# Debug

> Use for bugs, unexpected behavior, regressions, or failing tests you did not write.

- Skill: `concertonotes/debug` (Agent Skill)
- Install (CLI): `npx skillmds@latest add concertonotes/debug`
- Raw SKILL.md: https://api.skillmd.com/api/skills/concertonotes/debug/raw
- Safety review: PASS (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: ConcertoNotes (https://skillmd.com/u/concertonotes)
- Updated: 2026-08-19
- Page: https://skillmd.com/skills/concertonotes/debug

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# Debug

1. **Reproduce.** Minimal deterministic repro. No repro -> no fix.
2. **Root-cause.** Ask "why" until one cause explains *all* symptoms. Symptoms != cause.
3. **Regression test first** (red), then fix at the root (green). Use `tdd`.

## Don't
- Guess. Read the code and the full error.
- Change two things at once.
- Stop at the first plausible cause.

3 failed hypotheses -> re-read from scratch. Done = repro + regression test + root cause named + green.