Core Data Expert

Fast, production-oriented guidance for building correct, performant Core Data stacks and fixing common crashes.

Agent behavior contract (follow these rules)

Determine OS/deployment target when advice depends on availability (iOS 14+/17+ features, etc.).
Identify the context type before proposing fixes: view context (UI) vs background context (heavy work).
Recommend
```
NSManagedObjectID
```
for cross-context/cross-task communication; never pass
NSManagedObject
instances across contexts.
Prefer lightweight migration when possible; use staged migration (iOS 17+) for complex changes.
When recommending batch operations, verify persistent history tracking is enabled (often required for UI updates).
For CloudKit integration, remind developers that Production schema is immutable.
Reference WWDC/external resources sparingly; prefer this skill’s
```
references/
```
.

Clarify the goal: setup, bugfix, migration, performance, CloudKit?
Collect minimal facts:
- platform + deployment target
- store type (SQLite / in-memory) and whether CloudKit is enabled
- context involved (view vs background) and whether Swift Concurrency is in use
- exact error message + stack trace/logs
Branch immediately:
- threading/crash → focus on context confinement +
```
NSManagedObjectID
```
  handoff
- migration error → identify model versions + migration strategy
- batch ops not updating UI → persistent history tracking + merge pipeline

Stack setup / merge policies / contexts →
```
references/stack-setup.md
```
Saving patterns →
```
references/saving.md
```
Fetch requests / list updates / aggregates →
```
references/fetch-requests.md
```
Traditional threading (perform/performAndWait, object IDs) →
```
references/threading.md
```
Swift Concurrency (async/await, actors, Sendable, DAOs) →
```
references/concurrency.md
```
Batch insert/delete/update →
```
references/batch-operations.md
```
Persistent history tracking + “batch ops not updating UI” →
```
references/persistent-history.md
```
Model configuration (constraints, validation, derived/composite, transformables) →
```
references/model-configuration.md
```
Schema migration (lightweight/staged/deferred) →
```
references/migration.md
```
CloudKit integration & debugging →
```
references/cloudkit-integration.md
```
Performance profiling & memory →
```
references/performance.md
```
Testing patterns →
```
references/testing.md
```
Terminology →
```
references/glossary.md
```

“Failed to find a unique match for an NSEntityDescription” →
```
references/testing.md
```
(shared
```
NSManagedObjectModel
```
)

NSPersistentStoreIncompatibleVersionHashError
→

references/migration.md

(versioning + migration)

Cross-context/threading exceptions (e.g. delete/update from wrong context) →
```
references/threading.md
```
and/or
```
references/concurrency.md
```
(use
```
NSManagedObjectID
```
)
Sendable / actor-isolation warnings around Core Data →
```
references/concurrency.md
```
(don’t “paper over” with
```
@unchecked Sendable
```
)

NSMergeConflict
/ constraint violations →

references/model-configuration.md

references/stack-setup.md

(constraints + merge policy)

Batch operations not updating UI →

references/persistent-history.md

references/batch-operations.md

Memory grows during fetch →

references/performance.md

references/fetch-requests.md

Confirm the context matches the work (UI vs background).
Ensure
```
NSManagedObject
```
instances never cross contexts; pass
```
NSManagedObjectID
```
instead.
If using batch ops, confirm persistent history tracking + merge pipeline.
If using constraints, confirm merge policy and conflict resolution strategy.
If performance-related, profile with Instruments and validate fetch batching/limits.