---
description: Diagnose a failing test, build, or runtime error
argument-hint: "[error message, test name, or stack trace]"
subagent: true
fork: true
---
Diagnose the following failure: $@

Structure your answer as:
1. **What failed** — one-sentence summary; quote the exact error line
2. **Likely root cause** — ranked list, most probable first
3. **Minimal fix** — the smallest change that would resolve it
4. **How to verify** — the exact command or test to confirm the fix
5. **How to prevent** — a regression test, invariant, or guard to add

Be specific. If multiple causes are plausible, name each and give a quick disambiguation check (a log line, a flag, a one-line probe) to tell them apart. Do not speculate beyond what the error message and the code actually show — if you need more data, say so explicitly.