Test-driven development with red-green-refactor loop. Use when user wants to build features or fix bugs using TDD, mentions "red-green-refactor", wants integration tests, or asks for test-first development.

Test Driven Development Philosophy Core principle : Tests should verify behavior through public interfaces, not implementation details. Code can change entirely; tests shouldn't. Good tests are integration style: they exercise real code paths through public APIs. They describe what the system does, not how it does it. A good test reads like a specification "user can checkout with valid cart" tells you exactly what capability exists. These tests survive refactors because they don't care about internal structure. Bad tests are coupled to implementation. They mock internal collaborators, test private methods, or verify through external means (like querying a database directly instead of using the interface). The warning sign: your test breaks when you refactor, but behavior hasn't changed. If you rename an internal function and tests fail, those tests were testing implementation, not behavior. See [tests.md](tests.md) for examples and [mocking.md](mocking.md) for mocking guidelines. Anti Pattern: Horizontal Slices DO NOT write all tests first, then all implementation. This is "horizontal slicing" treating RED as "write all tests" and GREEN as "write all code." This produces crap tests : Tests written in bulk test imagined behavior, not actual behavior You end up testing the shape of things (data structures, function signatures) rather than user facing behavior Tests become insensitive to real changes they pass when behavior breaks, fail when behavior is fine You outrun your headlights, committing to test structure before understanding the implementation Correct approach : Vertical slices via tracer bullets. One test → one implementation → repeat. Each test responds to what you learned from the previous cycle. Because you just wrote the code, you know exactly what behavior matters and how to verify it. Workflow 1. Planning Before writing any code: [ ] Confirm with user what interface changes are needed [ ] Confirm with user which behaviors to test (prioritize) [ ] Identify opportunities for [deep modules](deep modules.md) (small interface, deep implementation) [ ] Design interfaces for [testability](interface design.md) [ ] List the behaviors to test (not implementation steps) [ ] Get user approval on the plan Ask: "What should the public interface look like? Which behaviors are most important to test?" You can't test everything. Confirm with the user exactly which behaviors matter most. Focus testing effort on critical paths and complex logic, not every possible edge case. 2. Tracer Bullet Write ONE test that confirms ONE thing about the system: This is your tracer bullet proves the path works end to end. 3. Incremental Loop For each remaining behavior: Rules: One test at a time Only enough code to pass current test Don't anticipate future tests Keep tests focused on observable behavior 4. Refactor After all tests pass, look for [refactor candidates](refactoring.md): [ ] Extract duplication [ ] Deepen modules (move complexity behind simple interfaces) [ ] Apply SOLID principles where natural [ ] Consider what new code reveals about existing code [ ] Run tests after each refactor step Never refactor while RED. Get to GREEN first. Checklist Per Cycle

tdd