Bun | Cap'n Web

Bun’s server-side WebSocket API uses callback-based handlers instead of the standard addEventListener interface. Cap’n Web provides newBunWebSocketRpcHandler(), which returns a handler object you can pass directly to Bun.serve().

import { RpcTarget, newBunWebSocketRpcHandler, newHttpBatchRpcResponse } from 'capnweb';

class MyApiImpl extends RpcTarget implements MyApi {
  // ... define API, same as above ...
}

// Create a WebSocket handler that manages RPC sessions automatically.
// The callback is invoked once per connection to create a fresh API instance.
let rpcHandler = newBunWebSocketRpcHandler(() => new MyApiImpl());

Bun.serve({
  async fetch(req, server) {
    let url = new URL(req.url);
    if (url.pathname === '/api') {
      // Upgrade WebSocket requests.
      if (req.headers.get('upgrade')?.toLowerCase() === 'websocket') {
        if (server.upgrade(req)) return;
        return new Response('WebSocket upgrade failed', { status: 500 });
      }

      // Handle HTTP batch requests.
      let response = await newHttpBatchRpcResponse(req, new MyApiImpl());
      response.headers.set('Access-Control-Allow-Origin', '*');
      return response;
    }

    return new Response('Not Found', { status: 404 });
  },

  // Pass the handler directly -- no manual wiring needed.
  websocket: rpcHandler,
});

Note that the callback passed to newBunWebSocketRpcHandler() runs once per connection, so each client gets its own API instance. That is usually what you want: per-connection state such as authentication lives naturally on that instance.

Payload limits

Bun’s Bun.serve() accepts maxPayloadLength in its websocket options. Set it when exposed to untrusted peers. See Security considerations.