Apply this skill when writing, reviewing, or planning tests for a Next.js + TypeScript + Drizzle application using Vitest, React Testing Library, MSW, and Playwright. Triggers on requests like "write tests for", "add test coverage", "test this component", "test this hook", "test this API route", "add E2E tests", "is this testable", or any time you are building features that need verification. Use proactively — all new features should include tests.