/reproduce

No fix without an observed failure. The deliverable is a regression test that fails before the fix and passes after it.

Preflight

• Find the repo's real test command (package.json scripts, Makefile, pyproject, go test, cargo test). Run it once on HEAD and record the baseline. If the baseline is already red elsewhere, note those failures. They aren't yours.
• Never stash, reset, or checkout over the user's working tree. History work happens in a separate worktree (step 4).

Decision tree

1. Classify the report

• Exact failing test or command given → run it, go to 3.
• Error message or stack trace → find the throw site (rg -n "<message>"), read its callers and existing tests, go to 2.
• Vague ("checkout is slow", "sometimes fails") → ask for one concrete input, environment, and expected vs. actual behavior. Max 3 questions, then stop until answered.