fix: wrap pipe union structured outputs

Signed-off-by: Yufeng He <40085740+he-yufeng@users.noreply.github.com>

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

@@ -677,6 +677,9 @@ def func_dict_str_int() -> dict[str, int]:  # pragma: no cover
     def func_union() -> str | int:  # pragma: no cover
         return "hello"
 
+    def func_pipe_union_containers() -> dict | list | str:  # pragma: no cover
+        return {"a": 1}
+
     def func_optional() -> str | None:  # pragma: no cover
         return None
 
@@ -706,6 +709,24 @@ def func_optional() -> str | None:  # pragma: no cover
         "title": "func_unionOutput",
     }
 
+    # Test PEP 604 union with containers
+    meta = func_metadata(func_pipe_union_containers)
+    assert meta.output_schema == {
+        "type": "object",
+        "properties": {
+            "result": {
+                "title": "Result",
+                "anyOf": [
+                    {"additionalProperties": True, "type": "object"},
+                    {"items": {}, "type": "array"},
+                    {"type": "string"},
+                ],
+            }
+        },
+        "required": ["result"],
+        "title": "func_pipe_union_containersOutput",
+    }
+
     # Test Optional
     meta = func_metadata(func_optional)
     assert meta.output_schema == {