tdd

Write one failing test that describes desired behavior through a public interface, then write only enough code to make it pass. Repeat one slice at a time, letting each cycle inform the next test rather than writing all tests upfront.

A seam is the public boundary where you observe behavior without reaching inside the code, such as an interface or public API. Tests live at seams agreed with the user before writing, never against private methods or internal collaborators.

Mock only at system boundaries such as external APIs, databases, time, randomness, and 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 computes its expected value the same way the implementation does, so it passes by construction and can never catch a bug. Expected values must come from an independent source such as a known literal, worked example, or specification.