Treat typing.Dict[str, X] return annotations like dict[str, X] (#3756)

Author: Kludex

docs/servers/structured-output.md

@@ -147,7 +147,7 @@ Ask for a two-day forecast and `structured_content` is `{"result": [{...}, {...}
 
 ## Dictionaries
 
-`dict[str, ...]` is the one generic that already *is* a JSON object, so it isn't wrapped:
+`dict[str, ...]` (or `typing.Dict[str, ...]`) is the one generic that already *is* a JSON object, so it isn't wrapped:
 
 ```python title="server.py" hl_lines="9"
 --8<-- "docs_src/structured_output/tutorial006.py"

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

@@ -4,7 +4,6 @@
 import sys
 from collections.abc import Awaitable, Callable, Sequence
 from itertools import chain
-from types import GenericAlias
 from typing import Annotated, Any, NotRequired, Union, cast, get_args, get_origin
 
 import anyio
@@ -512,10 +511,8 @@ def _create_output_model(original_annotation: Any, type_expr: Any, func_name: st
         model = _create_wrapped_model(func_name, original_annotation)
         wrap_output = True
 
-    # Handle GenericAlias types (list[str], dict[str, int], Union[str, int], etc.)
-    elif isinstance(type_expr, GenericAlias):
-        origin = get_origin(type_expr)
-
+    # Handle parameterized generics (list[str], typing.Dict[str, int], Union[str, int], etc.)
+    elif (origin := get_origin(type_expr)) is not None:
         if origin is dict:
             args = get_args(type_expr)
             if len(args) == 2 and args[0] is str:
@@ -558,8 +555,6 @@ def _create_output_model(original_annotation: Any, type_expr: Any, func_name: st
 
     # Handle any other types not covered above
     else:
-        # This includes typing constructs that aren't GenericAlias in Python 3.10
-        # (e.g., Union, Optional in some Python versions)
         model = _create_wrapped_model(func_name, original_annotation)
         wrap_output = True
 

tests/server/mcpserver/test_func_metadata.py

@@ -3,6 +3,7 @@
 # pyright: reportMissingParameterType=false
 # pyright: reportUnknownArgumentType=false
 # pyright: reportUnknownLambdaType=false
+import typing
 from collections.abc import Callable
 from dataclasses import dataclass
 from typing import TYPE_CHECKING, Annotated, Any, Final, NamedTuple, NotRequired, Required, TypedDict
@@ -293,6 +294,18 @@ def func_dict_list() -> dict[str, list[int]]:  # pragma: no cover
         "additionalProperties": {"type": "array", "items": {"type": "integer"}},
     }
 
+    def func_typing_dict() -> typing.Dict[str, str]:  # noqa: UP006
+        return {"name": "John"}
+
+    meta = func_metadata(func_typing_dict)
+    assert meta.output_schema == {
+        "type": "object",
+        "additionalProperties": {"type": "string"},
+    }
+    result = meta.convert_result(func_typing_dict())
+    assert isinstance(result, CallToolResult)
+    assert result.structured_content == {"name": "John"}
+
     # Test dict[int, str] - should be wrapped since key is not str
     def func_dict_int_key() -> dict[int, str]:  # pragma: no cover
         return {1: "a", 2: "b"}
@@ -762,6 +775,21 @@ def func_optional() -> str | None:  # pragma: no cover
         "required": ["result"],
     }
 
+    UserId = typing.NewType("UserId", int)
+
+    def func_new_type() -> UserId:
+        return UserId(1)
+
+    meta = func_metadata(func_new_type)
+    assert meta.output_schema == {
+        "type": "object",
+        "properties": {"result": {"type": "integer"}},
+        "required": ["result"],
+    }
+    result = meta.convert_result(func_new_type())
+    assert isinstance(result, CallToolResult)
+    assert result.structured_content == {"result": 1}
+
 
 def test_structured_output_dataclass():
     """Test structured output with dataclass return types"""