From cc2e919e6da17539f810e6aca66c0447165d1c9b Mon Sep 17 00:00:00 2001
From: Yufeng He <40085740+he-yufeng@users.noreply.github.com>
Date: Thu, 14 May 2026 22:42:08 +0800
Subject: [PATCH] fix: wrap structured union outputs explicitly

---
 .../mcpserver/utilities/func_metadata.py      |  2 +-
 tests/server/mcpserver/test_func_metadata.py  | 20 +++++++++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/src/mcp/server/mcpserver/utilities/func_metadata.py b/src/mcp/server/mcpserver/utilities/func_metadata.py
index 4a7610637..3ea9bf7ce 100644
--- a/src/mcp/server/mcpserver/utilities/func_metadata.py
+++ b/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]:
diff --git a/tests/server/mcpserver/test_func_metadata.py b/tests/server/mcpserver/test_func_metadata.py
index c57d1ee9f..0743dbf3f 100644
--- a/tests/server/mcpserver/test_func_metadata.py
+++ b/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[str, str] | list[str] | 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": {"type": "string"}},
+                    {"type": "array", "items": {"type": "string"}},
+                    {"type": "string"},
+                ],
+            }
+        },
+        "required": ["result"],
+        "title": "func_builtin_unionOutput",
+    }
+
 
 def test_structured_output_dataclass():
     """Test structured output with dataclass return types"""