tdd

Write one failing test at a public interface seam first, then write only enough code to make it pass, and repeat one vertical slice at a time. Do not write all tests upfront or add speculative features beyond the current failing test.

A seam is the public boundary where you observe behavior without reaching inside the code, such as a module's public interface. Tests live at seams and never against internals, so they survive refactors that change implementation but not behavior.

Mock only at system boundaries: external APIs, time, randomness, and sometimes databases or file systems. Never mock your own classes or internal collaborators, and prefer dependency injection plus SDK-style interfaces to make boundary mocking straightforward.

Tests break during 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 instead.

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