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 tosse
orstream
to only enable the respective transport (default: both)--endpoint
: Ifserver
is set tosse
orstream
, this option sets the endpoint path (default:/sse
or/mcp
)--sseEndpoint
: Set the SSE endpoint path (default:/sse
). Overrides--endpoint
ifserver
is set tosse
.--streamEndpoint
: Set the streamable HTTP endpoint path (default:/mcp
). Overrides--endpoint
ifserver
is set tostream
.--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
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,
});
close();
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