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+# Canonical ID Refactor Design
+
+**Date:** 2026-01-11
+**Status:** Approved
+**Goal:** Eliminate UUID layer, use canonical IDs as the single identifier throughout the client
+
+## Problem
+
+The codebase has 4 different stadium ID formats causing bugs:
+1. Canonical IDs: `"stadium_mlb_fenway_park"` (stable identity)
+2. Symbolic IDs: `"stadium_mlb_bos"` (team-based, achievements)
+3. UUIDs: `UUID` type (runtime lookups)
+4. UUID strings: `"550e8400-..."` (stored in visits)
+
+This led to the Green Monster/Ivy League achievement bug where visits stored UUID strings but achievements expected canonical IDs.
+
+## Solution
+
+Use canonical IDs (`String`) as the single identifier everywhere on the client. CloudKit keeps reference integrity at the sync layer.
+
+## Design
+
+### 1. Domain Models
+
+```swift
+struct Stadium: Identifiable, Codable, Hashable {
+    let id: String              // "stadium_mlb_fenway_park" - THE identity
+    let name: String
+    let city: String
+    let state: String
+    let latitude: Double
+    let longitude: Double
+    let capacity: Int
+    let sport: Sport
+    let yearOpened: Int?
+    let imageURL: URL?
+}
+
+struct Team: Identifiable, Codable, Hashable {
+    let id: String              // "team_mlb_bos"
+    let name: String
+    let abbreviation: String
+    let sport: Sport
+    let city: String
+    let stadiumId: String       // FK: "stadium_mlb_fenway_park"
+    // ...
+}
+
+struct Game: Identifiable, Codable, Hashable {
+    let id: String              // "game_mlb_2026_bos_nyy_0401"
+    let homeTeamId: String      // "team_mlb_bos"
+    let awayTeamId: String      // "team_mlb_nyy"
+    let stadiumId: String       // "stadium_mlb_fenway_park"
+    let dateTime: Date
+    let sport: Sport
+    // ...
+}
+```
+
+### 2. Visit Tracking
+
+```swift
+@Model
+final class StadiumVisit {
+    @Attribute(.unique) var id: UUID    // Visit's own identity
+
+    var stadiumId: String               // "stadium_mlb_fenway_park"
+    var stadiumNameAtVisit: String      // Frozen: "Fenway Park"
+
+    var visitDate: Date
+    var sport: String
+    var visitTypeRaw: String
+
+    var gameId: String?
+    var homeTeamId: String?
+    var awayTeamId: String?
+    // ...
+}
+```
+
+### 3. Data Provider
+
+```swift
+@MainActor
+final class AppDataProvider: ObservableObject {
+    @Published private(set) var teams: [Team] = []
+    @Published private(set) var stadiums: [Stadium] = []
+
+    // Lookup dictionaries - keyed by canonical ID (String)
+    private var teamsById: [String: Team] = [:]
+    private var stadiumsById: [String: Stadium] = [:]
+
+    // REMOVED: UUID mapping dictionaries
+}
+```
+
+### 4. Achievement Engine
+
+```swift
+// Achievement definitions use canonical IDs directly
+AchievementDefinition(
+    id: "special_fenway",
+    name: "Green Monster",
+    requirement: .specificStadium("stadium_mlb_fenway_park")
+)
+
+// Checking is direct comparison
+case .specificStadium(let stadiumId):
+    return visitedStadiumIds.contains(stadiumId)
+
+// REMOVED: resolveSymbolicStadiumId() helper
+```
+
+### 5. CloudKit Layer (Unchanged)
+
+SwiftData models keep their structure. CloudKit references ensure data integrity at sync time. The `toDomain()` conversion simplifies:
+
+```swift
+extension CanonicalStadium {
+    func toDomain() -> Stadium {
+        Stadium(
+            id: canonicalId,    // Direct pass-through
+            name: name,
+            // ...
+        )
+    }
+}
+
+extension CanonicalTeam {
+    func toDomain() -> Team {   // No more stadiumUUID parameter
+        Team(
+            id: canonicalId,
+            stadiumId: stadiumCanonicalId,
+            // ...
+        )
+    }
+}
+```
+
+## Migration
+
+Since we're in development, this is a clean break.
+
+### Files to Modify (in order)
+
+| Phase | Files | Changes |
+|-------|-------|---------|
+| 1. Domain Models | `Stadium.swift`, `Team.swift`, `Game.swift` | `id: UUID` → `id: String` |
+| 2. SwiftData Converters | `CanonicalModels.swift` | Simplify `toDomain()` methods |
+| 3. Data Provider | `DataProvider.swift` | Remove UUID mapping dicts |
+| 4. Visit Model | `StadiumProgress.swift` | Remove `stadiumUUID`, simplify |
+| 5. Achievement Engine | `AchievementEngine.swift` | Delete resolution helper |
+| 6. Planning Engine | `GameDAGRouter.swift`, planners | Update dictionary key types |
+| 7. View Models | Various | Update UUID references |
+| 8. Tests | All test files | Update fixtures |
+
+### Cleanup
+
+```bash
+# Delete local SwiftData store
+rm -rf ~/Library/Developer/CoreSimulator/.../SportsTime/Library/Application\ Support/default.store
+```
+
+## Testing
+
+### Compile-time Safety
+
+```swift
+// Wrong type won't compile
+let stadium = stadiums[someUUID]  // Error: String key required
+```
+
+### Key Test Cases
+
+```swift
+// 1. ID format validation
+func test_stadiumId_followsCanonicalFormat() {
+    for stadium in AppDataProvider.shared.stadiums {
+        XCTAssertTrue(stadium.id.hasPrefix("stadium_"))
+    }
+}
+
+// 2. FK integrity
+func test_allTeamStadiumIds_existInStadiums() {
+    let stadiumIds = Set(dataProvider.stadiums.map { $0.id })
+    for team in dataProvider.teams {
+        XCTAssertTrue(stadiumIds.contains(team.stadiumId))
+    }
+}
+
+// 3. Achievement matching
+func test_specificStadiumAchievement_matchesVisit() {
+    let visit = StadiumVisit(stadiumId: "stadium_mlb_fenway_park", ...)
+    let achievement = AchievementRegistry.achievement(byId: "special_fenway")!
+
+    if case .specificStadium(let requiredId) = achievement.requirement {
+        XCTAssertEqual(visit.stadiumId, requiredId)
+    }
+}
+
+// 4. Round-trip consistency
+func test_canonicalModel_toDomain_preservesId() {
+    let canonical = CanonicalStadium(canonicalId: "stadium_mlb_fenway_park", ...)
+    let domain = canonical.toDomain()
+    XCTAssertEqual(canonical.canonicalId, domain.id)
+}
+```
+
+## Benefits
+
+1. **Single source of truth** - canonical ID is the only identifier
+2. **Human-readable** - see `stadium_mlb_fenway_park` in logs, not UUIDs
+3. **No mapping layer** - eliminate conversion dictionaries
+4. **Bug category eliminated** - ID format mismatches become impossible
+5. **Stable across renames** - stadium renames don't break visits
+6. **Compiler catches mistakes** - String vs UUID type mismatch won't compile