With FastMCP.from_fastapi, how to pass lifespan to FastAPI app since it is already created?

[asaintsever]

In https://gofastmcp.com/deployment/asgi#fastapi-integration, we can see how to mount the FastMCP app under a path of our FastAPI app.

Problem is, when we leverage https://gofastmcp.com/patterns/fastapi to create our MCP Server from an existing FastAPI app, it is too late to pass the lifespan context from the FastMCP app to the FastAPI app (chicken and egg problem).

Is there any way to benefit from the ASGI FastAPI integration when we also use FastMCP.from_fastapi to create the MCP server (how to deal with the lifespan in this case)?

Thanks.

[ammmir]

Did you figure it out?

The docs warn:

For Streamable HTTP transport, you must pass the lifespan context from the FastMCP app to the resulting FastAPI app, as nested lifespans are not recognized. Otherwise, the FastMCP server’s session manager will not be properly initialized.

I'm having difficulty understanding this without an example, am I doing it correctly?

mcp = FastMCP('Hello')
mcp_app = mcp.http_app(path='/mcp')

@asynccontextmanager
async def lifespan(app: FastAPI):
    async with mcp_app.lifespan(app):
        # ...
        yield
        # ...

app = FastAPI(lifespan=lifespan)
app.mount("/mcp", mcp_app)

[asaintsever]

Yes I managed to have something working properly:

@asynccontextmanager
async def lifespan(app: FastAPI):
    async with create_mcp_server(app):
       # ...
       yield
      # ...

With create_mcp_server defined as:

def create_mcp_server(app: FastAPI):
    # Create an MCP server from the FastAPI app
    mcp = FastMCP.from_fastapi(app=app)

    # For Streamable HTTP transport, on "/mcp" path (default)
    mcp_app = mcp.http_app()

    # The MCP endpoint will be available at /mcp-server/mcp of the resulting FastAPI app.
    app.mount("/mcp-server", mcp_app)
    return mcp_app.lifespan(mcp_app)

By doing so my app is both a regular FastAPI app exposing REST endpoints and a FastMCP app exposing tools at /mcp-server/mcp.