---
name: debug
description: "Interactive debugging workflow with hypothesis-driven probe loop. Use when: unknown bugs, script errors, silent failures, troubleshooting. Not for: known bugs (use bug-fix), GitHub issue analysis (use issue-analyze), code understanding (use code-explore). Output: debug report with probe journal + root cause + fix."
allowed-tools: Read, Grep, Glob, Edit, Write, Bash, Skill, mcp__codex__codex, mcp__codex__codex-reply
---

# Debug Skill

## Trigger

- Keywords: debug, 除錯, troubleshoot, diagnose, 查問題, 找 bug, 為什麼不動, 為什麼不 work, something wrong, not working

## When NOT to Use

| Scenario | Alternative |
|----------|------------|
| GitHub Issue / PR thread 分析 | `/issue-analyze` |
| 已知根因、直接修復 | `/bug-fix` |
| 理解程式碼如何運作 | `/code-explore` |
| 需要雙視角確認邏輯 | `/code-investigate` |
| 部署後唯讀功能驗證 | `/feature-verify` |

## Prohibited Actions

```
❌ git add | git commit | git push — per @rules/git-workflow.md
```

This skill debugs and may fix code but does **not** commit. `/precommit` is a quality gate only. To commit, the user must invoke `/smart-commit --execute` separately.

## Workflow

```
Phase 0    Phase 1     Phase 2          Phase 3           Phase 4        Phase 5
Intake  →  Classify →  Probe Loop   →   Root Cause    →   Fix +       →  Report
+ Repro    (Taxonomy)  (max 6)          Confirmation      Verify
  │          │           │                 │                │              │
  ▼          ▼           ▼                 ▼                ▼              ▼
Execute    refs/       Bash            /seek-verdict    Edit+test     --export
reproduce  failure-    /codex-         /code-investigate /bug-fix      (optional)
           taxonomy    brainstorm      (optional)
```

## Phase 0: Intake + Reproduce

| Step | Action | Output |
|------|--------|--------|
| 0a | 解析問題描述（症狀、範圍、環境） | 結構化問題描述 |
| 0b | 嘗試重現（執行命令/腳本） | 預期 vs 實際結果 |
| 0c | 重現成功？ | Yes → Phase 1 / No → `⚠️ Need Human` |

**Repro Contract**: 根因聲稱必須基於可重現的觀察，不可基於靜態推測。

## Phase 1: Classify

根據觀察到的失敗模式，使用 Failure Taxonomy 分類並選擇 first-probe 策略。

See `references/failure-taxonomy.md` for:
- 6 problem types + detection signals + first-probe commands
- Classification decision tree
- Escalation paths

## Phase 2: Probe Loop

假設驅動的互動式探測迴圈。See `references/probe-protocol.md` for full rules.

每個 probe 是一個假設測試：

```
1. 定義假設 H
2. 設計探測命令 C
3. 預測「若 H 成立 → 期望 O」
4. 執行 C → 觀察實際結果
5. 更新假設集合
6. 選下一個最具鑑別力的探測
```

### Termination

| Condition | Action |
|-----------|--------|
| 根因已定位 + ≥1 執行結果佐證 | **Stop** → Phase 3 |
| ≥2 競爭假設無法區分 | **Brainstorm** → `/codex-brainstorm` 對抗辯論 |
| 連續 2 輪無新資訊 | **Escalate** → `⚠️ Need Human` |
| Max rounds (6) reached | **Escalate** → `⚠️ Need Human` |

### `/codex-brainstorm` Integration

When ≥2 equally credible hypotheses exist:
1. Pause probe loop
2. Invoke `/codex-brainstorm` via Skill tool — topic = hypothesis comparison
3. Use Nash equilibrium result as new primary hypothesis
4. Resume probe loop to verify

## Phase 3: Root Cause Confirmation

| Step | Action | Required |
|------|--------|----------|
| 3a | 總結根因（What + Why + Impact + Evidence） | ✅ |
| 3b | `/seek-verdict --intent confirm` — Codex 獨立驗證 | ✅ Mandatory |
| 3c | `/code-investigate` 雙視角深入驗證 | Optional |
| 3d | `/git-investigate` 追蹤引入點（若 regression） | Conditional |

### `/seek-verdict` Integration (Step 3b — Mandatory)

Invoke `/seek-verdict --intent confirm` for independent root cause verification.

