Vitest snapshot mismatch (--update blocked in CI) in CI

By Kaveh Alemi·Latchkey

When CI=true, Vitest will not create or update snapshots; it fails instead. This is deliberate: a snapshot that only exists because CI auto-wrote it would hide real regressions. Update snapshots locally with -u/--update, review the diff, and commit them.

What this error means

A snapshot test fails with a diff, or "New snapshot was not written" / "Snapshots cannot be updated in CI". The same test passes locally where updates are allowed.

 FAIL  src/render.test.ts > renders card
Error: Snapshot `renders card 1` mismatched

 New snapshot was not written, because we are running in CI. Run vitest -u locally.

Common causes

The committed snapshot is out of date

Output changed intentionally but the stored snapshot was not regenerated and committed, so CI reports a mismatch.

A test relies on CI auto-writing snapshots

Vitest blocks snapshot writes under CI, so a first-run snapshot that would be created locally simply fails in CI.

How to fix it

Update snapshots locally and commit

1. Run vitest -u locally to regenerate snapshots.
2. Review the snapshot diff to confirm the change is intended.
3. Commit the updated .snap files so CI matches.

vitest run -u
git add src/**/__snapshots__

Clean up obsolete snapshots

Remove stale snapshot entries locally so CI does not flag obsolete ones; do this locally, never by forcing updates in CI.

vitest run --update

How to prevent it

• Regenerate and commit snapshots with the change that alters output.
• Never pass --update/-u in the CI command.
• Review snapshot diffs in code review like any other change.

Frequently asked questions

Related guides

References

• GitHub Actions documentation