Function startSseServer

packages/mcp/src/server.js:22–121

({ port = 3001, sendAnalytics } = {})

 * @returns {Object} - Server instance and express app
 */
export async function startSseServer({ port = 3001, sendAnalytics } = {}) {
  // We can't use the SDK's createMcpExpressApp helper because it hard-codes
  // express.json's 100kb default and exposes no body-limit option. We
  // replicate its wiring here (json body parsing + Host-header validation
  // for DNS-rebinding protection) with a 4mb limit instead.
  const app = express()
  app.use(express.json({ limit: MAX_MESSAGE_SIZE }))
  app.use(hostHeaderValidation(['localhost', LOOPBACK_HOST, '[::1]']))

  // Create the MCP server instance.
  const server = new McpServer({
    name: 'serverless',
    version: '1.0.0',
  })

  // Add analytics tracking for tool executions if sendAnalytics function is provided
  wrapServerWithAnalytics(server, sendAnalytics)

  // Register all tools on the server
  registerTools(server)

  // Object to track active SSE transports keyed by sessionId.
  const activeTransports = {}

  /**
   * GET /sse
   * Establishes an SSE connection and stores the transport by its sessionId.
   */
  app.get('/sse', async (req, res) => {
    const transport = new SSEServerTransport('/messages', res)
    await server.connect(transport)

    // Store this connection in the activeTransports map.
    activeTransports[transport.sessionId] = transport
    console.error(`Client connected: ${transport.sessionId}`)

    // When the connection closes, remove it from the map.
    res.on('close', () => {
      delete activeTransports[transport.sessionId]
      console.error(`Client disconnected: ${transport.sessionId}`)
    })
  })

  /**
   * POST /messages
   * Routes incoming JSON‑RPC messages to the correct SSE transport based on sessionId.
   * Clients must include their sessionId in the query parameters.
   */
  app.post('/messages', async (req, res) => {
    const sessionId = req.query.sessionId
    if (!sessionId) {
      res.status(400).send('Missing sessionId in query parameters.')
      return
    }
    const transport = activeTransports[sessionId]
    if (!transport) {
      res.status(500).send('No active SSE connection for the given sessionId.')
      return