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

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