tdd

Write one failing test that describes desired behavior through a public interface, then write only enough code to make it pass. Repeat one vertical slice at a time, and keep refactoring out of the loop until the review stage.

A seam is the public boundary where you observe behavior without reaching inside a module. Tests should live only at pre-agreed seams confirmed with the user, never against private methods or internal collaborators.

Mock only at system boundaries such as external APIs, time, randomness, and sometimes databases or the file system. Never mock your own classes or internal collaborators, and prefer dependency injection plus SDK-style interfaces to make boundaries easy to mock.

Tests break on refactoring when they are coupled to implementation details, such as mocking internal collaborators, testing private methods, or asserting on call counts. Rewrite them to verify observable behavior through public interfaces so they survive internal changes.

A tautological test recomputes its expected value the same way the implementation does, so it passes by construction and can never catch a bug. Use independent expected values from known literals, worked examples, or the specification instead.