MCP Server transport for Hono applications

Overview

This project provides a transport for Hono application that needs to connect to MCP Server. An official TypeScript SDK is designed to be used with express. If you try and follow their instructions but change express with hono, you will find that it's not going to work. Even if you set HttpBinding as an env (see an example from @hono/node-server), it still doesn't work as some headers are being added to a response after it's sent.

This implementation is inspired by the following pull request

You can use this transport until there is a proper support for Hono in the official SDK.

Getting started

Installation

pnpm add -D hono-mcp-server-sse-transport

Usage

import { Hono } from 'hono';
import { serve } from '@hono/node-server';
import { streamSSE } from 'hono/streaming';
import { SSETransport } from 'hono-mcp-server-sse-transport';
import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';

const mcpServer = new McpServer(
  {
    name: 'your-mcp-server-name',
    version: '1.0.0',
  },
  {
    capabilities: {
      tools: {},
    },
  },
);

// here you add your tools
// ...

const app = new Hono();

// to support multiple simultaneous connections we have a lookup object from
// sessionId to transport
const transports: { [sessionId: string]: SSETransport } = {};

app.get('/sse', (c) => {
  return streamSSE(c, async (stream) => {
    const transport = new SSETransport('/messages', stream);

    transports[transport.sessionId] = transport;

    stream.onAbort(() => {
      delete transports[transport.sessionId];
    });

    await mcpServer.connect(transport);

    while (true) {
      // This will keep the connection alive
      // You can also await for a promise that never resolves
      await stream.sleep(60_000);
    }
  });
});

app.post('/messages', async (c) => {
  const sessionId = c.req.query('sessionId');
  const transport = transports[sessionId];

  if (transport == null) {
    return c.text('No transport found for sessionId', 400);
  }

  return transport.handlePostMessage(c);
});

serve(
  {
    fetch: app.fetch,
    port: 3000,
  },
);