Guides test-driven development using the red-green-refactor loop with vertical slices.
npx skills add https://github.com/scoots31/engineering-playbook --skill tdd-scoots31
Please help me install this Agent Skill. Skill: tdd Source: https://github.com/scoots31/engineering-playbook/tree/main/skills/tdd Command: npx skills add https://github.com/scoots31/engineering-playbook --skill tdd-scoots31
High-intent search queries and answers about installing and using this skill.
Write one failing test for a single behavior (red), write the minimal code to make it pass (green), then refactor while tests stay green. Repeat this cycle per behavior rather than writing all tests upfront.
Good tests verify observable behavior through public interfaces and survive internal refactors. Bad tests mock internal collaborators, test private methods, or assert on call counts, breaking whenever implementation changes without behavior changing.
Mock only at system boundaries such as external APIs, databases, time, and randomness. Never mock your own classes or internal collaborators, since that couples tests to implementation details.
Tests break on refactor when they are coupled to implementation details like internal function names or private methods. Rewrite them to exercise public interfaces and assert on observable outcomes instead of internal calls.
No. Writing all tests first is horizontal slicing and produces tests of imagined behavior. Use vertical slices instead: one test, one implementation, repeated, so each test responds to what the previous cycle revealed.
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