Traditional:     AI generates code → You hope it's correct → Ship → Find bugs
TDD with AI:     You write tests (spec) → AI generates code to pass → Proven correct

// Example: Designing a FavoriteManager
protocol FavoriteManaging {
    func add(_ item: Item) async throws
    func remove(_ item: Item) async throws
    func isFavorite(_ item: Item) -> Bool
    var favorites: [Item] { get }
    var count: Int { get }
}

import Testing
@testable import YourApp

@Suite("FavoriteManager")
struct FavoriteManagerTests {

    // 1. Construction
    @Test("starts with empty favorites")
    func startsEmpty() {
        let manager = FavoriteManager()
        #expect(manager.favorites.isEmpty)
        #expect(manager.count == 0)
    }

    // 2. Happy path
    @Test("can add a favorite")
    func addFavorite() async throws {
        let manager = FavoriteManager()
        let item = Item(id: "1", title: "Test")

        try await manager.add(item)

        #expect(manager.count == 1)
        #expect(manager.isFavorite(item))
    }

    // 3. State verification
    @Test("can remove a favorite")
    func removeFavorite() async throws {
        let manager = FavoriteManager()
        let item = Item(id: "1", title: "Test")
        try await manager.add(item)

        try await manager.remove(item)

        #expect(manager.count == 0)
        #expect(!manager.isFavorite(item))
    }

    // 4. Edge cases
    @Test("adding duplicate does not increase count")
    func addDuplicate() async throws {
        let manager = FavoriteManager()
        let item = Item(id: "1", title: "Test")

        try await manager.add(item)
        try await manager.add(item)

        #expect(manager.count == 1)
    }

    @Test("removing non-existent item does nothing")
    func removeNonExistent() async throws {
        let manager = FavoriteManager()
        let item = Item(id: "1", title: "Test")

        try await manager.remove(item)

        #expect(manager.count == 0)
    }

    // 5. Error handling
    @Test("throws when storage is full")
    func storageFullError() async {
        let manager = FavoriteManager(maxCapacity: 2)
        let items = (1...3).map { Item(id: "\($0)", title: "Item \($0)") }

        await #expect(throws: FavoriteError.capacityExceeded) {
            for item in items {
                try await manager.add(item)
            }
        }
    }

    // 6. Ordering
    @Test("favorites are in insertion order")
    func insertionOrder() async throws {
        let manager = FavoriteManager()
        let items = ["C", "A", "B"].map { Item(id: $0, title: $0) }

        for item in items {
            try await manager.add(item)
        }

        #expect(manager.favorites.map(\.title) == ["C", "A", "B"])
    }
}

Prompt to Claude: "Here are my failing tests for FavoriteManager.
Implement the FavoriteManager class to make all tests pass.
Follow the protocol FavoriteManaging."

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

@Suite("FavoriteManager Integration")
struct FavoriteManagerIntegrationTests {

    @Test("persists favorites across sessions")
    func persistence() async throws {
        let store = InMemoryStore()

        // Session 1: Add favorite
        let manager1 = FavoriteManager(store: store)
        try await manager1.add(Item(id: "1", title: "Test"))

        // Session 2: Verify it persists
        let manager2 = FavoriteManager(store: store)
        await manager2.loadFavorites()

        #expect(manager2.count == 1)
    }
}

RED    → Write one failing test (30 seconds - 2 minutes)
GREEN  → Make it pass with simplest code (1 - 5 minutes)
REFACTOR → Clean up while tests stay green (1 - 3 minutes)
REPEAT → Next test

@Suite("SearchViewModel")
struct SearchViewModelTests {
    @Test("starts in idle state")
    @Test("searching updates state to loading")
    @Test("successful search shows results")
    @Test("empty search shows empty state")
    @Test("failed search shows error")
    @Test("debounces rapid input")
    @Test("cancels previous search on new input")
}

@Suite("ItemRepository")
struct ItemRepositoryTests {
    @Test("fetches items from remote")
    @Test("caches fetched items locally")
    @Test("returns cached items when offline")
    @Test("syncs local changes to remote")
    @Test("handles conflict resolution")
    @Test("deletes expire cached items")
}

@Suite("SubscriptionManager")
struct SubscriptionManagerTests {
    @Test("free user has basic access")
    @Test("pro user has full access")
    @Test("expired subscription reverts to free")
    @Test("family member inherits subscription")
    @Test("trial period grants pro access")
    @Test("grace period maintains access after lapse")
}

## TDD Feature: [Feature Name]

### API Design
```swift
// Protocol / public interface

## Common Pitfalls

| Pitfall | Problem | Solution |
|---------|---------|----------|
| Writing too many tests before implementing | Overwhelming; can't see progress | Write 2-3 tests, implement, repeat |
| Tests that test implementation | Brittle; break on refactor | Test behavior and outcomes only |
| Skipping the refactor step | Accumulating technical debt | Refactor every 3-5 green cycles |
| AI implementing beyond the tests | Untested code in production | Only implement what tests require |
| Not running tests after each change | Silent regressions | `xcodebuild test` after every edit |

## References

- Kent Beck, *Test-Driven Development: By Example*
- `testing/test-contract/` — for protocol-level test suites
- `testing/test-data-factory/` — for reducing test setup boilerplate
- `generators/test-generator/` — for standalone test generation