Jest Snapshot Mismatch - "1 snapshot failed" / Obsolete in CI

A snapshot test compared rendered output against a stored .snap and they differ. Either the UI legitimately changed (update the snapshot), or the output is non-deterministic (dates, IDs) and changes every run.

What this error means

CI fails with "1 snapshot failed" and a diff of expected vs received. In --ci mode Jest will not write new snapshots - a missing snapshot is a failure, not an auto-create as it is locally.

● <Card /> renders › matches snapshot

  expect(received).toMatchSnapshot()

  Snapshot name: `<Card /> renders matches snapshot 1`
  - Snapshot  - 1
  + Received  + 1
  -   <span>Updated 2 days ago</span>
  +   <span>Updated 3 days ago</span>

Common causes

Intentional UI change not committed

The component changed on purpose but the stored snapshot was not updated, so every run diffs. Run --updateSnapshot and commit the new .snap.

Non-deterministic output

Snapshots containing dates, random IDs, or unsorted data differ on every run. They must be normalized or those values mocked.

CI mode never auto-writes snapshots

Locally a missing snapshot is created and passes; with --ci (or CI=true) Jest fails instead of writing, surfacing snapshots that were never committed.

How to fix it

Update and commit intentional changes

jest -u            # update snapshots locally
git add **/*.snap  # commit the regenerated snapshots

Make snapshots deterministic

1. Mock dates/clock (jest.useFakeTimers().setSystemTime(...)) and any random IDs.
2. Use property matchers: toMatchSnapshot({ id: expect.any(String) }).
3. Sort collections before snapshotting so order is stable.

How to prevent it

• Commit .snap files and review their diffs in PRs.
• Mock time and randomness in snapshot tests.
• Run CI with --ci so missing snapshots fail instead of silently writing.

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