systematic-debugging

Find a bug's root cause before changing code: build a tight red-capable feedback loop, gather evidence at component boundaries, rank falsifiable hypotheses, then fix once and prove it with a regression test. Use for any test failure, production bug, unexpected behavior, performance regression or build failure — especially under time pressure or after a fix that did not work. Not for exploratory "make it nicer" refactors, and not for a known mechanical change with an already-proven cause.