This puts tests first in your development workflow, red-green-refactor style. Write a failing test, make it pass with minimal code, then clean up. The real win is the Prove-It Pattern for bugs: reproduce the issue with a test before you fix anything, so you know it's actually fixed and won't regress. The guide includes practical stuff like the test pyramid (80% unit, 15% integration, 5% E2E), testing state over interactions, and when to use real implementations versus mocks. If you're working with an AI agent on a codebase without tests, you're flying blind. This gives you the proof that changes actually work.
npx skills add https://github.com/addyosmani/agent-skills --skill test-driven-development