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Watch agent sessions live

Obscura is headless, so an agent driving it through MCP, Puppeteer, or Playwright works invisibly. The CDP screencast surface lets you stream what the browser sees to a local tab while the agent works, which helps when supervising long tasks or debugging what an agent clicked.

This guide builds a small live viewer on top of obscura serve. It uses only Node's built-in modules and its native WebSocket client (Node 21+).

How it works

  1. Start the CDP server:

obscura serve --port 9222

  1. Run the viewer script below:

node watch.mjs

  1. Open http://localhost:8080 in a browser. Every page the agent navigates to appears there, updating as the page paints.

The script attaches to the CDP endpoint, captures the current page twice a second with Page.captureScreenshot, and forwards JPEG frames to your browser over Server-Sent Events.

// watch.mjs
// Usage: node watch.mjs [cdpPort] [httpPort]
import http from "node:http";

const cdpPort = process.argv[2] ?? 9222;
const httpPort = process.argv[3] ?? 8080;

const clients = new Set();
let latest = null;
let sessionId = null;
let ws = null;

const page = `
<!doctype html>
<html>
<head><meta charset="utf-8"><title>Obscura live</title>
<style>body{margin:0;background:#111;display:grid;place-items:center;height:100vh}
img{max-width:100%;max-height:100%}</style></head>
<body><img id="s" alt="live page">
<script>
const img = document.getElementById("s");
let old = null;
const es = new EventSource("/events");
es.onmessage = (e) => {
  const bytes = Uint8Array.from(atob(e.data), (c) => c.charCodeAt(0));
  const url = URL.createObjectURL(new Blob([bytes], { type: "image/jpeg" }));
  img.src = url;
  if (old) URL.revokeObjectURL(old);
  old = url;
};
</script></body>
</html>`;

const server = http.createServer((req, res) => {
  if (req.url === "/events") {
    res.writeHead(200, {
      "Content-Type": "text/event-stream",
      "Cache-Control": "no-store",
      Connection: "keep-alive",
    });
    res.socket.setNoDelay(true);
    if (latest) res.write(`data:${latest}\n\n`);
    clients.add(res);
    req.on("close", () => clients.delete(res));
  } else {
    res.writeHead(200, { "Content-Type": "text/html" });
    res.end(page);
  }
});

server.listen(httpPort, "127.0.0.1", () => {
  console.log(`live view: http://localhost:${httpPort}`);
});

function broadcast(base64) {
  latest = base64;
  for (const res of clients) {
    if (res.writable) res.write(`data:${base64}\n\n`);
  }
}

async function connect() {
  ws = new WebSocket(`ws://127.0.0.1:${cdpPort}/devtools/browser`);
  let id = 0;
  const pending = new Map();
  const call = (method, params = {}, sess) =>
    new Promise((resolve, reject) => {
      const mid = ++id;
      pending.set(mid, { resolve, reject });
      const msg = { id: mid, method, params };
      if (sess) msg.sessionId = sess;
      ws.send(JSON.stringify(msg));
    });

  ws.addEventListener("message", (ev) => {
    const msg = JSON.parse(ev.data);
    if (msg.id && pending.has(msg.id)) {
      const p = pending.get(msg.id);
      pending.delete(msg.id);
      msg.error ? p.reject(new Error(msg.error.message)) : p.resolve(msg.result);
    }
  });
  ws.addEventListener("close", () => {
    sessionId = null;
    setTimeout(() => connect().catch(retry), 2000);
  });
  ws.addEventListener("error", () => ws.close());

  await new Promise((resolve) => ws.addEventListener("open", resolve));

  // reuse the page target an agent session may already have created
  const { targetInfos } = await call("Target.getTargets");
  const existing = targetInfos.find((t) => t.type === "page");
  if (existing) {
    // pre-existing target: attach explicitly and use the returned session id
    const attached = await call("Target.attachToTarget", {
      targetId: existing.targetId,
      flatten: true,
    });
    sessionId = attached.sessionId;
  } else {
    // a freshly created target is auto-attached under a managed session id
    const created = await call("Target.createTarget", { url: "about:blank" });
    sessionId = `${created.targetId}-session`;
  }
  await call("Page.enable", {}, sessionId);

  // capture on an interval instead of Page.startScreencast: screencast
  // frames stream to the session that drives the page, so a passive
  // viewer stops receiving them once an agent takes over. An explicit
  // capture always reflects the current page state.
  const capture = async () => {
    if (ws.readyState !== WebSocket.OPEN || !sessionId) return;
    try {
      const shot = await call(
        "Page.captureScreenshot",
        { format: "jpeg", quality: 70 },
        sessionId
      );
      if (shot.data && shot.data.length > 100) broadcast(shot.data);
    } catch {
      // transient failures during navigation are normal, retry next tick
    }
    setTimeout(capture, 500);
  };
  capture();
}

function retry(err) {
  console.error(err.message);
  sessionId = null;
  setTimeout(() => connect().catch(retry), 2000);
}
connect().catch(retry);

For a packaged version with adaptive pacing (fast while painting, idle back-off) and multi-page handling, see tools/live-view.mjs.

Things worth knowing

  • Why captureScreenshot polling instead of screencast. Page.startScreencast frames stream to the CDP session that drives the page, so once an agent session takes over, a passive viewer session stops receiving frames. It also only produces frames when the page paints something new. An explicit Page.captureScreenshot always reflects the current page state regardless of which session navigated, which makes it the reliable choice for watching someone else's session.

  • Acknowledge every frame. Without Page.screencastFrameAck, frame delivery stops.

  • One client per page socket. If an agent's CDP client already holds the page's webSocketDebuggerUrl, connecting to it again fails with 503. Attach through the browser endpoint instead (ws://127.0.0.1:9222/devtools/browser) using Target.attachToTarget with flatten: true, then pass the returned session id as sessionId on every command.

  • Still images vs continuous view. The MCP server exposes browser_screenshot for one-shot captures. Screencast is the right tool when you want to watch continuously; it stays CDP-only by design.

Verifying

With obscura serve running and the viewer open:

  1. Navigate from another client, for example obscura fetch https://example.com through a separate worker, or any Puppeteer/Playwright/MCP session connected to port 9222.

  2. The tab shows the page within a second of it painting.

  3. Closing the viewer tab and reopening it resumes from the most recent frame.

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