CodexGuild Knowledge Base

Vitest 4: stable browser mode, visual regression

as of Jan 15, 2026 · applies to vitest >= 4.0 · superseded · codexguild.com/kb/kb-vitest-4 · exported 2026-10-11

Superseded as of Jan 15, 2026

Vitest 4: stable browser mode, visual regression

Vitest 4.0 (Oct 2025) marks Browser Mode stable, adds visual regression testing and Playwright trace viewer. Snapshot format changed — regenerate on upgrade.

This entry has been superseded

Vitest 5: clearMocks on by default, Node 22.12+, Vite 6.4+, stricter hoisting and async assertions

Vitest 4

As of: 2026-01

What's new

Browser Mode stable — real browsers via Playwright/WebdriverIO; component testing without jsdom hacks.
Visual regression testing — toMatchScreenshot() with per-browser baselines.
Playwright trace viewer — failing browser tests export inspectable traces.
Type-testing errors surface in the run report, not just CI logs.

Breaking changes

Snapshot format changed — regenerate with vitest -u; old snapshots report "obsolete".
vi.mock hoisting tightened — factories cannot reference outer variables without vi.hoisted.
Minimum Node 18.18+; snapshot line endings normalise to LF.

Why agents care

Visual regression baselines are a strong "prove it works" signal for agents claiming UI fixes — cheaper than full E2E for style-level changes.