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.opencode/skills/fix/references/prevention-gate.md

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+# Prevention Gate
+
+After fixing a bug, prevent the same class of issues from recurring. This step is MANDATORY.
+
+## Core Principle
+
+A fix without prevention is incomplete. The same bug pattern WILL recur if you only patch the symptom.
+
+## Prevention Requirements (Check All That Apply)
+
+### 1. Regression Test (ALWAYS required)
+
+Every fix MUST have a test that:
+- **Fails** without the fix applied (proves the test catches the bug)
+- **Passes** with the fix applied (proves the fix works)
+
+```
+If no test framework exists:
+  → Add inline verification or assertion at minimum
+  → Note in report: "No test framework — added runtime assertion"
+```
+
+### 2. Defense-in-Depth Validation (When applicable)
+
+Apply layered validation from `ck:debug` defense-in-depth technique:
+
+| Layer | Apply When | Example |
+|-------|-----------|---------|
+| **Entry point validation** | Fix involves user/external input | Reject invalid input at API boundary |
+| **Business logic validation** | Fix involves data processing | Assert data makes sense for operation |
+| **Environment guards** | Fix involves env-sensitive operations | Prevent dangerous ops in wrong context |
+| **Debug instrumentation** | Fix was hard to diagnose | Add logging/context capture for forensics |
+
+**Rule:** Not every fix needs all 4 layers. Apply what's relevant. But ALWAYS consider each.
+
+### 3. Type Safety (When applicable)
+
+| Scenario | Prevention |
+|----------|-----------|
+| Null/undefined caused the bug | Add strict null checks, use `??` or `?.` |
+| Wrong type passed | Add type guard or runtime validation |
+| Missing property | Add required field to interface/type |
+| Implicit any | Add explicit types |
+
+### 4. Error Handling (When applicable)
+
+| Scenario | Prevention |
+|----------|-----------|
+| Unhandled promise rejection | Add `.catch()` or try/catch |
+| Missing error boundary | Add error boundary component |
+| Silent failure | Add explicit error logging |
+| No fallback for external dependency | Add timeout + fallback |
+
+## Verification Checklist (Before Completing Step 5)
+
+```
+□ Pre-fix state captured? (error messages, test output)
+□ Fix applied to ROOT CAUSE (not symptom)?
+□ Fresh verification run? (exact same commands as pre-fix)
+□ Before/after comparison documented?
+□ Regression test added? (fails without fix, passes with fix)
+□ Defense-in-depth layers considered? (applied where relevant)
+□ No new warnings/errors introduced?
+□ Parallel verification passed? (typecheck + lint + build + test)
+```
+
+## Output Format
+
+```
+Prevention measures applied:
+- Regression test: [test file:line] — covers [specific scenario]
+- Guard added: [file:line] — [description of guard]
+- Type safety: [file:line] — [what was strengthened]
+- Error handling: [file:line] — [what was added]
+
+Before/After comparison:
+- Before: [exact error/failure]
+- After: [exact success output]
+```
+
+## Quick Mode Prevention
+
+For trivial issues (type errors, lint), abbreviated prevention:
+- Regression test: optional (type system IS the test)
+- Parallel verification: typecheck + lint only
+- Defense-in-depth: skip (not applicable for type fixes)
+- Still require before/after comparison of typecheck output