jcorpac fastmcp-stdio-proxy
Purpose
Routing tools via FastMCP requires a persistent stdio connection. Running an MCP application in a detached Docker container causes an immediate exit because it hits an EOF on standard input. Furthermore, an MCP router container needs a way to invoke other MCP containers via stdio locally without exposing network layers.
Usage
1. Keeping FastMCP Alive in Docker Detached Mode
When using docker-compose up -d, add stdin_open and tty to the service definition. This tricks FastMCP's stdio listener into staying alive in the background:
services:
mcp-router:
build: .
stdin_open: true
tty: true
2. The Docker-in-Docker Socket Proxy hack
If your containerized MCP Router needs to discover and spawn peer MCP servers using the stdio transport, it cannot natively launch them. Give the router access to the host's Docker engine:
docker-compose.yml:
volumes:
- /var/run/docker.sock:/var/run/docker.sock
Dockerfile: Install the Docker CLI so the python execution environment can call the socket over the OS layer:
RUN apt-get update && apt-get install -y docker.io
You can now use stdio_client with docker run -i --rm <target-image> inside your python router logic!
Resources
- FastMCP Stdio Transport Specs
- Docker Engine API