When Loki screenshot tests fail in GitHub Actions, first identify the exact failed workflow step. Then confirm Storybook is running before Loki, check the configured browser target and runner image, compare local and CI versions and settings, and inspect the failing story’s reference and rendering behavior. Don’t update snapshots until you have reviewed the image differences: the cause could be setup, environment drift, an unstable story, or a failure unrelated to Loki.
1. Find the first failing step and preserve its output
Open the failed GitHub Actions run and inspect the job logs before changing code or reference images. GitHub documents that failed steps are expanded in the run view. The setup logs for GitHub-hosted runners also record runner-image details and link to preinstalled tools. If ordinary output is insufficient, enable workflow debug logging or Loki’s verbose option, if available in your installed version. GitHub’s workflow-log guide
Identify whether the failure is actually in Loki. A dependency installation error, a Storybook startup failure, a browser launch error, or an unreachable Storybook URL needs a different fix from a screenshot comparison mismatch. Loki’s README says it does not start servers for you: Storybook must already be running when the tests begin. Loki README
What to record from the failed run
- The first failing step and its complete error output.
- Loki and Storybook versions, along with the configured Loki browser target.
- The runner image and browser or container version shown in the job logs.
- The Storybook URL and evidence that Storybook was ready before Loki ran.
- The checked-out revision of the reference images and the affected story names.
2. Verify Storybook is available before Loki runs
Check the workflow order: install dependencies, start Storybook, wait until it is reachable, then run Loki. A process that starts successfully is not necessarily ready to serve stories; use a readiness check appropriate to your project before invoking Loki.
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Loki’s documented workflow creates reference images with yarn loki update, runs comparisons with yarn loki test, and reviews current and difference images before approving any intentional visual change. Reference images are ordinarily checked into the repository; Loki’s documentation also describes Git LFS as an option. Loki getting started
If the error indicates a missing reference, first confirm the expected reference files are present on the commit being tested. If a comparison fails, inspect the current image and diff against the checked-in reference. Update the baseline only when the visual change is expected and reviewed—not just to turn the workflow green.
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3. Check Loki’s browser target and version-specific prerequisites
Compare the browser target configured in Loki with the one intended for CI. The Loki repository lists Chrome in Docker as its recommended target, alongside local Chrome, AWS Lambda, iOS Simulator, and Android Emulator targets. A mismatch between local and CI targets is a useful diagnostic lead, not proof of the cause. Loki README
Loki’s getting-started documentation lists Docker as an optional dependency for chrome.docker, Chrome 59 or later for chrome.app, and Node 16 or later. These are requirements stated by that documentation, not a guarantee of compatibility for every release. Check the documentation and configuration for the Loki version pinned by your project before changing runner dependencies. Loki getting started
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Compare local and CI on the same axes
- Loki and Storybook versions.
- Browser target and browser or container version.
- Operating environment and GitHub runner image.
- Storybook URL and readiness timing.
- Reference-image revision.
- Story data, asynchronous readiness, and animation state.
- Chrome concurrency, if using a Chrome target.
Change one variable at a time and rerun the affected test. That makes it easier to tell whether the difference tracks a browser, image, readiness, or concurrency change rather than a simultaneous set of workflow edits.
4. Investigate local-versus-CI image differences
Loki’s project rationale describes reproducibility across operating systems as a goal and recommends Docker Chrome. That goal does not establish pixel-identical output in every custom setup. The practical test is to record the local and CI environments and make them as comparable as your project allows.
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A community discussion about a Chromium upgrade reports small element-position differences. A reply describes tests passing locally but failing in GitHub Actions and suggests matching the Docker/browser approach and lowering chromeConcurrency to 1 or 2. This is anecdotal advice, not an official or universal fix. Treat it as a controlled experiment: compare the browser and container versions first, then test concurrency separately if the evidence points there. Loki Chromium discussion
5. Stabilize stories that render asynchronously or animate
A screenshot can capture an intermediate state if a story loads data or rerenders after Loki has begun capture. Loki’s flaky-test documentation describes an async callback pattern using @loki/create-async-callback so a story can signal that it is ready. Apply it to stories whose visible content is not complete at the initial render, following the API for the installed Loki version. Loki flaky tests
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Timing-sensitive animation can also produce inconsistent output. Loki says it disables common CSS transitions and animations, and requestAnimationFrame, by default. Its documentation identifies looped requestAnimationFrame, GIFs, SVG animations, native Lottie, and React Native’s Animated library as limitations that may need to be disabled by the project. If a story cannot meaningfully be tested as a static frame, Loki documents skipping it with loki: { skip: true }. Loki flaky tests
6. Keep screenshots and test results from failed runs
Upload the failure screenshots, difference images, test results, and relevant logs as GitHub Actions artifacts. Artifacts preserve files produced during a workflow run after the job ends and can make them available to another job; GitHub’s examples include test results and screenshots. GitHub artifact guide
For visual-regression diagnosis, retain the files that let someone compare the expected reference with the actual capture, plus enough logs to identify the runner and browser environment. This turns a transient CI failure into evidence you can inspect without rerunning immediately.
7. Common failure symptoms and targeted checks
| Symptom | First check | Targeted response |
|---|---|---|
| Loki cannot connect to Storybook or stories are unavailable | Whether Storybook started and was reachable before Loki ran. | Fix the earlier startup or readiness problem; Loki does not start the server itself. |
| Reference image missing or unexpectedly different | Whether reference images exist on the tested revision and whether the visual diff is intentional. | Restore or generate references as appropriate; approve only reviewed visual changes. |
| Images differ only in CI | Browser target/version, runner image, operating environment, and concurrency. | Compare environments and vary one factor at a time; do not assume a concurrency change is the cause. |
| Only stories with data or delayed rendering fail inconsistently | Whether the story signals readiness after asynchronous work finishes. | Use Loki’s documented async callback pattern where appropriate. |
| Animated stories are inconsistent | Whether the story uses an animation Loki does not disable. | Disable the project-level animation or skip the story if a static screenshot is not meaningful. |
| The error occurs before screenshot comparison | The first failed workflow step and its full log. | Resolve the dependency, server, browser launch, or workflow issue reported there before changing snapshots. |
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