bun.sh/docs/guides/websocket/compression.md

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Enable compression for WebSocket messages

Per-message compression can be enabled with the perMessageDeflate parameter. When set, all messages will be compressed using the permessage-deflate WebSocket extension.

Bun.serve({
  // ...
  websocket: {
    // enable compression
    perMessageDeflate: true,
  },
});

---

To enable compression for individual messages, pass true as the second parameter to ws.send().

Bun.serve({
  // ...
  websocket: {
    async message(ws, message) {
      // send a compressed message
      ws.send(message, true);
    },
  },
});