SDK doesn't support specifying tool metadata?

[johnson7788]

Some mcp tool, i don't want let LLM know, So i want transfer data by metadata, How can it do it? thank you !

Server example:

import datetime
from fastmcp import FastMCP, Context
from mcp.types import CallToolResult

mcp = FastMCP("EchoServer")

@mcp.tool()
async def echo_tool(text: str, ctx: Context) -> CallToolResult:
    # 从 ctx 获取 metadata
    meta_in = ctx.request_meta or {}
    print("[Server] meta_in:", meta_in)

    return CallToolResult(
        content=[{"type": "text", "text": text.upper()}],
        meta={
            "user_id": meta_in.get("user_id", "unknown"),
            "server_timestamp": datetime.datetime.utcnow().isoformat() + "Z"
        }
    )

if __name__ == "__main__":
    mcp.run(transport="sse")

Client Example

# client.py
from mcp.types import CallToolRequest, CallToolRequestParams, RequestParams
import asyncio
from fastmcp import Client
from mcp.types import CallToolRequestParams, RequestParams
import json

def simulate():
    # 构造调用请求，带上 meta 信息
    req = CallToolRequest(
        method="tools/call",
        params=CallToolRequestParams(
            name="echo_tool",
            arguments={"text": "hello MCP"},
            meta=RequestParams.Meta(  # 通过 alias="_meta" 被序列化为 "_meta"
                user_id="alice",
                session_id="sess-123",
            ),
        ),
    )

    # 序列化为 JSON-RPC 并发送
    payload = req.model_dump(by_alias=True)
    print("Sending:", json.dumps(payload, indent=2))
    # 这里你可以用 WebSocket/httpx/aiohttp 发送 payload 到 MCP server


# client_sse.py
async def main():
    # 指定 SSE 地址（在 server.run(transport="sse") 中默认挂载为 http://localhost:8000/sse）
    async with Client("http://localhost:8000/sse") as client:
        # 调用 echo_tool，自动使用 SSE transport
        result = await client.call_tool(
            "echo_tool",
            {"text": "hello MCP"},
            meta=RequestParams.Meta(
                user_id="alice",
                session_id="sess-123"
            )
        )
        print("Response content:", result.content)
        print("Response meta:", result.meta)

if __name__ == "__main__":
    asyncio.run(main())

[LucaButBoring]

There is actually a metadata parameter on all request/notification/response messages, e.g. here. I believe the Python SDK doesn't expose that, however - you should open a feature request on the Python SDK if you'd like this, I think.

[hesreallyhim]

I agree I think this is an SDK issue. I think I had to use the low-level server instead of fastmcp FWIW but it is doable.

EDIT: It looks like FastMCP 2 supports metadata on tools, just gave it a glance though:
https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/tools/tool.py

[pree-dew]

@dsp-ant Can I pick this issue?

[boijuny]

@pree-dew are you working on this ? I may be interested

[pree-dew]

@boijuny I definitely want to work on this, I thought maintainers will assign task for anyone to pick, was waiting for that. If that is not the case, I can pick this right away.

[boijuny]

k, good luck !