Playwright

This is a comprehensive rulebook for writing Playwright tests in Next.js apps, with 43 rules organized by priority from critical (test isolation, role-based selectors) to helpful (debugging config). You'd reach for this when setting up E2E tests, debugging flaky failures, or optimizing CI runs. The authentication state reuse patterns alone will save you minutes per test run. It's opinionated about avoiding XPath and hard waits, which is good because those are real footguns. The Next.js specific guidance covers hydration testing and App Router navigation, stuff that's easy to get wrong if you're coming from traditional client-side testing. Heavy on best practices, light on hand-holding, which feels right for a reference guide you'll actually consult mid-task.