Field alias is published in the tool schema but forwarded as an invalid Python keyword
Nobody has claimed this yet.
Assessment
- Difficulty
- 3/5
- Estimated time
- 1-2 days
- Newbie friendliness
- 68/100
Research direction
Start by reading the Tool.from_function and Tool.run entry points and run the provided reproduction with an aliased parameter. Trace how the schema name externalX is passed to the function, then add coverage showing that schema-valid input invokes f with x and returns successfully.
Written by the indexing model from the issue text.
Description
Initial Checks
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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
- Dominant language
- Python
- Stars
- 24.3k
- Forks
- 4k
- Avg merge
- 1d 11h
- Merged PRs (30d)
- 30
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
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- Open a pull request that references the issue number.
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