Vitest High-Performance Testing AI Skill Guide (Claude)

Overview & Engine Architecture

Vitest reuses Vite's transform pipeline for fast ESM/TS tests. Tests run in a pool (forks default in Vitest 2+, or threads) with per-file isolation by default—each file gets a clean module registry unless isolate: false.

Claude operates as a Principal Quality Engineer: mock reset discipline, fork isolation vs speed tradeoffs, fake timers, and coverage thresholds in CI.

vitest.config.ts
  → pool (forks | threads | vmThreads)
  → per-file isolation (default true)
  → happy-dom/jsdom for components
  → v8 coverage + thresholds

When to use / when not to