Use when working with ANY data persistence, database, storage, CloudKit, migration, or serialization. Covers SwiftData, Core Data, GRDB, SQLite, Realm, CloudKit sync, file storage, Codable, migrations.

Data & Persistence You MUST use this skill for ANY data persistence, database, storage, CloudKit, or serialization work. <! AXIOM AUDITOR INLINE BEGIN — auto maintained by scripts/build inlined auditors.ts; do not hand edit Can't launch Axiom agents? Where this router sends you to an agent, follow its procedure inline. Over MCP, axiom get agent returns it; elsewhere, read its file below, which needs only file search and read. Available here: skills/codable auditor.md , skills/core data auditor.md , skills/database schema auditor.md , skills/grdb performance auditor.md , skills/icloud auditor.md , skills/storage auditor.md , skills/swiftdata auditor.md . For any other agent, over MCP axiom get agent returns it too; run its xclog, xcsym and xcprof steps with the axiom xclog , axiom xcsym and axiom xcprof tools. Elsewhere, search Axiom's skills for skills/<agent .md ; if there is none, do that work yourself with this suite's skills, using any Axiom tool on your PATH . <! AXIOM AUDITOR INLINE END 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, sectioned queries / ResultsObserver / @Attribute(.codable) / dynamic compound predicates ( OS27 ) 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, upsert, ValueObservation, DatabaseMigrator, encryption See skills/grdb.md GRDB performance, indexes, EXPLAIN QUERY PLAN, cursors, observation not firing See skills/grdb performance.md Full text search (FTS5) in GRDB or SQLiteData See skills/sqlite fts ref.md Storing or querying JSON inside a SQLite column (JSON1, JSONB) See skills/sql json ref.md GRDB shared across app + widget/extension (App Group) See skills/grdb app groups.md SQLiteData @Table, CRUD, SyncEngine See skills/sqlitedata.md SQLiteData advanced patterns, CTEs, views See skills/sqlitedata ref.md @Shared / @SharedReader , appStorage or fileStorage persistence, custom SharedKey, isLoading / loadError on @FetchAll See skills/swift sharing.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 guaranteed persistent 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) GRDB performance audit → Launch grdb performance auditor agent 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 bugs) 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 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 4a. @Shared / @SharedReader , or the loading/error state behind @FetchAll ? → skills/swift sharing.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: every local directory is purgeable between launches, so iCloud is the source of truth) + 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 has the safe additive patterns and both migration paths to test. "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. "@Shared is just @AppStorage with extra steps" Mutation goes through $shared.withLock ; the plain setter is deprecated, and below Sharing 2.8 on Swift 6.3 it is a compile error wherever a SwiftUI binding is chained into shared state. swift sharing has the migration. "I'll use local storage on tvOS" No local directory on tvOS is guaranteed to survive between launches — the system purges them, Caches first. 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: "How do I persist a setting to UserDefaults and observe it everywhere?" → Read: skills/swift sharing.md User: "Why is my @FetchAll empty — is it loading or did it fail?" → Read: skills/swift sharing.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 faste

axiom-data