tdd

Start by agreeing on the public seams to test, then write one failing test that describes observable behavior, implement only enough code to pass it, and repeat one vertical slice at a time. Refactoring happens later in code review, not inside the loop.

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

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 boundary mocking straightforward.

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

A tautological test recomputes the 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.