Mounting of fastmcp proxy

[s0214lx]

I created a fastmcp proxy and want to mount it with an already running fastapi service. How can I do this? I found that in the case of the document, the proxy is created first, and then the fastapi service is created with the proxy context as a parameter. If my fastapi service already exists, how can I do this?

[jerrygergov]

You can keep the existing FastAPI app and compose its lifespan before the ASGI server starts:

from contextlib import asynccontextmanager

mcp_app = proxy.http_app(path="/")
previous_lifespan = app.router.lifespan_context

@asynccontextmanager
async def combined_lifespan(app):
    async with previous_lifespan(app) as state:
        async with mcp_app.lifespan(app):
            yield state

app.router.lifespan_context = combined_lifespan
app.mount("/mcp", mcp_app)

Here app is your existing FastAPI instance and proxy is your FastMCP proxy. This preserves the existing startup/shutdown context and its yielded state; the MCP endpoint is /mcp/. The MCP session manager needs its lifespan entered in addition to mounting the app, as covered in the FastAPI integration guide.

If “already running” means the process has completed startup, apply this during app setup and restart/redeploy; changing the lifespan function afterward does not run it retroactively. I verified this with FastMCP 2.13.1 and FastAPI 0.141.1: a stateful MCP initialization and proxied tool call succeeded, and the original startup, shutdown, and lifespan state were preserved. I have not tested your remote backend.

AI-assisted answer; the ASGI test was executed.