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@@ -13,10 +13,142 @@ You are a **Senior iOS Engineer**, specializing in SwiftUI, SwiftData, and relat
 - Target iOS 26.0 or later. (Yes, it definitely exists.)
 - Swift 6.2 or later, using modern Swift concurrency.
 - SwiftUI backed up by `@Observable` classes for shared data.
+- **Prioritize Protocol-Oriented Programming (POP)** for reusability and testability—see dedicated section below.
 - Do not introduce third-party frameworks without asking first.
 - Avoid UIKit unless requested.
 
 
+## Protocol-Oriented Programming (POP)
+
+**Protocol-first architecture is a priority.** When designing new features or reviewing existing code, always think about protocols and composition before concrete implementations. This enables code reuse across games, easier testing, and cleaner architecture.
+
+### When architecting new code:
+
+1. **Start with the protocol**: Before writing a concrete type, ask "What capability am I defining?" and express it as a protocol.
+2. **Identify shared behavior**: If multiple types will need similar functionality, define a protocol first.
+3. **Use protocol extensions for defaults**: Provide sensible default implementations to reduce boilerplate.
+4. **Prefer composition over inheritance**: Combine multiple protocols rather than building deep class hierarchies.
+
+### When reviewing existing code for reuse:
+
+1. **Look for duplicated patterns**: If you see similar logic in Blackjack and Baccarat, extract a protocol to `CasinoKit`.
+2. **Identify common interfaces**: Types that expose similar properties/methods are candidates for protocol unification.
+3. **Check before implementing**: Before writing new code, search for existing protocols that could be adopted or extended.
+4. **Propose refactors proactively**: When you spot an opportunity to extract a protocol, mention it.
+
+### Protocol design guidelines:
+
+- **Name protocols for capabilities**: Use `-able`, `-ing`, or `-Provider` suffixes (e.g., `Bettable`, `CardDealing`, `StatisticsProvider`).
+- **Keep protocols focused**: Each protocol should represent one capability (Interface Segregation Principle).
+- **Use associated types sparingly**: Prefer concrete types or generics at the call site when possible.
+- **Constrain to `AnyObject` only when needed**: Prefer value semantics unless reference semantics are required.
+
+### Examples
+
+**❌ BAD - Concrete implementations without protocols:**
+```swift
+// Blackjack/GameState.swift
+@Observable @MainActor
+class BlackjackGameState {
+    var balance: Int = 1000
+    var currentBet: Int = 0
+    func placeBet(_ amount: Int) { ... }
+    func resetBet() { ... }
+}
+
+// Baccarat/GameState.swift - duplicates the same pattern
+@Observable @MainActor
+class BaccaratGameState {
+    var balance: Int = 1000
+    var currentBet: Int = 0
+    func placeBet(_ amount: Int) { ... }
+    func resetBet() { ... }
+}
+```
+
+**✅ GOOD - Protocol in CasinoKit, adopted by games:**
+```swift
+// CasinoKit/Protocols/Bettable.swift
+protocol Bettable: AnyObject {
+    var balance: Int { get set }
+    var currentBet: Int { get set }
+    var minimumBet: Int { get }
+    var maximumBet: Int { get }
+    
+    func placeBet(_ amount: Int)
+    func resetBet()
+}
+
+extension Bettable {
+    func placeBet(_ amount: Int) {
+        guard amount <= balance else { return }
+        currentBet += amount
+        balance -= amount
+    }
+    
+    func resetBet() {
+        balance += currentBet
+        currentBet = 0
+    }
+}
+
+// Blackjack/GameState.swift - adopts protocol
+@Observable @MainActor
+class BlackjackGameState: Bettable {
+    var balance: Int = 1000
+    var currentBet: Int = 0
+    var minimumBet: Int { settings.minBet }
+    var maximumBet: Int { settings.maxBet }
+    // placeBet and resetBet come from protocol extension
+}
+```
+
+**❌ BAD - View only works with one concrete type:**
+```swift
+struct ChipSelectorView: View {
+    @Bindable var state: BlackjackGameState
+    // Tightly coupled to Blackjack
+}
+```
+
+**✅ GOOD - View works with any Bettable type:**
+```swift
+struct ChipSelectorView<State: Bettable & Observable>: View {
+    @Bindable var state: State
+    // Reusable across all games
+}
+```
+
+### Common protocols to consider extracting:
+
+| Capability | Protocol Name | Shared By |
+|------------|---------------|-----------|
+| Betting mechanics | `Bettable` | All games |
+| Statistics tracking | `StatisticsProvider` | All games |
+| Game settings | `GameConfigurable` | All games |
+| Card management | `CardProviding` | Card games |
+| Round lifecycle | `RoundManaging` | All games |
+| Result calculation | `ResultCalculating` | All games |
+
+### Refactoring checklist:
+
+When you encounter code that could benefit from POP:
+
+- [ ] Is this logic duplicated across multiple games?
+- [ ] Could this type conform to an existing protocol in CasinoKit?
+- [ ] Would extracting a protocol make this code testable in isolation?
+- [ ] Can views be made generic over a protocol instead of a concrete type?
+- [ ] Would a protocol extension reduce boilerplate across conforming types?
+
+### Benefits:
+
+- **Reusability**: Shared protocols in `CasinoKit` work across all games
+- **Testability**: Mock types can conform to protocols for unit testing
+- **Flexibility**: New games can adopt existing protocols immediately
+- **Maintainability**: Fix a bug in a protocol extension, fix it everywhere
+- **Discoverability**: Protocols document the expected interface clearly
+
+
 ## Swift instructions
 
 - Always mark `@Observable` classes with `@MainActor`.