How to set / get client_id?

I'd like to be able to retrieve the client_id from inside a tool call on an MCP server. However, it is always None. What do I need to set in the MCP client in order to be able to retrieve it on the server?

The client code is basically taken from the documentation:

from mcp import ClientSession, StdioServerParameters, types
from mcp.client.stdio import stdio_client

server_params = StdioServerParameters(
    command="python",  # Executable
    args=["example_server.py"],  # Optional command line arguments
    env=None,  # Optional environment variables
)


async def run():
    async with stdio_client(server_params) as (read, write):
        async with ClientSession(
            read, write
        ) as session:
            # Initialize the connection
            await session.initialize()

            # Call a tool
            result = await session.call_tool("my_tool")

The tool function is as follows:

mcp = FastMCP("docint")

@mcp.tool()
async def my_tool(ctx: Context) -> Optional[str]:
    """
    Extracts client ID and request ID from the context.

    Returns:
        client ID and request ID from the context.
    """
    return f"{ctx.client_id} {ctx.request_id}" # <----- THIS

This prints None 2 (client_id as None).

What am I missing?

Thank you.

I have the exact same question, but i think that this is a WIP feature that will be implemented based on this draft.

Can one of the contributors or maintainers confirm if this is supposed to work, please?

Or is it broken / WIP as the above comment suggests?

Thank you.

ctx.client_id isn't populated by the SDK: it just echoes a client_id key from the request's _meta (both on v2 main and the 1.x FastMCP Context), and clients don't send one by default, hence the None. To have it set, pass it per call: await session.call_tool("my_tool", meta={"client_id": "my-app"}) (the meta= argument on call_tool has been there since 1.19 and is on v2's Client/ClientSession too). If what you're actually after is an authenticated client's identity, that's the OAuth client_id on the access token: get_access_token().client_id, covered in The caller's identity.

Example (v2)

import anyio

from mcp import Client
from mcp.server.mcpserver import Context, MCPServer

mcp = MCPServer("demo")


@mcp.tool()
def my_tool(ctx: Context) -> str:
    return f"client_id={ctx.client_id!r}"


async def main() -> None:
    async with Client(mcp) as client:
        print((await client.call_tool("my_tool")).content[0].text)
        print((await client.call_tool("my_tool", meta={"client_id": "my-app"})).content[0].text)


anyio.run(main)

Prints client_id=None, then client_id='my-app'.

Closing this as part of a wider backlog cleanup around the v2 release. Feel free to reopen if this is still relevant.

_{AI Disclaimer}

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How to set / get `client_id`? #373