Session Screencast

Records browser sessions as .webm videos. Videos are displayed in the DevTools UI alongside the snapshot and DOM mutation views.

Note: Screencast recording is supported for WebdriverIO and Selenium WebDriver today. Nightwatch.js support is planned for a future release.

Demo

Setup

Screencast encoding requires ffmpeg on PATH and the fluent-ffmpeg package:

# Install ffmpeg — https://ffmpeg.org/download.html
brew install ffmpeg        # macOS
sudo apt install ffmpeg    # Ubuntu/Debian

# Install fluent-ffmpeg
npm install fluent-ffmpeg

Configuration

services: [
  [
    'devtools',
    {
      screencast: {
        enabled: true,
        captureFormat: 'jpeg',
        quality: 70,
        maxWidth: 1280,
        maxHeight: 720,
      }
    }
  ]
]

Options

Option	Type	Default	Description
enabled	boolean	false	Enable session recording
captureFormat	'jpeg' | 'png'	'jpeg'	Frame image format. Chrome/Chromium only — controls the format Chrome sends over CDP. Ignored in polling mode (Firefox, Safari) where screenshots are always PNG. Does not affect the output video container, which is always .webm
quality	number	70	JPEG compression quality 0–100. Only applies in Chrome/Chromium CDP mode with captureFormat: 'jpeg'
maxWidth	number	1280	Maximum frame width in pixels. Chrome/Chromium only — Chrome scales frames before sending over CDP. Ignored in polling mode
maxHeight	number	720	Maximum frame height in pixels. Chrome/Chromium only — same as above
pollIntervalMs	number	200	Screenshot interval in milliseconds for non-Chrome browsers (polling mode). Lower = smoother video but more WebDriver round-trips during test execution

Browser Support

Recording works across all major browsers using automatic mode selection:

Browser	Mode	Notes
Chrome / Chromium / Edge	CDP push	Chrome pushes frames over the DevTools Protocol. Efficient — no impact on test command timing
Firefox / Safari / others	BiDi polling	Falls back to calling browser.takeScreenshot() at pollIntervalMs intervals. Works wherever WebDriver screenshots are supported; adds a small overhead proportional to the interval

No configuration change is needed to switch modes — the service detects browser capabilities automatically and logs which mode is active.

Behaviour

• Recording starts when the browser session opens and stops when it closes.
• Leading blank frames (captured before the first URL navigation) are automatically trimmed so videos begin at the first meaningful page action.
• If browser.reloadSession() is called mid-run, the service finalises the current recording and starts a fresh one for the new session. Each session produces its own .webm file.
• When multiple recordings exist, the DevTools UI shows a Recording N dropdown to switch between them.
• Output files are written to the directory containing wdio.conf.ts (WDIO's rootDir) or outputDir if explicitly configured.

Output Files

File	Description
wdio-trace-{sessionId}.json	Full trace: DOM mutations, commands, screenshots, console logs, network requests
wdio-video-{sessionId}.webm	Screencast video (only produced when screencast.enabled: true)