Percy snapshots can differ between local and CI runs because the runs captured different page states or rendering inputs, or because Percy compared them with different baselines. A visual diff alone does not identify which happened. Check project and branch context first, then snapshot identity, capture conditions, and page readiness before deciding whether the UI truly changed.
What a Percy diff does—and does not—tell you
A Percy diff means the captured snapshot differs from the baseline Percy selected. It does not, by itself, prove that the application changed: the baseline may be mismatched, or the captures may represent different states. Treat baseline selection and rendering differences as separate diagnostic paths. Renaming a snapshot will not make two differently rendered pages match, and ignoring a region will not repair a wrong baseline.
Diagnose the runs in the right order
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Confirm project and upload context
Check that local and CI runs use the Percy token for the intended project and the expected Percy CLI or integration invocation. Verify that the token is available in each environment. A token pointing to another project can separate the uploads from the baseline you expected; a missing or invalid token can prevent the expected upload. Percy’s integration guidance shows the project token supplied through the execution environment: Percy CLI integration.
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Verify branch and baseline selection
Inspect the build’s branch and target-branch context, then confirm which approved baseline Percy selected. Do not assume two builds compare against the same reference merely because they came from the same repository. Baseline-selection behavior and baseline-management strategies are distinct from how the page was rendered; BrowserStack describes baseline workflows in its Percy baseline documentation.
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Compare snapshot names and variants
Compare the full snapshot names in both runs, including any page or state suffixes. Keep names stable and make variants explicit. A changed name can be treated as a different snapshot identity and may leave Percy without the baseline you expected. Locale, country, brand, and other variant matrices should have corresponding baseline structures rather than pairing unlike variants. See BrowserStack’s locale guidance and the Percy client documentation for snapshot behavior and naming practices.
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Match the capture inputs
Compare the inputs that can affect the visible page:
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- Viewport widths and browser or test configuration.
- Locale, timezone, feature flags, and application configuration.
- Test data, account state, session state, and network responses.
- Whether the same fonts, images, stylesheets, and other assets are available.
Percy’s documentation describes rendering submitted snapshots across configured browsers and responsive widths. Your application’s data and session inputs still need to be checked in your own test setup.
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Capture at the same settled point
Call the Percy snapshot API only after the page has reached the intended visible state. Wait for relevant asynchronous requests and lazy-loaded content, and ensure fonts and assets have loaded. Control animations, clocks, random identifiers, rotating banners, and personalized content if they vary between runs. Percy’s client documentation describes snapshot capture options; use the options supported by your integration and test framework.
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Classify the result before changing the baseline
If the snapshot is missing a baseline or paired with the wrong variant, repair naming, branch context, or baseline organization. If the baseline is correct but visible content differs, investigate application state and rendering inputs. If the difference is an intentional product change, review and approve it through your team’s baseline process. Ignore regions only when the content is genuinely expected to vary; they do not fix baseline selection.
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Compare a local run with CI systematically
Use the same snapshot and page state as your unit of comparison. Record what each run actually used rather than relying on assumptions about “local” and “CI.”
| Axis | Compare | What a mismatch can indicate |
|---|---|---|
| Project and build | Percy project token, branch, target branch, and selected baseline | Uploads in different project contexts or comparison against an unexpected baseline |
| Snapshot identity | Full snapshot name, page/state, locale, and variant structure | A new or differently paired snapshot, including a no-baseline case |
| Capture inputs | Viewport widths, browser/test configuration, flags, test data, and user/session state | Different application content or layout |
| Rendering readiness | Fonts and assets, asynchronous and lazy content, and dynamic elements | A capture taken before the intended state settles, or content that varies across runs |
| Decision | Whether the evidence shows a true UI change, a baseline mismatch, or expected dynamic content | The appropriate next action: fix setup, approve a change, or selectively ignore expected volatility |
Why Percy’s rendering may not match your local browser
Percy receives snapshots through test integrations and renders them in its service environment. A local browser’s conditions are therefore not necessarily identical to the conditions used for Percy’s comparison. The browser matrix and responsive widths Percy is configured to use matter, as do the state and inputs represented by the submitted snapshot. Compare those settings and the captured application state rather than treating a local screenshot as a guaranteed one-to-one reference.
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Common symptoms and fixes
- “No baseline” or an unexpected new snapshot: Compare full snapshot names, locale or other variant labels, and branch/baseline context. Restore stable identity or establish the intended baseline structure.
- A diff appears although no UI change was intended: Confirm the selected baseline, then compare test data, account state, flags, locale, viewport, and timing. A diff is evidence of a visual difference, not proof of a code regression.
- Local looks right but CI differs: Check whether CI has the same assets, fonts, responses, browser configuration, viewport widths, and settled page state. Inspect waits for asynchronous and lazy-loaded content.
- Only a small region keeps changing: Determine whether it is expected dynamic content, such as a rotating or personalized element. Stabilize it where possible; use an ignore region only if the variation is intentional and irrelevant to the comparison.
- The upload is absent or appears under an unexpected project: Verify the token’s presence and project, then confirm the Percy integration command used in that environment.
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Frequently Asked Questions
Does a Percy diff mean the UI changed?
No. It means the snapshot differs from the selected baseline; inspect baseline selection and captured state before concluding that the UI changed.
Are local screenshots rendered the same way as Percy CI snapshots?
Not necessarily. Percy renders submitted snapshots in its service environment, so compare configured browsers, responsive widths, and capture inputs.
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