**Anti-anchoring contract** (per `@skills/seek-verdict/SKILL.md`):
- **Fresh thread**: new `mcp__codex__codex` call, never reuse
- **No Claude conclusions**: prompt must not contain Claude's probe findings or root cause judgment
- **Finding packet only**: provide symptoms, files, observed behavior — let Codex judge independently

**Result routing**:

| Result | Action |
|--------|--------|
| ACTIONABLE (confirm) | → Phase 4 |
| NON_ACTIONABLE (high confidence) | Re-enter Phase 2 or `⚠️ Need Human` |
| UNCERTAIN | → Phase 4 (conservative), note low confidence |

## Phase 4: Fix + Verify

| Condition | Path |
|-----------|------|
| Simple fix (≤3 lines) | Edit + regression test + verify |
| Complex fix (multi-file) | Delegate to `/bug-fix` |
| Architecture-level change | `⛔ Need Human` — report root cause + recommendation |

**All fix paths require regression test** per `@rules/testing.md` evidence model. Bug type → test level mapping per `@skills/bug-fix/SKILL.md`.

Simple fix path:
1. Edit fix
2. Write regression test
3. Re-execute Phase 0 repro command (verify fix)
4. If code file changed → enter review loop (auto-loop)

## Phase 5: Debug Report

Output format: see `references/report-template.md`.

Default: conversation-only output. With `--export [path]`, write to file (with redaction per Probe Safety Rules).

## Probe Safety Rules

| Rule | Description |
|------|-------------|
| Read-first default | 預設探測為唯讀：`cat`, `curl -s`, `grep`, `ls`, `git log` |
| Write-probe gate | 可能修改狀態的探測標記 `[WRITE_PROBE]`，非 sandbox 環境需用戶確認 |
| Timeout | 每個探測命令 timeout ≤ 30 秒 |
| Output budget | 單次探測輸出 ≤ 500 行（超出 truncate） |
| Redaction | 禁止記錄 API keys, tokens, passwords（per `@rules/security.md`），以 `[REDACTED]` 取代 |
| Deny list | 禁止：`rm`, `drop`, `delete`, `truncate`, 任何 destructive 操作 |

## Config

| Key | Default | Description |
|-----|---------|-------------|
| `debug.max_probe_rounds` | 6 | Maximum probe loop iterations |

## Review Loop

**MUST re-review after fix until PASS** (per @rules/auto-loop.md)

```
Fix → Review → Issues found → Fix again → ... → ✅ Pass → Next step
```

## Doc Sync

Doc Sync is governed by `@rules/auto-loop.md` (behavior-layer rule). After precommit pass, triggers conditionally when changes map to `docs/features/`.

## Output

```markdown
## Debug Report: <title>
- **Type**: <classification>
- **Root Cause**: <what + why>
- **Fix**: <change description>
- **Verdict**: <seek-verdict result>
- **Probe Rounds**: <N>
```

## Verification

- [ ] Phase 0 reproduced the issue
- [ ] Problem classified using failure taxonomy
- [ ] Probe journal recorded for each round
- [ ] `/seek-verdict --intent confirm` executed (Phase 3)
- [ ] Root cause has ≥1 execution evidence
- [ ] Regression test written (if fix applied)
- [ ] Review loop completed (if code changed)
- [ ] No `git add/commit/push` executed

## References

| File | Purpose | When to Read |
|------|---------|--------------|
| `references/failure-taxonomy.md` | Problem classification + first-probe routing | Phase 1 |
| `references/probe-protocol.md` | Probe Loop rules + termination criteria | Phase 2 |
| `references/report-template.md` | Debug Report template + export format | Phase 5 |

## Examples

### Script Bug

```
Input: /debug bash scripts/deploy.sh 回傳 exit code 1
Phase 0: 執行 bash scripts/deploy.sh → 確認失敗
Phase 1: Script Bug
Phase 2: R1: bash -x trace → 發現 line 42 curl 失敗
         R2: 直接 curl endpoint → 404
         R3: 檢查 URL → 路徑缺少 /api prefix → Stop
Phase 3: /seek-verdict confirm → ACTIONABLE
Phase 4: Edit line 42 + regression test
Phase 5: Debug Report
```

### API Error

```
Input: /debug API 回傳空陣列但資料庫有資料
Phase 0: curl API endpoint → 確認回傳 []
Phase 1: Silent Failure（表面正常但結果錯誤）
Phase 2: R1: 檢查 query filter → 發現 status 欄位名稱不符
         R2: 直接 DB query → 有資料 → Stop
Phase 3: /seek-verdict confirm → ACTIONABLE
Phase 4: 修正 field name + unit test
Phase 5: Debug Report
```

### Silent Failure

```
Input: /debug ks-status.sh 回傳 ready:false 但 deployment 已就緒
Phase 0: 執行 bash ks-status.sh → ready:false, 缺少 replicas 欄位
Phase 1: Silent Failure（404 被 fallback 吞掉）
Phase 2: R1: 直接 curl API → invalid JSON → R2: 原始回應 → 404
         R3: 改用 apps/v1 路徑 → 200 + 完整資料 → Stop
Phase 3: /seek-verdict confirm → ACTIONABLE
Phase 4: api/v1 → apps/v1（1 word fix）+ test
Phase 5: Debug Report
```