tdd

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

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 and SDK-style interfaces to make boundaries mockable.

Implementation-coupled tests mock internal collaborators or verify through side channels, so they break on refactors without behavior changes. Tautological tests recompute the expected value the same way the code does, so they pass by construction and can never catch a bug.

Skip tests for assignments, logic-free getters and setters, constants, pass-through wiring, framework behavior, and anything already covered through a higher stable seam. Before writing a test, name the meaningful bug it would catch; if you cannot, do not write it.