Agent Skills: TDD Refactor Guard

TDD Refactor Guard

A safety gate that runs before any AI-assisted refactoring. Ensures tests exist to catch regressions. If coverage is insufficient, generates characterization tests first.

When This Skill Activates

Use this skill when the user:

• Says "refactor this" or "clean this up"
• Wants AI to "restructure" or "reorganize" code
• Asks to "extract", "inline", "rename", or "simplify" existing code
• Plans to migrate between patterns (e.g., MVC → MVVM, CoreData → SwiftData)
• Wants AI to "improve" or "modernize" existing code

Why This Guard Exists

Without guard:               With guard:
"Claude, refactor this"      "Claude, refactor this"
  → AI rewrites code           → Guard checks for tests
  → Something breaks           → Tests missing → generate them
  → You don't know what        → Tests pass → proceed with refactor
  → Debugging nightmare         → Tests fail → exact regression found

Process

Step 1: Identify Scope

What code is being refactored?

Ask user: "Which files/classes are you planning to refactor?"

Or detect from context:

Glob: [files mentioned in conversation]
Read: each file to understand scope

Determine:

• [ ] Files involved
• [ ] Public API surface (methods, properties)
• [ ] Dependencies (what calls this code, what it calls)
• [ ] Side effects (network, disk, notifications, UI updates)

Step 2: Check Existing Test Coverage

Grep: "class.*Tests.*XCTestCase|@Suite.*Tests|struct.*Tests" in test targets
Grep: "@Test|func test" that reference the classes being refactored

Coverage Assessment

| Level | Criteria | Action | |-------|----------|--------| | Good (> 80%) | Most public methods have tests for happy + error paths | Proceed with refactoring | | Partial (40-80%) | Some methods tested, missing edge cases | Add characterization tests for uncovered methods | | Minimal (< 40%) | Few or no tests | Generate full characterization test suite first | | None (0%) | No test file exists | STOP — create characterization tests before any refactoring |

Step 3: Evaluate Test Quality

Even if tests exist, check quality:

// ❌ Low-quality test — doesn't actually verify behavior
@Test("test items")
func testItems() {
    let vm = ViewModel()
    vm.loadItems()
    #expect(true)  // Always passes, tests nothing
}

// ❌ Tests implementation, not behavior
@Test("calls repository")
func testCallsRepo() {
    let vm = ViewModel()
    vm.loadItems()
    #expect(mockRepo.fetchCallCount == 1)  // Brittle
}

// ✅ Tests observable behavior
@Test("loads items into published array")
func testLoadsItems() async {
    let vm = ViewModel(repo: MockRepo(items: [.sample]))
    await vm.loadItems()
    #expect(vm.items.count == 1)
    #expect(vm.items.first?.title == "Sample")
}

Quality checklist:

• [ ] Tests verify outputs/state, not implementation details
• [ ] Both happy path and error paths covered
• [ ] Edge cases (empty, nil, boundary) tested
• [ ] Async behavior properly awaited
• [ ] No #expect(true) or always-passing assertions

Step 4: Generate Missing Tests

If coverage is insufficient, use characterization-test-generator/:

Before refactoring [ClassName], generate characterization tests:
1. Read the source file
2. Identify all public methods
3. Generate tests that capture CURRENT behavior
4. Run and verify all pass

Step 5: Green Light / Red Light

Green Light — Proceed

## Refactor Guard: ✅ PROCEED

**Files**: ItemManager.swift, ItemRepository.swift
**Test coverage**: Good (12 tests covering 8/9 public methods)
**Test quality**: Adequate — tests verify behavior, not implementation

Safe to refactor. Run tests after each change:
`xcodebuild test -scheme YourApp`

Yellow Light — Needs Work

## Refactor Guard: ⚠️ NEEDS TESTS

**Files**: ItemManager.swift, ItemRepository.swift
**Test coverage**: Partial (5 tests covering 4/9 public methods)
**Missing coverage**:
- `ItemManager.sort()` — no test
- `ItemManager.filter(by:)` — no test
- `ItemRepository.sync()` — no test
- Error paths for `save()` and `delete()` — not tested

**Action**: Generate characterization tests for uncovered methods before refactoring.
Use `characterization-test-generator` skill.

Red Light — Stop

## Refactor Guard: 🔴 STOP

**Files**: ItemManager.swift, ItemRepository.swift
**Test coverage**: None (0 tests found)

**Action**: Do NOT refactor until characterization tests exist.
Refactoring without tests is like surgery without monitoring —
you won't know if you killed the patient.

Run `characterization-test-generator` on:
1. ItemManager (7 public methods)
2. ItemRepository (5 public methods)

Then re-run this guard.

Step 6: Post-Refactor Verification

After refactoring is complete:

# Run all tests
xcodebuild test -scheme YourApp

# Check specifically for regressions
xcodebuild test -scheme YourApp \
  -only-testing "YourAppTests/ItemManagerCharacterizationTests"

• [ ] All pre-existing tests still pass
• [ ] Characterization tests still pass
• [ ] No new warnings or errors
• [ ] Public API surface unchanged (unless intentionally changed)

Refactoring Safeguards

Safe Refactorings (Low Risk)

These typically don't change behavior:

• Rename variable/method/type
• Extract method/function
• Inline temporary variable
• Move method to another type (keeping same behavior)
• Extract protocol from class

Risky Refactorings (Need Good Coverage)

These can change behavior:

• Change method signature
• Reorder operations
• Replace algorithm
• Merge/split classes
• Change data structure
• Introduce async/await to sync code
• Migration between frameworks (CoreData → SwiftData)

Migration Refactorings (Need Comprehensive Coverage)

Full contract tests recommended:

• Architecture change (MVC → MVVM)
• Framework migration (UIKit → SwiftUI)
• Storage migration (UserDefaults → SwiftData)
• Concurrency model change (GCD → async/await)

Output Format

## Refactor Guard Report

**Scope**: [Files/classes to be refactored]
**Verdict**: ✅ PROCEED / ⚠️ NEEDS TESTS / 🔴 STOP

### Coverage Summary
| Class | Public Methods | Tests | Coverage | Verdict |
|-------|---------------|-------|----------|---------|
| ItemManager | 9 | 12 | 89% | ✅ |
| ItemRepository | 5 | 2 | 40% | ⚠️ |

### Missing Coverage
- [ ] `ItemRepository.sync()` — needs characterization test
- [ ] Error path for `ItemRepository.delete()` — untested

### Recommendations
1. [Action item]
2. [Action item]

References

• testing/characterization-test-generator/ — generates tests this guard may require
• testing/tdd-feature/ — for building new code during refactor
• Martin Fowler, Refactoring: Improving the Design of Existing Code