TDD (Red-Green-Refactor)

Use this skill whenever you need reliable, incremental implementation with test-first development.

When to use

• Building new behavior where correctness matters
• Fixing bugs with a reproducible failing test
• Refactoring safely with behavior locked by tests
• Working in codebases where regression risk is high

Workflow

1. Red

   • Write or update a test that describes the intended behavior.
   • Run tests and verify the new/changed test fails for the right reason.

2. Green

   • Implement the smallest possible code change to make the failing test pass.
   • Run the smallest relevant test scope first, then broader tests.

3. Refactor

   • Improve naming, remove duplication, simplify logic.
   • Keep behavior unchanged and run tests again.

4. Repeat

   • Continue in small cycles until acceptance criteria are complete.

Guardrails

• Never skip the failing test step.
• Keep each cycle small and reviewable.
• Prefer deterministic tests over timing/network fragile tests.
• If behavior is unclear, stop and ask for clarification before coding.

Deliverables

• Final passing tests for all new/changed behavior
• Minimal implementation changes
• Brief summary of what each red-green-refactor cycle achieved