Field alias is published in the tool schema but forwarded as an invalid Python keyword

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK.
• I confirm that I searched the issue tracker before opening this issue.

Description

For a function parameter annotated with Field(alias="externalX"), the generated tool schema correctly requires externalX. At runtime, however, that wire name is forwarded directly to the original function, so the call fails with:

ToolError: Error executing tool f: f() got an unexpected keyword argument 'externalX'

I expected the schema-valid input to be mapped back to the Python parameter x before invocation.

Example Code

import asyncio
from typing import Annotated

try:
    from mcp.server.mcpserver.tools.base import Tool
except ImportError:  # MCP Python SDK 1.x
    from mcp.server.fastmcp.tools.base import Tool

from pydantic import Field


AliasInt = Annotated[int, Field(alias="externalX", ge=1)]


def f(x: AliasInt) -> int:
    return x


async def main() -> None:
    tool = Tool.from_function(f)
    print(tool.parameters)
    print(await tool.run({"externalX": 1}, None))


asyncio.run(main())

The printed schema requires externalX, then the call fails with the error above.

Python & MCP Python SDK

Stable reproduction:
- Python 3.13.13
- mcp 1.28.1
- pydantic 2.13.4

Current main reproduction:
- modelcontextprotocol/python-sdk commit 2713b53b127afc094dc97d6067df9f69b647661c
- mcp 2.0.0b2
- mcp-types 2.0.0b2
- Python 3.13.13

Commencez par lire les points d’entrée Tool.from_function et Tool.run et exécutez la reproduction fournie avec un paramètre doté d’un alias. Suivez la façon dont le nom de schéma externalX est transmis à la fonction, puis ajoutez une couverture montrant qu’une entrée valide selon le schéma appelle f avec x et retourne correctement.