debug-root-cause

Debug a concrete, existing failure with method instead of guesses — reproduce and isolate it, gather evidence from logs, traces, diffs and environment, run a hypothesis ledger with the smallest possible experiments, fix the root cause only after a failing test proves it ("red"), verify the fix turns green without breaking the suite, and record a short bug report. Use this skill for any actual failure — a red test, a build or CI failure, a runtime error or stack trace, a flaky test, a "worked before, broken now" regression, "test đỏ", "lỗi chỉ fail trên CI", "sao lại lỗi" — before proposing or writing any fix. Skip it for net-new features, refactors without a failure, open-ended investigation or research breadth-first (that is research-swarm territory), code review of a healthy diff, and problems outside the codebase's control such as vendor or infrastructure outages, for which you escalate and stop.