Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx agentmods add skills/markdavidgan/apple-dev-skills/core-data-migrationsnpx skills add markdavidgan/apple-dev-skills --skill core-data-migrationsgit clone --depth 1 https://github.com/markdavidgan/apple-dev-skillsWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/markdavidgan/apple-dev-skills/core-data-migrations)<a href="https://agentmods.dev/skills/markdavidgan/apple-dev-skills/core-data-migrations"><img src="https://agentmods.dev/badge/skills/markdavidgan/apple-dev-skills/core-data-migrations.svg" alt="Measured on agentmods" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00162 | $0.02447 |
| Opus 5 | $0.00081 | $0.01223 |
| Sonnet 5 | $0.00032 | $0.00489 |
| Haiku 4.5 | $0.00016 | $0.00245 |
Grade A, and why
core-data-migrations scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 139 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Core Data Migrations
Changing a shipped Core Data model is a data-safety operation, not an edit. The model on disk belongs to the user; the model in the repo is a proposal for how to reinterpret it. Every rule here exists because the failure mode is silent — a bad migration usually does not throw, it just hands back nil where the bytes used to be.
Read this before touching any .xcdatamodel that has shipped to anyone, including TestFlight.
The one rule that prevents most incidents
Never edit a model version that has shipped. Add a new one.
A .xcdatamodeld is a bundle of versions plus an .xccurrentversion pointer. Editing the current version in place changes the store's expected shape without changing its version hashes, so Core Data has no idea a migration is needed and no way to describe one. Symptoms range from "store won't open" to attributes silently reading nil.
Near.xcdatamodeld/
Near 18.xcdatamodel/contents ← shipped, frozen forever
Near 19.xcdatamodel/contents ← shipped, frozen forever
Near 20.xcdatamodel/contents ← the new one you edit
.xccurrentversion ← points at "Near 20.xcdatamodel"
Copy the current version's directory, rename it, edit the copy, and repoint .xccurrentversion. If the project is generated (XcodeGen, Tuist), regenerate afterwards — a new version directory that the project does not reference produces a build that silently keeps using the old model.
What lightweight migration actually supports
Inferred (lightweight) migration handles a specific list. Anything outside it needs a mapping model or a manual pass:
Supported — adding an attribute (optional, or non-optional with a default); removing an attribute; removing an entity; adding an entity; changing an attribute from non-optional to optional; changing optional to non-optional with a default; adding or removing a relationship; changing a relationship's cardinality; toggling allowsExternalBinaryDataStorage.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 2d ago First seen · 139 lines · 162 tokens per session scan A 7d3ace39dfbb
core-data-migrations is a skill published in the GitHub repository markdavidgan/apple-dev-skills (5 stars, last pushed 6d ago), licensed MIT. It adds 162 tokens to every session and 2,447 once invoked, about $0.0008 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-09-03.
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