Document Snapshot behavior on CI

[hi-ogawa]

Discussed in #9693

^{Originally posted by snebjorn February 19, 2026}
I'm trying to figure out how snapshots behave when running in CI.

This doesn't seem to be covered by https://vitest.dev/guide/snapshot.html
Is there another place this is documented?

I found #7963 which seems to suggest that obsolete snapshot will causes failed tests when running in CI. Though I'm not exactly sure what obsolete means here :) But never the less this is good.

Where can I find out how the snapshots behave when running in CI?

[hi-ogawa]

It looks like we are missing docs. We detect process.env.CI variable and such, then automatically flip this --update flag to be false https://vitest.dev/config/update.html This should be documented in https://vitest.dev/guide/snapshot.html#updating-snapshots

vitest/packages/vitest/src/node/config/resolveConfig.ts

Lines 564 to 575 in 8169fbd

	const UPDATE_SNAPSHOT = resolved.update || process.env.UPDATE_SNAPSHOT
	resolved.snapshotOptions = {
	expand: resolved.expandSnapshotDiff ?? false,
	snapshotFormat: resolved.snapshotFormat || {},
	updateSnapshot:
	UPDATE_SNAPSHOT === 'all' || UPDATE_SNAPSHOT === 'new'
	? UPDATE_SNAPSHOT
	: isCI && !UPDATE_SNAPSHOT ? 'none' : UPDATE_SNAPSHOT ? 'all' : 'new',
	resolveSnapshotPath: options.resolveSnapshotPath,
	// resolved inside the worker
	snapshotEnvironment: null as any,
	}