anti-bug-prevention

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• SKILL.md

SKILL.md (first lines)

# Anti-bug prevention pipeline

> Sanitized example skill. Each pattern below cost a real production incident or a near-miss.

Apply this on any change larger than a single working day — not "when I remember."

## Pattern 1 — Re-review your own fixes is a separate step

**Problem:** "Fixed per review" ≠ "Reviewed." Your own fixes can contain fresh bugs. In one real case, review round 1 found 6 blockers; after applying the fixes, round 2 (same reviewers, focused on the fixes) found **2 more CRITICAL + 4 HIGH — inside the fixes**. Deploying after round 1 would have been a production failure.

**Rule:**
1. Round 1 review (type reviewer / DB reviewer / QA / prompt engineer)
2. Apply fixes
3. **Round 2 review** — same agents, explicit focus "check my fixes"
4. Only after round 2 — deploy

**When mandatory:**
- Any review found ≥ 2 CRITICAL/HIGH blockers → re-review required
- Migration / DB schema / threading-related fixes → re-review even on 1 blocker
- Multi-agent review (3+ agents found different things) → re-review required — fixes can conflict with each other

**Anti-pattern:** "typecheck green after my fixes = ready to deploy." Typecheck catches syntax, not semantics. Re-review catches semantics.

## Pattern 2 — Anti-bug catalogue at design phase, not in code

**Problem:** "Tests will catch bugs" — no. Tests catch what you predicted. Categories of bugs you didn't foresee are the mine under the floor.

**Rule:** before writing code, ask a design agent (architect / board) an explicit question: "find 10 potential bugs in this plan." Each bug → assigned to a specific sprint day + a defensive recipe in code from day one.

**Template:**
```
| # | Bug | Mitigation |
|---|---|---|
| B1 | CVPixelBuffer use-after-free (Vision async) | Day 2: pixelBuffer.deepCopy(via: pool) as the first line |
| B2 | Priority inversion: audio thread waits on vision queue lock | Day 3: os_unfair_lock_trylock or an SPSC ring-buffer |
| ... | ... | ... |
```

**When mandatory:**
- Sprint > 3 days
- Touches threading / memory ownership / IPC / migrations
- Extends a hot-path module (audio engine, video encoder, ML inference)

**Anti-pattern:** "we'll solve it if it comes up." In production "comes up" = an incident with an emergency hotfix.

## Pattern 3 — Quality gates between days: atomic vs safe-rollback

**Problem:** "On day 5 something broke" in a sprint without gates = roll back all 5 days at once. An atomic block for days 1-4 plus safe stops after days 5/7 gives you two rollback points instead of one all-or-nothing.

**Rule:** for a sprint > 5 days:

1. **Atomic block** — a sequence of days that CANNOT be stopped mid-way. Usually: DB migration + code-wide rename + critical refactor. If you stop, old code + new DB = production crash.

2. **Safe rollback points** — points where you can stop the sprint and ship a smaller version. Usually: after Phase A (without Phase B), after core-feature (without UI polish).

3. **Quality gate after each day** — a specific grep / SQL / unit-test that is green and BLOCKS the start of the next day.

**Template:**
```
| Day | Quality gate (blocker for next day) | Checkpoint |
|---|---|---|
| 1 | grep "OldTableName" = 0 matches in src/, integration test green | — |
| 2 | unit tests green + audio invariant manual check | — |
| 3 | AudioMixerNoObserverTest green (regression FIRST) | ⚠️ checkpoint #1 |
```

**Anti-pattern:** "we'll check everything on day 10." By day 10 it's unclear when what broke. Gates localize it.

## Pattern 4 — Surprise zones with ENV fallbacks

**Problem:** confidence in new patterns is rarely 100%. Active-speaker-detection accuracy may be 70% on your data instead of 85% in the research paper. Without a fallback = redeploy.

**Rule:** every zone of uncertainty (new ML threshold, performance assumption, third-party API behavior) → an ENV switch in code from day one. Default = your hypothesis. If there's a problem — `ENV=value` in one minute, no redeploy.

**Template:**
```
- ACTIVE_SPEAKER_HEURISTIC_DISABLED=true  → fall back to a simple rule
- SCREEN_FACE_MATCH_THRESHOLD=0.92        → tune threshold for screen
- VOICE_MATCH_THRESHOLD=0.85              → tune voice threshold
- FUSION_FEATURE_ENABLED=false            → kill-switch the whole feature without a code rollback
```

**Document in the sprint file** under a "Surprise zones" section — which zones, which ENV for each, and the trigger to activate it (e.g. "if accuracy < 70% on the first 5 records").

**Anti-pattern:** hardcoded magic numbers. In production you'll learn that 0.7 is too high / too low — and have nothing to tune.

## Pattern 5 — Idempotent SQL migrations = default

**Problem:** a hotfix once exposed two historical migrations that fail on a clean DB (`column "isPersonal" does not exist`, `relation "MediaAsset" does not exist`). Cause: `prisma db push` created columns/tables without migrations; it survived on prod but failed on a fresh dev/staging DB.

**Rule:** all DDL operations are idempotent by default.

```sql
-- ALWAYS:
ALTER TABLE "X" ADD COLUMN IF NOT EXISTS "y" TEXT;
CREATE TABLE IF NOT EXISTS "Z" (...);
CREATE INDEX IF NOT EXISTS "X_idx" ON "X"(...);

-- FK constraints via DO-block (no native IF NOT EXISTS):
DO $$ BEGIN
  ALTER TABLE "X" ADD CONSTRAINT "X_y_fkey" FOREIGN KEY ...;
EXCEPTION WHEN duplicate_object THEN NULL; END $$;
```

**Cost on prod:** zero (no-op if it already exists).
**Benefit on dev/staging:** works from any DB snapshot, safe to re-run.

**Anti-pattern:** `ALTER TABLE x ADD COLUMN y` without `IF NOT EXISTS`. It passes once; the second time = `column already exists` → migration failed, and downstream migrations too.

## Pattern 6 — Test files → exclude from production tsconfig

**Problem:** a production build once failed on the deploy host with `Cannot find module 'vitest'` from `src/foo.test.ts`. Cause: the host doesn't install devDependencies, `tsc` tries to compile ALL `.ts` under src/ including tests, and vitest is unresolved.

**Rule:** in every `tsconfig.json` that compiles for production:

```json
{
  "include": ["src/**/*.ts"],
  "exclude": ["src/**/*.test.ts", "src/**/*.integration.test.ts"]
}

From signalo-rust/ai-guardrails, MIT.