Filed by swiftcraft · Jun 12, 2026
✓ Fastest verified fix
Make the migration consistent — wrapper and model class must move together — and add a task-brief rule that property-wrapper changes are all-or-nothing per model. Then clear the preview cache, which otherwise keeps serving the crash.
grep -rn --include='*.swift' -e 'ObservableObject' -e '@Bindable' Sources/ | sort✓ What worked
The fix above. Previews stable for six weeks across two agent-heavy projects. The grep in step 1 is now a pre-commit check.
swiftcraft · 2mo ago
✕ What did not work
Folk remedy, doesn't touch the cached preview build products. Crash survives restart, reboot, and one full reinstall of Xcode before I found the simctl --set previews incantation.
junipers · 2mo ago
| Agent | Version | Model | OS | Framework | Outcome | × |
|---|---|---|---|---|---|---|
| Codex | 0.45.1 | GPT-5.5 Codex | macOS 15.5 | — | Reproduced | 2 |
| Claude Code | 2.4.0 | Claude Opus 5 | macOS 15.5 | — | Reproduced | 6 |
| Claude Code | 2.3.5 | Claude Sonnet 5 | macOS 15.5 | — | Reproduced | 3 |
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// before
final class CartModel: ObservableObject {
@Published var items: [Item] = []
}
// after
@Observable final class CartModel {
var items: [Item] = []
}xcrun simctl --set previews delete all && rm -rf ~/Library/Developer/Xcode/DerivedData/*/Build/Intermediates.noindex/PreviewsStrong evidence gets promoted into the record above.
Confirmed from the Codex side, and one addition: if you use xcodebuild-mcp, add a preview smoke check to the agent's verification loop so it catches this itself before handing back:
xcrun xcodebuild -scheme App -destination 'platform=iOS Simulator,name=iPhone 16' \
build-for-testing -only-testing:AppPreviewTests 2>&1 | tail -5We wrap previews in a snapshot test target precisely so agents get a machine-readable failure instead of a broken canvas the human finds later.
The general lesson transfers beyond Swift: agents apply the new idiom confidently and partially. Same failure shape as half-done React class→hooks migrations and half-done null-safety migrations in Flutter. 'All-or-nothing per unit' is a great brief rule everywhere.
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Related records
'Builds fine, only previews break' cost me two days because I kept debugging my code instead of the cache. The preview host being a separate compiler service with its own cache should be on a poster somewhere.