regression-root-cause-analyzer
Installation
SKILL.md
Regression Root Cause Analyzer
A regression has one advantage over a greenfield bug: somewhere in history there is a commit where it worked. The entire skill is exploiting that advantage. Do not read code first — read history first.
Step 1 — Establish the good/bad bounds
| What you know | Good bound | Bad bound |
|---|---|---|
| "Worked in v2.3, broken in v2.4" | v2.3 tag |
v2.4 tag |
| "Broke sometime this week" | Monday's first commit | HEAD |
| "Test X just started failing in CI" | Last green CI SHA | First red CI SHA |
| "Broke after I pulled" | ORIG_HEAD or reflog |
HEAD |
| "No idea when" | Oldest tag — or bail (§ edge cases) | HEAD |
Verify both bounds before bisecting. Check out good → run → confirm green. Check out bad → run → confirm red. If either bound is wrong, bisect will converge on garbage.
Step 2 — Bisect
git bisect is log₂(N), so even 1000 commits is 10 runs. Automate if the oracle is scriptable: