Defense in Depth

Philosophy

MULTIPLE INDEPENDENT VALIDATION LAYERS
Each layer catches what others miss

One check can fail silently. Four layers rarely do.

The Four Layers

Layer 1: Type/Syntax

What: Static analysis, types, linting
Catches: Typos, syntax errors, type mismatches
Run: First, before anything else

Layer 2: Unit Tests

What: Individual function/module tests
Catches: Logic errors, edge cases, regressions
Run: After each code change

Layer 3: Integration Tests

What: Component interaction tests
Catches: Interface mismatches, data flow errors
Run: After unit tests pass

Layer 4: End-to-End

What: Full system tests, user scenarios
Catches: Missing requirements, workflow bugs
Run: Before claiming complete

The Pipeline

Code Change
    ↓
[Layer 1: Lint/Type Check]
    ↓ PASS?
[Layer 2: Unit Tests]
    ↓ PASS?
[Layer 3: Integration Tests]
    ↓ PASS?
[Layer 4: E2E / Manual Check]
    ↓ PASS?
Done

Fail at any layer → FIX before proceeding

Why Every Layer Matters

Without This Layer	What Escapes
No type checks	Undefined calls crash at runtime
No unit tests	Logic bugs slip through
No integration	Components don't work together
No E2E	User workflows broken

Implementation Patterns

For Code Changes

1. Make change
2. Run linter immediately
3. Run affected unit tests
4. Run integration suite
5. Test the actual feature

For Bug Fixes

1. Write failing test first
2. Fix the bug
3. Run all tests (regression check)
4. Verify original symptom gone

For New Features

1. Write unit tests for new logic
2. Implement feature
3. Add integration tests
4. Verify against requirements

Layer-Specific Commands

Configure per project:

layer_1:
  command: "npm run lint && npm run typecheck"
  when: "On save, before commit"
  
layer_2:
  command: "npm run test:unit"
  when: "After code changes"
  
layer_3:
  command: "npm run test:integration"
  when: "After unit tests pass"
  
layer_4:
  command: "npm run test:e2e"
  when: "Before claiming done"

Recovery Procedures

Layer 1 Fails

→ Syntax/type error in your code
→ Fix immediately, don't proceed
→ Never commit with lint errors

Layer 2 Fails

→ Logic error or regression
→ Check: Did you break something?
→ Check: Is your new test wrong?
→ Fix root cause, not symptoms

Layer 3 Fails

→ Interface/contract broken
→ Check: API changes?
→ Check: State management issues?
→ May need to update multiple files

Layer 4 Fails

→ User-visible problem
→ Trace back through layers
→ Ask: Which layer SHOULD have caught this?
→ Add test to that layer

Integration with Checker Agents

• pre-action-verifier → Runs before each layer
• assumption-checker → Validates test assumptions
• scope-boundary-checker → Ensures tests cover scope
• rollback-planner → Ready if pipeline fails