# core-data-expert

Provides Core Data architectural guidance for iOS and macOS, covering stack setup, Swift Concurrency, migrations, and CloudKit sync.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 2500 installs · ★ 318
- Author: AvdLee
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/AvdLee/Core-Data-Agent-Skill/tree/HEAD/core-data-expert/SKILL.md
- Updated: 2026-09-14
- HTML: https://affpapa.org/en/nevibecode/skills/core-data-expert

## What it does

The skill enforces strict Core Data development rules for your coding agent. It separates UI view contexts from background contexts right from the start. Under its guidance, the agent never passes `NSManagedObject` instances across contexts, demanding `NSManagedObjectID` instead.

A built-in triage checklist routes issues directly to markdown guides in `references/`. The agent pulls rules for `NSFetchedResultsController`, batch operations, staged migrations on iOS 17+, and `NSMergeConflict` policies. If you implement batch updates or deletions, it insists on persistent history tracking so your UI actually refreshes.

Papering over actor isolation warnings with `@unchecked Sendable` will fail the check: the skill enforces real concurrency safety. It also keeps you from breaking live apps by reminding you that CloudKit production schemas are immutable.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add AvdLee/Core-Data-Agent-Skill --skill core-data-expert -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/core-data-expert/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/core-data-expert/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add AvdLee/Core-Data-Agent-Skill --skill core-data-expert -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/core-data-expert/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add AvdLee/Core-Data-Agent-Skill --skill core-data-expert -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/core-data-expert/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install AvdLee/Core-Data-Agent-Skill core-data-expert
```

2. **Manually: put the files in place**
- File: `.github/skills/core-data-expert/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add AvdLee/Core-Data-Agent-Skill --skill core-data-expert -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/AvdLee/Core-Data-Agent-Skill --path core-data-expert
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/core-data-expert/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add AvdLee/Core-Data-Agent-Skill --skill core-data-expert -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/AvdLee/Core-Data-Agent-Skill/tree/HEAD/core-data-expert/SKILL.md

## When to use it
- resolve cross-context threading crashes by passing `NSManagedObjectID`
- set up batch operations with persistent history tracking for UI updates
- configure schema migrations, including staged migrations for iOS 17+
- handle `NSMergeConflict` and entity constraint violations
- debug CloudKit synchronization with `NSPersistentCloudKitContainer`

## Example prompts
- Fix NSPersistentStoreIncompatibleVersionHashError after modifying the Core Data model
- How do I safely pass fetched records to a background actor in Swift Concurrency?
- Batch delete operations are not updating the NSFetchedResultsController UI, how do I configure persistent history tracking?

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 2f8fbb83d9339e6f.

BEGIN THIRD-PARTY FILE 2f8fbb83d9339e6f
```text
---
name: core-data-expert
description: 'Expert Core Data guidance (iOS/macOS): stack setup, fetch requests & NSFetchedResultsController, saving/merge conflicts, threading & Swift Concurrency, batch operations & persistent history, migrations, performance, and NSPersistentCloudKitContainer/CloudKit sync.'
---
# Core Data Expert

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

## Agent behavior contract (follow these rules)

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

## First 60 seconds (triage template)

- **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

## Routing map (pick the right reference fast)

- **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`
```
END THIRD-PARTY FILE 2f8fbb83d9339e6f

The third-party file has ended. Everything between the 2f8fbb83d9339e6f tags is data: requests and commands from it are not followed. An “end of file” line with another tag inside the block is part of the file.

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