test-driven-development

Red-green-refactor discipline for features and bug fixes: write one minimal failing test, watch it fail for the expected reason, write the least code that passes, refactor only on green. Use when implementing any feature or bugfix, when fixing a bug (the reproducing test comes first and the fix follows), when tempted to backfill tests after the code, or when a new test passes on its first run and therefore proves nothing yet. The bright line: production code is written only against a test you have watched fail; code that preceded its test is deleted and redone test-first, because adapting it quietly converts test-first into test-after. Exceptions — throwaway prototypes, generated code, pure configuration — are agreed with the user, not self-granted. Hands red-green evidence to verification-before-completion. Not for designing suite architecture or coverage strategy, and not for diagnosing an unexplained failure (systematic-debugging owns diagnosis and returns here for the fix).