@wdio/devtools-service

@wdio/devtools-service

A WebdriverIO service that provides a developer tools UI for running, debugging, and inspecting browser automation tests. Features include DOM mutation replay, per-command screenshots, network request inspection, console log capture, and session screencast recording.

Quick Start

1. Install the package as a dev dependency:

npm install @wdio/devtools-service --save-dev
# or
pnpm add -D @wdio/devtools-service

2. Add the service to your WebdriverIO config:

// wdio.conf.ts
export const config = {
  services: ['devtools'],
}

3. Run your tests as you normally do — the DevTools UI opens automatically, showing each test's commands, screenshots, console logs, and network activity in real time.

Configuration

Options are passed as the second element of the service tuple:

services: [['devtools', options]]

All options are optional — the service runs with sensible defaults out of the box. See Reference for the full options table.

Live Mode

Test Runner

// wdio.conf.ts
export const config = {
  services: ['devtools'],
}

Standalone

import { remote } from 'webdriverio'
import { setupForDevtools } from '@wdio/devtools-service'

const browser = await remote(setupForDevtools({
  capabilities: { browserName: 'chrome' }
}))
await browser.url('https://example.com')
await browser.deleteSession()

Trace Mode

Set mode: 'trace' to skip the live UI window and write a portable trace.zip (or, with traceFormat: 'ndjson-directory', an unpacked directory) under a test-results/ directory at session end — meant for CI, offline replay, and agentic diffing. See the full trace mode docs.

Open a trace in the first-party player:

pnpm show-trace path/to/trace.zip     # from this repo
npx show-trace path/to/trace.zip      # in a project that installs an adapter

The show-trace bin ships with this service (and with the Selenium/Nightwatch adapters), so it's available wherever one is installed — no extra dependency. It boots the DevTools UI in a dedicated player mode:

• DOM time-travel — the page pane rebuilds the real DOM as of the selected action from the captured mutation stream (not just a screenshot), replaying form-field state (typed values, checked/selected) so each step shows the page exactly as it was at that moment.
• A11y tab — the accessibility tree (roles + accessible names) captured for the selected action; the semantic view a screen reader sees, distinct from the raw DOM in the snapshot pane.
• Element overlay ("pick locator") — every locator the test interacted with is outlined on the replayed page. Hover a box to highlight the matching A11y row; click to copy a resilient locator to the clipboard. Hovering an A11y row highlights the element back — the two views are linked bidirectionally.
• Transcript tab + Copy-for-LLM — the run's transcript.md rendered in-panel, with a one-click Copy prompt that bundles the transcript and any failing-command errors into paste-ready LLM context.
• Timeline — categorized action markers (navigation / input / assertion / query / …), a dense filmstrip when filmstrip is enabled, Network and Console tracks, a draggable playhead, and playback controls (play/pause, step, speed). Cucumber runs nest as Feature → Scenario → Step.
• Errors, Console, Network, and Source tabs — the same workbench tabs as live mode.

The same trace.zip also opens in other compatible standalone trace viewers, and its on-disk format is what an Allure report's embedded trace viewer reads (Allure ≥ 2.35).

Allure integration

When @wdio/allure-reporter is installed, trace-mode artifacts are attached to the Allure report automatically (and emitArtifactsManifest is auto-enabled, so the run also writes the devtools-artifacts-<sessionId>.json index):

• traceGranularity: 'test' — each test's trace (application/zip, a download that opens in pnpm show-trace), screenshot (image/png, inline) and video (video/webm, inline) attach to that test's card. This is the mode to use for a per-test Allure report.
• traceGranularity: 'session' / 'spec' — a session/spec-spanning trace.zip is written to disk and enumerated in devtools-artifacts-<sessionId>.json (the artifacts manifest, listing every artifact + each test's state), but it is not attached to individual test cards.

Why session/spec traces aren't attached per test

The reporter's addAttachment targets the currently-running test. A session/spec trace is only finalized after all its tests have run — by which point their Allure cards are already closed — so there is no open test to attach it to. Per-test attachment therefore requires traceGranularity: 'test', where each slice is written during its own test hook while the card is still open.

To surface a session/spec trace in Allure anyway, post-process the manifest in your own onComplete hook (copying the trace.zip into allure-results/ and appending it to the result files). This is deliberately left to userland — baking it into the adapter would couple it to Allure's on-disk result format.

