tdd

Write one failing test against a public interface first, then implement only enough code to make it pass, and repeat one slice at a time. Never write all tests upfront or add speculative features beyond what the current failing test demands.

A seam is the public boundary where you observe behavior without reaching inside the code, such as an exported function of a module. Tests should live only at pre-agreed seams confirmed with the user, never against internal helpers or private methods.

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

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.