Data & Persistence

You MUST use this skill for ANY data persistence, database, storage, CloudKit, or serialization work.

When to Use

Use this skill when working with:

• Databases (SwiftData, Core Data, GRDB, SQLiteData)
• Schema migrations
• CloudKit sync
• File storage (iCloud Drive, local storage)
• Data serialization (Codable, JSON)
• Storage strategy decisions
• Keychain / secure credential storage
• Encryption, signing, key management (CryptoKit)

Quick Reference

Symptom / Task	Reference
SwiftData @Model, @Query, ModelContext	See skills/swiftdata.md
SwiftData schema migration, VersionedSchema	See skills/swiftdata-migration.md
SwiftData migration crashes, data loss	See skills/swiftdata-migration-diag.md
Migrating from Realm to SwiftData	See skills/realm-migration-ref.md
SwiftData vs SQLiteData decision	See skills/sqlitedata-migration.md
GRDB queries, ValueObservation, DatabaseMigrator	See skills/grdb.md
SQLiteData @Table, CRUD, SyncEngine	See skills/sqlitedata.md
SQLiteData advanced patterns, CTEs, views	See skills/sqlitedata-ref.md
Core Data stack, relationships, concurrency	See skills/core-data.md
Core Data migration crashes, thread errors	See skills/core-data-diag.md
ANY schema migration safety	See skills/database-migration.md
Codable, JSON encoding/decoding	See skills/codable.md
Cloud sync architecture, offline-first	See skills/cloud-sync.md
CloudKit, CKSyncEngine, CKRecord	See skills/cloudkit-ref.md
iCloud Drive, ubiquitous containers	See skills/icloud-drive-ref.md
Cloud sync errors, conflict resolution	See skills/cloud-sync-diag.md
Storage strategy, where to store data	See skills/storage.md
Storage issues, files disappeared	See skills/storage-diag.md
Storage management, disk pressure	See skills/storage-management-ref.md
Keychain / secure credential storage	See axiom-security (skills/keychain.md)
Keychain errors (errSecDuplicateItem)	See axiom-security (skills/keychain-diag.md)
Keychain API reference	See axiom-security (skills/keychain-ref.md)
Encryption / signing / key management	See axiom-security (skills/cryptokit.md)
CryptoKit API reference	See axiom-security (skills/cryptokit-ref.md)
File protection, NSFileProtection	See axiom-security (skills/file-protection-ref.md)
tvOS data persistence (no local storage)	See axiom-swift (skills/tvos.md)
tvOS + CloudKit SyncEngine	See skills/sqlitedata.md

Automated Scanning

Core Data audit → Launch core-data-auditor agent or /axiom:audit core-data (safety violations, architectural gaps — migration options, thread-confinement, N+1 queries, merge policies, context isolation) Codable audit → Launch codable-auditor agent or /axiom:audit codable (safety violations, semantic gaps — try? swallowing errors, JSONSerialization, date handling, silent field drops, wrapper-hidden fallbacks, cross-file strategy drift, enum future-case crashes) iCloud audit → Launch icloud-auditor agent or /axiom:audit icloud (entitlement checks, file coordination, incomplete CKError matrix coverage, missing account-change observation, polling vs CKSubscriptions, SwiftData + CloudKit unsupported features, compound risks like uncoordinated I/O across extensions) Storage audit → Launch storage-auditor agent or /axiom:audit storage (wrong file locations, missing backup exclusions, sensitive data on disk vs Keychain, missing App Group containers, unbounded cache growth, orphan files, compound risks like user data in tmp/ + critical content) Database schema audit → Launch database-schema-auditor agent or /axiom:audit database-schema (unsafe ALTER TABLE, DROP operations, missing idempotency, FK constraints declared but not enforced, incomplete upgrade paths, compound risks like INSERT OR REPLACE on FK-referenced tables) SwiftData audit → Launch swiftdata-auditor agent or /axiom:audit swiftdata (struct models, missing schema registration, array relationships without defaults, background context misuse, N+1 patterns, stale predicates, CloudKit conformance gaps, compound risks like struct model + array relationship)

