Logging in lifespan never emmitted

[ion-elgreco]

Description

Logging in the lifespan is never emitted. With a fastapi server this doesn't happen.

import logging
from fastmcp import FastMCP
from contextlib import asynccontextmanager


logging.basicConfig(
    level=logging.INFO,
    format="%(asctime)s - %(name)s - %(levelname)s - %(message)s",
)
logger = logging.getLogger(__name__)

@asynccontextmanager
async def lifespan(app: FastMCP):  # noqa: ANN201, ARG001
    """Application lifespan handler"""
    logger.info("Starting MCP API...")
    yield
    logger.info("Closing MCP API...")

mcp = FastMCP(
    name="MCP",
    host="0.0.0.0",
    port=8050,
    stateless_http=True,
    lifespan=lifespan,
)

mcp.run(transport="streamable-http", show_banner=False, log_level="INFO")

Example Code

Version Information

FastMCP version:                                                             2.12.4
MCP version:                                                                 1.14.1
Python version:                                                              3.12.9
Platform:                                              macOS-15.6.1-arm64-arm-64bit

[ion-elgreco]

Seems to be discussed before: #775

I suggest renaming lifespan to something else or documenting it differently

[strawgate]

works just fine for me with your sample code. lifespan is entered on request and prints to console

[ion-elgreco]

works just fine for me with your sample code. lifespan is entered on request and prints to console

It works per request session indeed, it however doesnt on global app level akin to FastApi

[strawgate]

The current behavior is whatever the python sdk does

We are hoping to offer a "server_lifespan" argument in an upcoming version of FastMCP, there's an example implementation here #2013

[strawgate]

Would you like to try the branch in #2013 and see if that works for your use-case? You'd pass your lifespan to server_lifespan

[ion-elgreco]

@strawgate gave it a try and indeed solves it. Also much simpler then having to mount mcp_app in another fastapi app just for the server lifespan logging

[ion-elgreco]

@strawgate I have noticed a difference, when using server_lifespan logging is emitted before:

INFO: Started server process [34130]
INFO: Waiting for application startup.

But with fastapi and combined lifespan its done after those lines