FastMCP Mount

ASGI Middleware to fix FastMCP endpoint paths when mounted under a sub-path in frameworks like FastAPI or Starlette.

The Problem

When using the official MCP SDK, the FastMCP server generates an SSE event: endpoint message containing the path for the client to post messages back (e.g., /messages/). However, if you mount the FastMCP().sse_app() ASGI application under a sub-path in your main framework (like FastAPI), for example at /mcp/my-server, the endpoint path sent to the client remains /messages/ instead of the correct, fully qualified path /mcp/my-server/messages/. This causes client post requests to fail with a 404 Not Found error.

The Solution

This library provides a simple ASGI middleware, MountFastMCP, that intercepts the SSE response stream. It specifically looks for the event: endpoint message and automatically prepends the correct root_path (the path where the app is mounted) to the endpoint before sending it to the client.

Installation

pip install fastmcp-mount

Or using UV:

uv pip install fastmcp-mount

You will also need the official MCP python SDK and a compatible ASGI framework like fastapi or starlette.

pip install mcp[cli] fastapi uvicorn[standard]
# or
uv pip install mcp[cli] fastapi uvicorn[standard]

Usage

Simply wrap the FastMCP().sse_app() with MountFastMCP before mounting it in your main application.

from fastapi import FastAPI
from mcp.server.fastmcp import FastMCP
from fastmcp_mount import MountFastMCP # Import the middleware
import uvicorn

# 1. Create your FastMCP instance and define tools (as usual)
mcp_server = FastMCP(title="My Mounted Server")

@mcp_server.tool()
def my_tool(param: str) -> str:
    return f"Processed: {param}"

# 2. Get the raw SSE ASGI app
sse_app = mcp_server.sse_app()

# 3. Create your main FastAPI/Starlette app
app = FastAPI(title="Main API")

# 4. Mount the *wrapped* app at your desired sub-path
app.mount("/mcp/my-server", app=MountFastMCP(app=sse_app), name="my_mcp_server")

@app.get("/")
def read_root():
    return {"message": "Main API running"}

# Run with: uvicorn your_module:app --reload
# Connect your MCP client to: http://localhost:8000/mcp/my-server/sse

Now, when an MCP client connects to /mcp/my-server/sse, the endpoint it receives from the server will be correctly prefixed with /mcp/my-server/..., allowing the client to post back successfully.

Compatibility

• Python: 3.8+
• Frameworks: Starlette, FastAPI (and likely other Starlette-based ASGI frameworks)
• Dependencies: starlette

Contributing

Contributions are welcome! Please feel free to open an issue or submit a pull request on the GitHub repository.

1. Fork the repository.
2. Create a virtual environment: python -m venv .venv && source .venv/bin/activate
3. Install development dependencies: pip install -e ".[dev,test]"
4. Make your changes and add tests.
5. Run tests: pytest
6. Format, lint, and type check: ruff format . && ruff check . && mypy .
7. Commit and push your changes.
8. Open a pull request.

License

This project is licensed under the MIT License - see the LICENSE file for details.