MCP Proxy

A TypeScript streamable HTTP and SSE proxy for MCP servers that use stdio transport.

[!NOTE] CORS is enabled by default.

[!NOTE] For a Python implementation, see mcp-proxy.

[!NOTE] MCP Proxy is what FastMCP uses to enable streamable HTTP and SSE.

Installation

npm install mcp-proxy

Quickstart

Command-line

npx mcp-proxy --port 8080 --shell tsx server.js

This starts a server and stdio server (tsx server.js). The server listens on port 8080 and /mcp (streamable HTTP) and /sse (SSE) endpoints, and forwards messages to the stdio server.

options:

• --server: Set to sse or stream to only enable the respective transport (default: both)
• --endpoint: If server is set to sse or stream, this option sets the endpoint path (default: /sse or /mcp)
• --sseEndpoint: Set the SSE endpoint path (default: /sse). Overrides --endpoint if server is set to sse.
• --streamEndpoint: Set the streamable HTTP endpoint path (default: /mcp). Overrides --endpoint if server is set to stream.
• --stateless: Enable stateless mode for HTTP streamable transport (no session management). In this mode, each request creates a new server instance instead of maintaining persistent sessions.
• --port: Specify the port to listen on (default: 8080)
• --debug: Enable debug logging
• --shell: Spawn the server via the user's shell

Passing arguments to the wrapped command

When wrapping a command that takes arguments starting with -, you must use -- to prevent mcp-proxy from interpreting them as its own options. Everything after -- is passed directly to the wrapped command.

For example, to wrap a command that uses the -v flag:

# Wrong: mcp-proxy will try to parse -v as its own option
npx mcp-proxy --port 8080 my-command -v

# Correct: use -- to pass -v to my-command
npx mcp-proxy --port 8080 -- my-command -v

Stateless Mode

By default, MCP Proxy maintains persistent sessions for HTTP streamable transport, where each client connection is associated with a server instance that stays alive for the duration of the session.

Stateless mode (--stateless) changes this behavior:

• No session management: Each request creates a new server instance instead of maintaining persistent sessions
• Simplified deployment: Useful for serverless environments or when you want to minimize memory usage
• Request isolation: Each request is completely independent, which can be beneficial for certain use cases

Example usage:

# Enable stateless mode
npx mcp-proxy --port 8080 --stateless tsx server.js

# Stateless mode with stream-only transport
npx mcp-proxy --port 8080 --stateless --server stream tsx server.js

[!NOTE] Stateless mode only affects HTTP streamable transport (/mcp endpoint). SSE transport behavior remains unchanged.

When to use stateless mode:

• Serverless environments: When deploying to platforms like AWS Lambda, Vercel, or similar
• Load balancing: When requests need to be distributed across multiple instances
• Memory optimization: When you want to minimize server memory usage
• Request isolation: When you need complete independence between requests
• Simple deployments: When you don't need to maintain connection state

Node.js SDK

The Node.js SDK provides several utilities that are used to create a proxy.

proxyServer

Sets up a proxy between a server and a client.

const transport = new StdioClientTransport();
const client = new Client();

const server = new Server(serverVersion, {
  capabilities: {},
});

proxyServer({
  server,
  client,
  capabilities: {},
});

In this example, the server will proxy all requests to the client and vice versa.

startHTTPServer

Starts a proxy that listens on a port, and sends messages to the attached server via StreamableHTTPServerTransport and SSEServerTransport.

import { Server } from "@modelcontextprotocol/sdk/server/index.js";
import { startHTTPServer } from "mcp-proxy";

const { close } = await startHTTPServer({
  createServer: async () => {
    return new Server();
  },
  eventStore: new InMemoryEventStore(),
  port: 8080,
  stateless: false, // Optional: enable stateless mode for streamable HTTP transport
});

close();

Options:

• createServer: Function that creates a new server instance for each connection
• eventStore: Event store for streamable HTTP transport (optional)
• port: Port number to listen on
• host: Host to bind to (default: "::")
• sseEndpoint: SSE endpoint path (default: "/sse", set to null to disable)
• streamEndpoint: Streamable HTTP endpoint path (default: "/mcp", set to null to disable)
• stateless: Enable stateless mode for HTTP streamable transport (default: false)
• onConnect: Callback when a server connects (optional)
• onClose: Callback when a server disconnects (optional)
• onUnhandledRequest: Callback for unhandled HTTP requests (optional)

startStdioServer

Starts a proxy that listens on a stdio, and sends messages to the attached sse or streamable server.

import { ServerType, startStdioServer } from "./startStdioServer.js";

await startStdioServer({
  serverType: ServerType.SSE,
  url: "http://127.0.0.1:8080/sse",
});

tapTransport

Taps into a transport and logs events.

import { tapTransport } from "mcp-proxy";

const transport = tapTransport(new StdioClientTransport(), (event) => {
  console.log(event);
});

Development

Running MCP Proxy with a local server

tsx src/bin/mcp-proxy.ts --debug tsx src/fixtures/simple-stdio-server.ts