| name | test-driven-development |
| description | Apply a focused red-green-refactor workflow for behavior changes and bug fixes. Use when the user asks for TDD, regression tests, test-first implementation, bug fixes with tests, or when a change has clear observable behavior that should be protected by tests. |
Test Driven Development
Use this skill when the safest path is to lock behavior down before implementation.
Red-Green-Refactor
- Identify the observable behavior to protect. Keep the test scope narrow enough to fail for the intended reason.
- Find the repository's existing test style, framework, fixtures, and naming conventions before adding a test.
- Write or update a focused failing test first. For a bug fix, the test should reproduce the bug. For a feature, it should encode the requested behavior.
- Run the targeted test and confirm it fails for the expected reason. If it passes, improve the test before editing production code.
- Implement the smallest production change that can make the test pass.
- Run the targeted test again. Then run the nearest relevant broader test or lint command when the blast radius justifies it.
- Refactor only after the behavior is green, keeping tests green after each meaningful cleanup.
When Test-First Is Not Practical
If a test-first loop is not practical, state why before implementation. Good reasons include missing test infrastructure, changes limited to prompt text or documentation, a purely mechanical configuration change, or a test that would require unsafe external services.
When skipping a test, still choose the best available verification: static check, existing suite, manual reproducer, CLI smoke test, or diff review.
Guardrails
- Do not add brittle tests that assert incidental implementation details.
- Prefer one focused regression over a broad snapshot unless the project already uses snapshots for this surface.
- Keep fixtures minimal and local to the test unless the repository has shared fixture patterns.
- Do not rewrite existing tests just to fit a new style.
