Posting here to see if folks have workarounds or if I have a misunderstanding of SwiftData supported types. In adopting SwiftData, I have swiftData properties of collection type (Array or Set - both have this issue). E.g: @Model final class Item { var timestamp: Date var strings = ["aa", "bb"] var display: String { strings.joined(separator: " ") } init(timestamp: Date) { self.timestamp = timestamp } } So far in development I haven't had issues on iOS 17, but on the iOS 18 betas 4-5 the app logs show the following error: "fault: Could not materialize Objective-C class named "Array" from declared attribute value type "Array<String>" of attribute named strings" It happens immediately in my app when creating an object with a collection attribute. In a minimal test example, the error log appears only after a few minutes and doesn't seem to affect the template app's basic functionality. Anyone else running into this? Was filed as FB14397250

Running into this new issue in beta 3. All of our immutable model attributes are now triggering swift compiler error from the title (tied to swift 6 - warning in swift 5). Neither warning or error show up on previous builds. Anyone else running into this? Hopefully not a limitation on swiftData that attributes must be mutable? filed as: FB14379932 (new in Xcode 16 beta3: "cannot expand accessors on variable declared with 'let'" in SwiftData Models)