Flaky Loki screenshot tests usually point to nondeterministic story state or rendering conditions—not simply an image-comparison problem. Make asynchronous work finish explicitly, stop uncontrolled motion, and run baseline creation and CI in the same pinned browser environment before adjusting diff sensitivity.
1. Make story readiness explicit
A story that fetches data, computes on mount, or re-renders after an event may be captured before it reaches its intended state. Loki’s Handling Flaky Tests guide recommends an async callback pattern to signal when that work is complete. Loki handles most network traffic and image loading, but other asynchronous work may still need an explicit completion signal.
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Use the callback pattern documented for your installed Loki version in stories with asynchronous setup. Signal completion only after the state that should appear in the screenshot has rendered. Avoid replacing a readiness signal with an arbitrary sleep: a fixed delay can be too short on a slow run and unnecessarily long on a fast one.
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Loki disables common CSS transitions and animations and requestAnimationFrame by default. Those defaults do not cover every moving visual: Loki documents limitations involving looping requestAnimationFrame, GIFs, SVG animation, Lottie, and React Native Animated content.
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For those cases, make the story deterministic: render a test-only static state or disable the animation in the story. Loki documents both a context/decorator approach and Loki-targeted CSS for controlling animation. If a story cannot meaningfully be represented by one still frame, selectively skip that story rather than treating every capture as a failed assertion.
3. Pin the capture environment
Generate references and run tests using the same controlled environment. Playwright’s visual comparison guidance warns that host operating system, browser version, settings, hardware, power source, and headless mode can change screenshot output.
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- Keep the browser build, operating-system image, headless mode, viewport, fonts, and relevant settings consistent between baseline creation and CI.
- Loki’s CLI exposes a Docker image option; its configuration supports named targets and viewport dimensions. Treat sample Docker image values in older docs as examples, not universal current versions, and verify compatibility with your installed Loki and browser.
- Use a static Storybook build in CI, as Loki’s CI guide recommends, so the test does not depend on a changing development server.
4. Check failed requests and capture boundaries
Loki fails a test by default when a story makes a network request that fails. Investigate the failed request and make the fixture or dependency deterministic. Configure fetchFailIgnore only for a deliberately understood URL pattern; using it to silence an unexplained failure can conceal a broken story.
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Also verify chromeSelector if you use it to constrain the capture. Loki describes this as cropping to the selector’s dimensions, not as a DOM-only screenshot. An absolutely positioned element outside or above that area may still appear in the captured image.
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5. Choose a diff engine from representative changes
Loki documents three diff engines. Their trade-offs differ, so compare them against known intentional changes and accidental visual noise before changing engines or tolerance.
| Engine | Documented behavior | Trade-off to check |
|---|---|---|
pixelmatch |
More sensitive to changes on large images and less susceptible to anti-alias flakiness. | Check whether its sensitivity catches the real changes your project cares about. |
gm (GraphicsMagick) |
Generally faster; tolerance is based on the overall image. | Requires GraphicsMagick as an external dependency. An overall-image tolerance may treat localized changes differently from a per-pixel approach. |
looks-same |
JavaScript-only, slower, and measures tolerance across neighboring pixels; this may help with differing pixel densities. | Compare its behavior and runtime on your representative screenshots. |
These descriptions come from Loki’s configuration documentation, whose related pages display an August 27, 2024 update date. No universal safe tolerance is established there: choose thresholds from your own diffs, preserving detection of meaningful UI changes.
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6. Use retries and tolerance narrowly
Loki’s CLI documentation lists chromeRetries with a default of zero, chromeLoadTimeout with a default of 60,000 milliseconds, and chromeTolerance defaulting to zero. Check the installed version’s documentation before relying on a flag or default, and remember that tolerance behavior depends on the selected diff engine.
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Fix state, timing, or environment variation first. A retry can help with an occasional capture failure, but persistent differing pixels call for a deterministic fixture or rendering setup. Keep tolerance small and document which expected noise it is meant to exclude.
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7. Review reference updates like code changes
Loki separates testing, updating, and approving references, and stores reference images, current screenshots, and diffs in configurable directories. Its CI guidance recommends requiring references; a missing baseline can make CI behavior misleading. Review the expected image, actual capture, and diff before approving or updating a reference.
Playwright likewise documents explicit snapshot updates and recommends committing and reviewing screenshot changes. A baseline update should record an intentional design change, not convert an unexplained failure into the new accepted output.
Troubleshooting by symptom
| Symptom | Likely cause | What to check |
|---|---|---|
| The screenshot shows a loading or intermediate state. | Asynchronous work or a re-render has not finished. | Add Loki’s async completion callback to the story and signal it after the intended state renders. |
| Only animated stories vary between runs. | The animation type is outside Loki’s automatic controls. | Use a static test state, disable motion with the documented Loki approach, or selectively skip a story that has no meaningful still state. |
| Local captures pass but CI differs. | Browser, OS, fonts, headless mode, viewport, or other rendering conditions differ. | Compare and pin the capture environment; use a static Storybook build in CI. |
| A test fails despite an apparently correct image. | A story request may be failing, or capture boundaries may include unexpected content. | Inspect network failures and the chromeSelector crop behavior before suppressing failures. |
| Many unrelated pixels differ. | The environment or story state may be unstable, or the chosen engine may react differently to rendering noise. | Stabilize state and environment first, then compare engines on representative diffs. |
| A reference appears unexpectedly absent or CI behaves inconsistently. | The reference may not exist or may not be required in CI. | Require references and review the current capture and diff before creating or approving a baseline. |
Or skip the browser setup
If your goal is to capture a page rather than stabilize a Loki component story, ScreenshotNeo offers a one-request screenshot API. For example, this cURL request saves a WebP screenshot of the target URL; see the ScreenshotNeo API documentation for options and response details:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpScreenshotNeo accepts cookie and consent banners like a visitor and removes more than 60 known consent platforms, newsletter popups, and chat widgets before capture; each cleanup step can be turned off. Bot checks, blank pages, timeouts, failed loads, and cache hits cost nothing, with the page verdict and billing status included in response headers. Its MCP server provides take_screenshot, get_page_info, and capture_pdf for Claude, Cursor, and other MCP clients. The free plan includes 1,000 screenshots a month with no card; paid plans start at $5 for 3,000.
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