legacy-debugging

Debug code you didn't write and can't fully trust — unfamiliar, poorly documented, or untested subsystems — using history archaeology, boundary instrumentation, minimal harnesses, and history bisection, while resisting the urge to rewrite. Trigger this when a failure lives in code that is foreign to you AND lacks reliable documentation or tests: inherited services, vendored code, an unowned module, "nobody knows how this works". For failures in code you understand and can navigate normally, use debugging-methodology instead — this skill adds the archaeology layer for when reading the code alone doesn't tell you what it's supposed to do.