Data & Persistence

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

Cursor Subagent Routing

Delegate to the appropriate Cursor subagent when this router calls for a specialized auditor.

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

Automated Scanning

Core Data audit → Delegate to the core-data-auditor subagent or /axiom-audit core-data (safety violations, architectural gaps — migration options, thread-confinement, N+1 queries, merge policies, context isolation) Codable audit → Delegate to the codable-auditor subagent 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 → Delegate to the icloud-auditor subagent 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 → Delegate to the storage-auditor subagent 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 → Delegate to the database-schema-auditor subagent 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) GRDB performance audit → Delegate to the grdb-performance-auditor subagent or /axiom-audit grdb-performance (raw SQL string interpolation, missing FK indexes in raw SQL, missing PRAGMA optimize for raw-GRDB apps, journal mode mismatch for app-group DBs, missing observesSuspensionNotifications for shared DBs, prefix-redundant indexes in raw SQL, legacy Record subclass, INSERT OR REPLACE misused as upsert, observation on WITHOUT ROWID tables, WITHOUT ROWID upsert below GRDB 7.11) SwiftData audit → Delegate to the swiftdata-auditor subagent 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 3a. GRDB perf, slow query, schema design for perf? → skills/grdb-performance.md 3b. FTS5 (full-text search, any layer)? → skills/sqlite-fts-ref.md 3c. DB shared across app + extension/widget/Live Activity? → skills/grdb-app-groups.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) 23a. Want GRDB performance/app-group scan? → grdb-performance-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

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: "How do I make my GRDB queries faster?" → Read: skills/grdb-performance.md

User: "Add search to my GRDB-backed app" / "Add search to SQLiteData app" → Read: skills/sqlite-fts-ref.md

User: "My widget needs to read the same database as the app" → Read: skills/grdb-app-groups.md

User: "My widget shows stale data from the database" → Read: skills/grdb-app-groups.md

User: "Audit my GRDB code for performance issues" → Launch: grdb-performance-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)