APFS to create a separate partition Install Sequoia in new partition Install macOS 26 Beta (enable betas) Xcode Coding Assistant works.

First suggestion seems to require an addition to the RealityKitContent > Package. ...], swiftLanguageModes: [.v5] You may need to Clean. Xcode seems to incorrectly remember all the Build fails.

It would be good if sample code was updated with version controlled embedded source so everybody could actually understand the diffs.

You can use a PersistentModel object in XCTest. You have to create a ModelContainer as a property of XCTestCase - see the default template code that is supporting the Item model. Seems you have to pretend with @MainActor. For the config enabled store in memory only. Use container.mainContext.insert() to add objects to the model.