# Build a simple WebSocket server (/docs/guides/websocket/simple)

<!-- agent-signals: reading_time_min: 1 · est_tokens: 234 · updated: 2026-07-28 -->
Related: [Build a publish-subscribe WebSocket server](/docs/guides/websocket/pubsub.md), [Set per-socket contextual data on a WebSocket](/docs/guides/websocket/context.md), [Enable compression for WebSocket messages](/docs/guides/websocket/compression.md), [Streaming HTTP Server with Node.js Streams](/docs/guides/http/stream-node-streams-in-bun.md)

Start a WebSocket server with [`Bun.serve`](/runtime/http/server).

Inside `fetch`, `server.upgrade()` attempts to upgrade incoming `ws:` or `wss:` requests to WebSocket connections.

```ts server.ts icon="/icons/typescript.svg"
const server = Bun.serve({
  fetch(req, server) {
    const success = server.upgrade(req);
    if (success) {
      // Bun automatically returns a 101 Switching Protocols
      // if the upgrade succeeds
      return undefined;
    }

    // handle HTTP request normally
    return new Response("Hello world!");
  },
  websocket: {
    // TypeScript: specify the type of ws.data like this
    data: {} as { authToken: string },

    // this is called when a message is received
    async message(ws, message) {
      console.log(`Received ${message}`);
      // send back a message
      ws.send(`You said: ${message}`);
    },
  },
});

console.log(`Listening on ${server.hostname}:${server.port}`);
```