typing.Dict[str, X] return annotation is wrapped in {"result": ...}, unlike dict[str, X] #3745

Initial Checks

• I confirm that I'm using the newest release of my line (the latest 2.x, or the latest 1.x if I'm still on v1)
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Release line

2.x (current stable)

Description

-> typing.Dict[str, Any] and -> dict[str, Any] are the same type, but MCPServer gives them different output schemas and different structuredContent:

• -> dict[str, Any]: outputSchema is the object itself ({"type": "object", "additionalProperties": true, ...}) and the result is {"ok": true}.
• -> typing.Dict[str, Any]: the result is wrapped, so outputSchema is {"properties": {"result": {...}}, "required": ["result"], ...} and the result is {"result": {"ok": true}}.

So modernising annotations changes what a tool returns on the wire. Ruff's UP006 (and pyupgrade) rewrite Dict to dict automatically, and that silently changed one of our tools' output shape. A client that had cached the tool's earlier output schema then rejected its results.

Expected: both spellings behave like dict[str, Any] does today (and typing.List[...], typing.Tuple[...] like their builtin forms).

Cause: in _create_output_model (src/mcp/server/mcpserver/utilities/func_metadata.py), the generic branch is elif isinstance(type_expr, GenericAlias):, with GenericAlias from types. typing.Dict[str, Any] is a typing._GenericAlias, not a types.GenericAlias, so it skips the origin is dict case and falls through to the final else, which wraps it.

Possible fix: test the origin instead of the alias class, e.g. elif get_origin(type_expr) is not None:. The other generics that reach the final else today (Union, Optional, Literal, ...) all have an origin other than dict, so they would still be wrapped as now. I'm happy to send a PR with tests if a maintainer assigns this to me.

Related, but different: #3725 is about bare dict/list/tuple without parameters.

Example Code

import asyncio
import typing

from mcp.server.mcpserver import MCPServer

server = MCPServer(name="repro")


@server.tool()
def typing_dict() -> typing.Dict[str, typing.Any]:
    return {"ok": True}


@server.tool()
def builtin_dict() -> dict[str, typing.Any]:
    return {"ok": True}


async def main() -> None:
    tools = {tool.name: tool for tool in await server.list_tools()}
    for name in ("typing_dict", "builtin_dict"):
        result = await server.call_tool(name, {})
        print(name, tools[name].output_schema, result.structured_content)


asyncio.run(main())

Output:

typing_dict {'properties': {'result': {'additionalProperties': True, 'title': 'Result', 'type': 'object'}}, 'required': ['result'], 'title': 'typing_dictOutput', 'type': 'object'} {'result': {'ok': True}}
builtin_dict {'additionalProperties': True, 'title': 'builtin_dictDictOutput', 'type': 'object'} {'ok': True}

Python & MCP Python SDK

Python 3.12
mcp 2.3.0 (also on main: func_metadata.py still uses isinstance(type_expr, GenericAlias))

Prepared with the help of Claude Code.