Decision Tree

1. SwiftData? → skills/swiftdata.md, skills/swiftdata-migration.md
2. Core Data? → skills/core-data.md, skills/core-data-diag.md
3. GRDB? → skills/grdb.md
4. SQLiteData? → skills/sqlitedata.md, skills/sqlitedata-ref.md
5. ANY schema migration? → skills/database-migration.md (ALWAYS — prevents data loss)
6. Realm migration? → skills/realm-migration-ref.md
7. SwiftData vs SQLiteData? → skills/sqlitedata-migration.md
8. Cloud sync architecture? → skills/cloud-sync.md
9. CloudKit? → skills/cloudkit-ref.md
10. iCloud Drive? → skills/icloud-drive-ref.md
11. Cloud sync errors? → skills/cloud-sync-diag.md
12. Codable/JSON serialization? → skills/codable.md
13. File storage strategy? → skills/storage.md, skills/storage-diag.md, skills/storage-management-ref.md
14. File protection? → See axiom-security (skills/file-protection-ref.md)
15. Keychain / storing tokens, passwords, secrets securely? → See axiom-security (skills/keychain.md), See axiom-security (skills/keychain-diag.md), See axiom-security (skills/keychain-ref.md)
16. SecItem errors (errSecDuplicateItem, errSecItemNotFound, errSecInteractionNotAllowed)? → See axiom-security (skills/keychain-diag.md)
17. Encryption, signing, Secure Enclave, CryptoKit? → See axiom-security (skills/cryptokit.md), See axiom-security (skills/cryptokit-ref.md)
18. Quantum-secure cryptography, HPKE, ML-KEM? → See axiom-security (skills/cryptokit.md)
19. Want Core Data safety scan? → core-data-auditor (Agent)
20. Want Codable anti-pattern scan? → codable-auditor (Agent)
21. Want iCloud sync audit? → icloud-auditor (Agent)
22. Want storage location audit? → storage-auditor (Agent)
23. Want database schema/migration safety scan? → database-schema-auditor (Agent)
24. Want SwiftData code audit? → swiftdata-auditor (Agent)
25. tvOS data persistence? → See axiom-swift (skills/tvos.md) (CRITICAL: no persistent local storage) + skills/sqlitedata.md (CloudKit SyncEngine)
26. SwiftData @MainActor / background context threading? → /skill axiom-concurrency
27. Structured data generation with Foundation Models? → /skill axiom-ai

Sync patterns

• HealthKit anchored/observer queries as a generalizable change-tracking pattern → See axiom-health (skills/sync-and-background.md)

Anti-Rationalization

Thought	Reality
"Just adding a column, no migration needed"	Schema changes without migration crash users. database-migration prevents data loss.
"I'll handle the migration manually"	Manual migrations miss edge cases. database-migration covers rollback and testing.
"Simple query, I don't need the skill"	Query patterns prevent N+1 and thread-safety issues. The skill has copy-paste solutions.
"CloudKit sync is straightforward"	CloudKit has 15+ failure modes. cloud-sync-diag diagnoses them systematically.
"I know Codable well enough"	Codable has silent data loss traps (try? swallows errors). codable skill prevents production bugs.
"I'll use local storage on tvOS"	tvOS has NO persistent local storage. System deletes Caches at any time. See axiom-swift (skills/tvos.md) for the iCloud-first pattern.
"UserDefaults is fine for this token"	UserDefaults is unencrypted, backed up to iCloud, and visible to MDM profiles. One audit catches it. keychain stores tokens securely.
"I'll encrypt it myself with CommonCrypto"	CryptoKit replaced CommonCrypto's buffer-management nightmares with one-line APIs. cryptokit prevents misuse.

Critical Pattern: Migrations

ALWAYS read skills/database-migration.md when adding/modifying database columns.

This prevents:

• "FOREIGN KEY constraint failed" errors
• "no such column" crashes
• Data loss from unsafe migrations

Example Invocations

User: "I need to add a column to my SwiftData model" → Read: skills/database-migration.md (critical - prevents data loss)

User: "How do I query SwiftData with complex filters?" → Read: skills/swiftdata.md

User: "CloudKit sync isn't working" → Read: skills/cloud-sync-diag.md

User: "Should I use SwiftData or SQLiteData?" → Read: skills/sqlitedata-migration.md

User: "Check my Core Data code for safety issues" → Launch: core-data-auditor agent

User: "Scan for Codable anti-patterns before release" → Launch: codable-auditor agent

User: "Audit my iCloud sync implementation" → Launch: icloud-auditor agent

User: "Check if my files are stored in the right locations" → Launch: storage-auditor agent

User: "Audit my database migrations for safety" → Launch: database-schema-auditor agent

User: "Check my SwiftData models for issues" → Launch: swiftdata-auditor agent

User: "How do I persist data on tvOS?" → Invoke: See axiom-swift (skills/tvos.md) + Read: skills/sqlitedata.md

User: "My tvOS app loses data between launches" → Invoke: See axiom-swift (skills/tvos.md)

User: "How do I store an auth token securely?" → Invoke: See axiom-security (skills/keychain.md)

User: "errSecDuplicateItem but I checked and the item doesn't exist" → Invoke: See axiom-security (skills/keychain-diag.md)

User: "How do I encrypt data with AES in Swift?" → Invoke: See axiom-security (skills/cryptokit.md)

User: "I need to sign data with the Secure Enclave" → Invoke: See axiom-security (skills/cryptokit.md)

User: "What's ML-KEM and should I use it?" → Invoke: See axiom-security (skills/cryptokit.md)