Report noise

In trace mode the service captures a per-action snapshot (a takeScreenshot WebDriver command) to build the trace timeline; @wdio/allure-reporter logs every WebDriver command as a step and attaches a screenshot per takeScreenshot. Silence that flood with the reporter's own options — the trace.zip / screenshot / video attachments are unaffected:

reporters: [
  ['allure', {
    outputDir: 'allure-results',
    disableWebdriverStepsReporting: true,
    disableWebdriverScreenshotsReporting: true
  }]
]

Screencast Recording

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

Available across all three adapters — WebdriverIO uses CDP push for Chrome (and polling fallback otherwise); see the Nightwatch and Selenium READMEs for their adapter-specific modes.

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)

Performance API capture

After every navigation command (url, navigateTo, etc.), the service runs the shared CAPTURE_PERFORMANCE_SCRIPT from @wdio/devtools-core to read window.performance.getEntriesByType('navigation' | 'resource'), cookies, and document info. The result is attached to the command entry in the Actions tab so you see loadTime / domReady / responseTime / resource counts per navigation. Same script and applyPerformanceData post-processing used by selenium-devtools and nightwatch-devtools — uniform dashboard fields across all three adapters.

Reference

services: [['devtools', options]]

Option	Type	Default	Description
port	number	random	Port the DevTools UI server listens on
hostname	string	'localhost'	Hostname the DevTools UI server binds to
devtoolsCapabilities	Capabilities	Chrome 1600×1200	Capabilities used to open the DevTools UI window
screencast	ScreencastOptions	—	Session video recording (live mode only — see below; for trace mode use video)
mode	'live' | 'trace'	'live'	'live' opens the DevTools UI window; 'trace' skips the UI and writes a trace-<sessionId>.zip under a test-results/ directory (base dir resolved from the test file's dir → config dir → cwd) at session end. See Trace mode
traceFormat	'zip' | 'ndjson-directory'	'zip'	Trace mode only. Output layout — 'zip' writes a single archive; 'ndjson-directory' unpacks the same files into trace-<id>/ (one less unzip step for scripted/agentic consumers). Both open in show-trace and other compatible viewers.
traceGranularity	'session' | 'spec' | 'test'	'session'	Trace mode only. How traces are partitioned — one per worker session / spec file / test. 'test' is required for per-test Allure attachments (trace, screenshot, video).
tracePolicy	TraceRetentionPolicy	'on'	Trace mode only. Which traces to keep: 'on' | 'retain-on-failure' | 'retain-on-first-failure' | 'on-first-retry' | 'on-all-retries' | 'retain-on-failure-and-retries'. The retry-aware policies pair best with traceGranularity: 'test'.
screenshot	'off' | 'on' | 'only-on-failure'	'off'	Trace mode + traceGranularity: 'test'. Per-test screenshot, attached inline to Allure (image/png). WDIO-service-specific.
video	'off' | TraceRetentionPolicy	'off'	Trace mode + traceGranularity: 'test'. Per-test screencast video, retained per the given policy, attached inline to Allure (video/webm). WDIO-service-specific.
filmstrip	boolean	true	Trace mode only. Records a dense, continuous screencast filmstrip into the trace so the player scrubs smooth playback — dense frames are added alongside the per-action frames (not one frame per action). Frames are thinned (≥100 ms apart, ~600 max) and content-addressed (identical frames — a static wait — collapse to one resource); windowed per slice at any traceGranularity. Runs the screencast recorder (CDP push on Chrome, polling elsewhere).
emitArtifactsManifest	boolean	false	Trace mode only. Writes devtools-artifacts-<sessionId>.json next to the trace — a generic index of every produced artifact (trace/screenshot/video) plus each test's state, for reporters/CI to consume. Off by default; auto-enabled when @wdio/allure-reporter is in the config.
captureAssertions	boolean	true	Capture assertions as command/action rows — node:assert plus passing and failing expect-webdriverio matchers, folded into single expect.<matcher> rows (e.g. toHaveText, toExist). Set false to opt out.

How It Works

Most of this service's capture + reporting logic now lives in @wdio/devtools-core and is consumed by all three adapters: SessionCapturerBase, ScreencastRecorderBase, TestReporterBase, loadInjectableScript/pollUntilReady, processTracePayload, captureSource, sendCommand/sendReplaceCommand, errorMessage/toError/serializeError, RetryTracker, mapChromeBrowserLogs, attachBidiHandlers, finalizeScreencast, encodeToVideo, suite-helpers, test-discovery. This service contains only WDIO-specific glue (BiDi event listeners via WDIO's native browser.on, the WDIO reporter integration, beforeCommand/afterCommand hook wiring, Cucumber UID branching).

License

MIT