Tdd Bug Fix Skill for Claude

Without TDD:           With TDD:
Bug reported           Bug reported
  → AI generates fix     → Write failing test (RED)
  → Deploy               → AI generates fix (GREEN)
  → Hope it works        → Test passes — confirmed fixed
  → Bug returns later    → Test prevents regression forever

Grep: "[relevant keyword]" to find the source
Read: the suspected file(s)

import Testing
@testable import YourApp

@Suite("Bug Fix: [Brief description]")
struct BugFix_DescriptionTests {

    @Test("should [expected behavior] — was [actual behavior]")
    func reproduceBug() {
        // Arrange — set up the conditions that trigger the bug
        let calculator = PriceCalculator()

        // Act — perform the operation that's broken
        let result = calculator.applyDiscount(price: 100, percent: 50)

        // Assert — what it SHOULD return (this will FAIL now)
        #expect(result == 50.0)  // Currently returns 0.5 (bug: divides by 100 twice)
    }
}

@Test("should return cached items when network fails — was crashing")
func reproduceBug() async throws {
    let mockNetwork = MockNetworkClient(shouldFail: true)
    let mockCache = MockCache(items: [.sample])
    let service = DataService(network: mockNetwork, cache: mockCache)

    // This should return cached data, but currently crashes
    let items = try await service.fetchItems()

    #expect(items.count == 1)
}

@Test("should update count after deletion — was showing stale count")
func reproduceBug() {
    let manager = ItemManager()
    manager.addItem(Item(title: "Test"))

    manager.deleteItem(at: 0)

    #expect(manager.count == 0)     // Currently still returns 1
    #expect(manager.items.isEmpty)  // Currently still contains item
}

@Test("should handle empty input — was crashing with index out of range")
func reproduceBug() {
    let parser = CSVParser()

    // This crashes with empty string
    let result = parser.parse("")

    #expect(result.rows.isEmpty)
    #expect(result.columns.isEmpty)
}

@Test("should show error state when API fails — was showing infinite spinner")
func reproduceBug() async {
    let failingAPI = MockAPI(shouldFail: true)
    let viewModel = ListViewModel(api: failingAPI)

    await viewModel.loadData()

    #expect(viewModel.state == .error)  // Currently stuck on .loading
    #expect(viewModel.isLoading == false)
}

xcodebuild test -scheme YourApp \
  -only-testing "YourAppTests/BugFix_DescriptionTests"

Prompt to Claude: "Here's a failing test that reproduces a bug.
Fix the source code to make this test pass without breaking
any existing tests."

# Run the bug fix test
xcodebuild test -scheme YourApp \
  -only-testing "YourAppTests/BugFix_DescriptionTests"

# Run ALL tests to check for regressions
xcodebuild test -scheme YourApp

@Suite("Bug Fix: Discount calculation errors")
struct BugFix_DiscountCalculationTests {

    @Test("50% discount on $100 should be $50")
    func fiftyPercentDiscount() {
        let calc = PriceCalculator()
        #expect(calc.applyDiscount(price: 100, percent: 50) == 50.0)
    }

    @Test("discount on $0 should return $0")
    func zeroPrice() {
        let calc = PriceCalculator()
        #expect(calc.applyDiscount(price: 0, percent: 50) == 0.0)
    }
}

## Bug Fix: [Description]

### Bug Summary
- **Expected**: [What should happen]
- **Actual**: [What was happening]
- **Root cause**: [Why it was broken]

### Test (RED)

### Fix (GREEN)
**File**: `path/to/file.swift:XX`

### Verification
- [x] Bug test fails before fix
- [x] Bug test passes after fix
- [x] All existing tests still pass
- [x] Edge cases covered: [list]

### Regression Prevention
Test added: `Tests/BugFixes/BugFix_DescriptionTests.swift`
This test will catch any future regression of this bug.