tdd

Test-driven development with red-green-refactor cycles involves pinning an observable behavior with a failing test, writing minimal code to pass it, then refactoring while green. This workflow automates that disciplined cycle around vertical slices of a feature.

Vertical slice testing in TDD exercises the public contract of a feature end-to-end with a single test per task plan. This approach verifies external behavior at API boundaries or module interfaces rather than internal implementation details.

Use TDD for feature development when tasks have clear external contracts, such as API boundaries or module interfaces. It requires defining a public interface upfront so tests can pin observable behavior and guide incremental implementation.

Start a TDD cycle by defining the public interface, writing a tracer bullet, then pinning one behavior with a failing test. Implement the minimum code to make the test green before refactoring, capturing each step in a task plan.

You should never refactor while tests are red because failing tests indicate incomplete or incorrect behavior. Refactoring must only occur while green to ensure the external contract remains intact and observable behavior does not break.

TDD with this workflow focuses on public-interface testing rather than internal module details. Tests verify external behavior at API boundaries or module interfaces, which enables safe refactoring without breaking pinned observable contracts.