Content
# MCP Proxy
A TypeScript streamable HTTP and SSE proxy for [MCP](https://modelcontextprotocol.io/) servers that use `stdio` transport.
> [!NOTE]
> CORS is enabled by default.
> [!NOTE]
> For a Python implementation, see [mcp-proxy](https://github.com/sparfenyuk/mcp-proxy).
> [!NOTE]
> MCP Proxy is what [FastMCP](https://github.com/punkpeye/fastmcp) uses to enable streamable HTTP and SSE.
## Installation
```bash
npm install mcp-proxy
```
## Quickstart
### Command-line
```bash
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:
```bash
# 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:
```bash
# 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.
```ts
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`.
```ts
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.
```ts
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.
```ts
import { tapTransport } from "mcp-proxy";
const transport = tapTransport(new StdioClientTransport(), (event) => {
console.log(event);
});
```
## Development
### Running MCP Proxy with a local server
```bash
tsx src/bin/mcp-proxy.ts --debug tsx src/fixtures/simple-stdio-server.ts
```
