I will just summarise solution, which works to us (thanks a lot @AGDq ) : combination attributeValueClassName: [String], valueTransformerName: NSSecureUnarchiveFromData does not work now, works combination attributeValueClassName: NSArray, valueTransformerName: NSSecureUnarchiveFromData. The @NSManaged variable could still be array of strings, like: @NSManaged public internal(set) var routes: [String]? if you use own valueTransformer, like attributeValueClassName: MyLocation. valueTransformerName: LocationValueTransformer, both classes should be marked as @objc(name of class). Without the names there will be warnings in the Xcode log if use set valueTransformerName = "NSSecureUnarchiveFromData", you need to set some class name to attributeValueClassName property, either NSObject, NSArray or own, otherwise the app will periodically crashes. Before it could be empty and it worked as NSObject

Finally in Xcode 16 beta 6 I have no warnings and no crashes. @AGDq interesting, but our 'Declared Objective-C type "[String]" ...' disappear in beta 6...