Example agent

// A minimal X Calls agent: joins a call as your bot, plays a greeting tone
// (stand-in for your TTS), stops talking when someone interrupts, and logs
// what happens. Node.js 22+ with no dependencies.
//
//   USER_ACCESS_TOKEN=<your bot's OAuth 2.0 user token> node agent.mjs <call_id>

const API = "https://api.x.com/2";
const RATE = 16000; // Hz, PCM16 mono, both directions

async function join(callId) {
  const res = await fetch(`${API}/calls/${callId}/join`, {
    method: "POST",
    headers: {
      Authorization: `Bearer ${process.env.USER_ACCESS_TOKEN}`,
      "Content-Type": "application/json",
    },
    body: JSON.stringify({
      media: { type: "websocket", audio: { encoding: "pcm16", sample_rate: RATE } },
    }),
  });
  const body = await res.json();
  if (!res.ok) throw new Error(`join failed: ${res.status} ${JSON.stringify(body)}`);
  return body.data;
}

// PCM16 little-endian, whatever the host's byte order.
function tone(hz, seconds) {
  const n = Math.round(RATE * seconds);
  const view = new DataView(new ArrayBuffer(n * 2));
  for (let i = 0; i < n; i++) {
    view.setInt16(i * 2, Math.round(6000 * Math.sin((2 * Math.PI * hz * i) / RATE)), true);
  }
  return view.buffer;
}

function level(buffer) {
  const view = new DataView(buffer);
  let sum = 0;
  for (let i = 0; i + 1 < view.byteLength; i += 2) sum += view.getInt16(i, true) ** 2;
  return Math.round(Math.sqrt(sum / Math.max(1, view.byteLength / 2)));
}

function runAgent(session) {
  const url = `${session.media_url}?token=${encodeURIComponent(session.media_token)}`;
  const ws = new WebSocket(url);
  ws.binaryType = "arraybuffer";
  const send = (command) => ws.send(JSON.stringify(command));
  let frames = 0;
  let loudest = 0;

  ws.onmessage = ({ data }) => {
    if (data instanceof ArrayBuffer) {
      // 20 ms of everyone else in the call: feed it to your model here.
      loudest = Math.max(loudest, level(data));
      if (++frames % 50 === 0) {
        console.log(`heard ${frames / 50}s, loudest ${loudest}`);
        loudest = 0;
      }
      return;
    }
    const event = JSON.parse(data);
    console.log(event.type, JSON.stringify(event));
    switch (event.type) {
      case "session.joined":
        ws.send(tone(440, 2)); // any chunk size; X paces it into the call
        send({ type: "audio.mark", name: "greeting" });
        break;
      case "playback.mark":
        if (event.name === "greeting" && !event.cleared) console.log("greeting played");
        break;
      case "speech.started":
        send({ type: "audio.clear" }); // let the person talk
        break;
      case "session.ended":
        ws.close();
        break;
    }
  };
  ws.onclose = () => process.exit(0);
  process.on("SIGINT", () => {
    if (ws.readyState === WebSocket.OPEN) send({ type: "leave" });
    else process.exit(0);
  });
}

runAgent(await join(process.argv[2]));