devtools

Use CDP at localhost:9222. For exploration, use tmux with websocat, or wscat, or uv run --with playwright,markdownify,httpx,readability python -m asyncio (starts Python REPL with top-level await). For automation, npm install playwright and code in node.

Capabilities:

• For DOM HTML to Markdown: turndown, markdownify, or uvx markitdown file.html
• Log progress via console: page.on("console", msg => console.log("LOG:", msg.type(), msg.text())). Use CDP's Console.enable for replay
• Log screenshots via (page||locator).screenshot({ path, fullPage, type, quality, ... })
• Intercept fetch/XHR, parse JSON directly, cache for repeat runs
• HAR traces: context.tracing.start({ screenshots: true, snapshots: true }) ... context.tracing.stop({ path: "trace.zip" })
• Inject JS into existing tabs via a blob: URL created in the page context. CSP may block inline scripts.
  • url = URL.createObjectURL(new Blob([code], { type: "text/javascript" }))
  • Append <script src="blob:...">
  • Avoid page.addScriptTag({content: ...}) on CSP-heavy sites (e.g. WhatsApp, Google apps).

Uses:

• Debug/test using inspection (DOM, cookies, storage), screenshots, console logs, breakpoints, JS execution, network intercepts (modify headers, mock responses)
• Automate (research, scrape, ...) using navigation, form-filling, print to PDF
• Refactor: remove dead/unused JS, CSS, HTML
• Replay test/automation scripts: capture flow as scripts
• Monitor performance, audit using Lighthouse, axe-core
• Emulate devices, screen size, dark mode, network speed, geo, time zone, color blindness, touch devices
• Harden via cookie audits, pen-testing
• Parallelize using multiple tabs
• Browse safely using separate profiles / incognito mode

Tips:

• When scraping, collect 8-10 diverse variants of the target structure to cover edge cases before implementing selectors.
• Generate a selector bundle per element. Include role+name, text substring, stable attributes, and a fallback position. Try them in order and remember which one works.
• Combine screenshots with DOM snapshots and accessibility tree (since CSS can be brittle) for better context.
• Annotate with colored borders, labels, or numbers before full-page screenshot and use that for visual context.
• On failure, use screenshot, console logs, recent network requests, localStorage/cookies, DOM for diagnosis.
• Record golden HAR/screenshots/state. Helps spot regression errors, missing headers, caching quirks, and third-party blockers quickly.