find_context_parameter() fails to detect Context parameter in callable class instances

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

When registering a callable class instance as an MCP tool via FastMCP.add_tool(), the ctx: Context parameter is incorrectly exposed as an externally visible tool parameter instead of being injected by the framework. This is because find_context_parameter() uses typing.get_type_hints() which doesn't introspect the call method of callable class instances.

Suggested Fix
Update find_context_parameter() to handle callable class instances:

def find_context_parameter(fn: Callable[..., Any]) -> str | None:
    from mcp.server.fastmcp.server import Context

    # Handle callable class instances by using __call__ method
    target = fn
    if not (inspect.isfunction(fn) or inspect.ismethod(fn)):
        if callable(fn) and hasattr(fn, "__call__"):
            target = fn.__call__

    try:
        hints = typing.get_type_hints(target)
    except Exception:
        return None

    # ... rest of function unchanged

Example Code

from typing import Annotated, Any
from mcp.server.fastmcp import Context, FastMCP
from mcp.server.session import ServerSession
from starlette.requests import Request
import pydantic

class MyTool:
    def __init__(self, name: str):
        self.__name__ = name

    async def __call__(
        self,
        query: Annotated[str, pydantic.Field(description="Search query")],
        ctx: Context[ServerSession, Any, Request],
    ) -> str:
        """Performs a search."""
        return f"Results for: {query}"

mcp = FastMCP(name="Test Server")
mcp.add_tool(MyTool(name="my_search"), name="my_search", description="Search tool")

# When listing tools, 'ctx' appears as a required parameter in the schema

Python & MCP Python SDK

1.26.0

FastMCP.add_tool() を通じてツールを登録する際に使用される find_context_parameter() から始めます。通常の関数と呼び出し可能なクラスインスタンスに対する処理を比較し、呼び出し可能なインスタンスを登録したときに、例で ctx がツールパラメーターとして公開されなくなっていることを確認します。