclass return types"""