wrapped tools using annotations fail

[stevenh]

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

If you try to use a tool a which is wrapped and uses from __future__ import annotations setup fails with and InvalidSignature exception e.g.

mcp.server.fastmcp.exceptions.InvalidSignature: Unable to evaluate type annotation ForwardRef("Literal['literal'] | None")

The cause of this is that the global name space used for the lookup is the wrapper namespace and not the original function namespace.

Example Code

from __future__ import annotations

import logfire
from mcp.server.fastmcp import FastMCP

@logfire.instrument
def test_tool(location_type: Literal["literal"] | None = None) -> str:
    """Test tool."""
    return f"a is {location_type}"

mcp = FastMCP(
    "Wrap test",
    tools=[
        Tool.from_function(test_tool),
    ],
)

Python & MCP Python SDK

SDL: 1.14.1
Python: 3.13.7

[ochafik]

Updated test case for self-contained, safe repro:

#!/bin/sh 
'''exec' docker run --rm -it --workdir /test -v $PWD:/test:ro ghcr.io/astral-sh/uv:debian uv run "$0" "$@"' '''
# /// script
# dependencies = [
#     "logfire",
#     "mcp",
# ]
# ///
from __future__ import annotations
from typing import Literal

from mcp.server.fastmcp.tools import Tool

import logfire
from mcp.server.fastmcp import FastMCP

@logfire.instrument
def test_tool(location_type: Literal["literal"] | None = None) -> str:
    """Test tool."""
    return f"a is {location_type}"

mcp = FastMCP(
    "Wrap test",
    tools=[
        Tool.from_function(test_tool),
    ],
)

Failure:

$ ./test2.py
Installed 48 packages in 77ms
Traceback (most recent call last):
  File "/test/./test2.py", line 25, in <module>
    Tool.from_function(test_tool),
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/.cache/uv/environments-v2/test2-3771963631ca83ad/lib/python3.11/site-packages/mcp/server/fastmcp/tools/base.py", line 66, in from_function
    func_arg_metadata = func_metadata(
                        ^^^^^^^^^^^^^^
  File "/root/.cache/uv/environments-v2/test2-3771963631ca83ad/lib/python3.11/site-packages/mcp/server/fastmcp/utilities/func_metadata.py", line 208, in func_metadata
    sig = _get_typed_signature(func)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/.cache/uv/environments-v2/test2-3771963631ca83ad/lib/python3.11/site-packages/mcp/server/fastmcp/utilities/func_metadata.py", line 475, in _get_typed_signature
    typed_params = [
                   ^
  File "/root/.cache/uv/environments-v2/test2-3771963631ca83ad/lib/python3.11/site-packages/mcp/server/fastmcp/utilities/func_metadata.py", line 480, in <listcomp>
    annotation=_get_typed_annotation(param.annotation, globalns),
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/.cache/uv/environments-v2/test2-3771963631ca83ad/lib/python3.11/site-packages/mcp/server/fastmcp/utilities/func_metadata.py", line 466, in _get_typed_annotation
    raise InvalidSignature(f"Unable to evaluate type annotation {annotation}")
mcp.server.fastmcp.exceptions.InvalidSignature: Unable to evaluate type annotation ForwardRef("Literal['literal'] | None")

[stevenh]

To clarify is that failure from current state or from my linked PR #1392?

[maxisbey]

@ochafik's comment is just showing another way of running your example code you gave on the current state of the codebase, not from your PR.

[stevenh]

Thanks for clarifying, I wanted to check as would typically expect that as a review comment, so wanted to ensure I understood 😄