The error is due to the macro for @Observable converting prop0 to be a computed property. One alternative is this: @Observable final class Model { var prop0: Int { didSet { guard oldValue != self.prop0 else { return } UserDefaults.standard.set(self.prop0, forKey: "prop0") } } private init() { prop0 = UserDefaults.standard.integer(forKey: "prop0") } }