starlette-mount

Embed an MCP server inside an existing Starlette (or FastAPI) app at a sub-path, next to your own routes. mcp.streamable_http_app() returns a mountable ASGI app; the two things to get right are the path (the default streamable_http_path="/mcp" stacks under your mount prefix) and the lifespan (Starlette does not run a mounted sub-app's lifespan, so the parent must enter mcp.session_manager.run()).

Run it

# HTTP — the client self-hosts the mounted app on a free port at /api/, runs,
# then tears it down
uv run python -m stories.starlette_mount.client --http

# against a server you run yourself (real uvicorn on :8000)
uv run python -m stories.starlette_mount.server --port 8000 &
SERVER_PID=$!
curl http://127.0.0.1:8000/health        # → {"status":"ok"}
uv run python -m stories.starlette_mount.client --http http://127.0.0.1:8000/api/
kill "$SERVER_PID"

What to look at

• client.py main — opens with async with Client(target, mode=mode) as client:. Nothing on the client side knows about the mount: the /api/ URL handed in as target is just another streamable-HTTP endpoint.
• server.py streamable_http_path="/" — without this the endpoint would be /api/mcp; with it, Mount("/api", ...) serves MCP at /api/ (trailing slash required — Starlette's Mount forwards /api as an empty path that the inner / route won't match).
• server.py lifespan — mcp.session_manager.run() must be entered by the parent app. Forget it and every MCP request fails immediately with a 500 (RuntimeError: Task group is not initialized. Make sure to use run().) — the sub-app's own lifespan never fires under Mount.
• server.py Route("/health", ...) — non-MCP routes live alongside the mount; FastAPI users do the same with app.mount("/api", mcp_app).

Caveats

• DNS-rebinding protection is on by default; the example passes transport_security=NO_DNS_REBIND because the in-process test client sends no Origin header. Remove it (or configure allowed hosts) for a real deployment.
• The parent-lifespan dance is a known SDK ergonomics gap (other SDKs mount with no extra ceremony); tracked for the beta reshape. The recipe shown here is what works today.

Spec

Streamable HTTP transport

See also

stateless_legacy/ (the one-liner mcp.streamable_http_app() without a parent app), json_response/, legacy_routing/. TS-SDK equivalent: examples/hono/.