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+---
+name: Build a simple WebSocket server
+---
+
+Start a simple WebSocket server using [`Bun.serve`](/docs/api/http).
+
+Inside `fetch`, we attempt to upgrade incoming `ws:` or `wss:` requests to WebSocket connections.
+
+```ts
+const server = Bun.serve<{ authToken: string }>({
+  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: {
+    // 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 localhost:\${server.port}`);
+```