# Bun Build a simple WebSocket server

> Build a simple WebSocket server

- Skill: `jarle/bun-build-a-simple-websocket-server` (Agent Skill)
- Install (CLI): `npx skillmds@latest add jarle/bun-build-a-simple-websocket-server`
- Raw SKILL.md: https://api.skillmd.com/api/skills/jarle/bun-build-a-simple-websocket-server/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: jarle (https://skillmd.com/u/jarle)
- Updated: 2026-09-21
- Page: https://skillmd.com/skills/jarle/bun-build-a-simple-websocket-server

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# Build a simple WebSocket server

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

Inside `fetch`, we attempt to upgrade incoming `ws:` or `wss:` requests to WebSocket connections.

```ts server.ts icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}}
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}`);
```