The documented template for @AppEntity(schema: .messages.message) at Integrating your messaging app with Apple Intelligence and on the MessagesEntity.message reference page shows: var reaction: <#ReadReaction#>? with <#ReadReaction#> rendered as a placeholder (the Xcode placeholder syntax). A natural reading is "fill in your own type that you want to use here". I tried several: Tapback? where Tapback is @AppEnum(schema: .messages.customReaction) MessageReaction? (same, renamed) MessageReaction? with manual AppEnum conformance (no schema decoration) AttributedString? AttributedString (non-optional) field omitted entirely Each variant produced one of: error: Property 'reaction' type does not match required AppSchemaEntity property type 'ReadReactionCases:(Schema<Tapback> | AttributedString)' error: Required AppSchemaEntity property 'reaction' must be optional error: Missing required property 'reaction' from AppSchemaEntity 'messages.message' The phrase ReadReactionCases:(Schema<Tapback> | AttributedString) is opaque — ReadReactionCases isn't a public type, Schema<Tapback> isn't constructable from outside, and the documentation doesn't mention @UnionValue in this context. The actual working pattern, which I only found by downloading the UnicornChat sample and reading MessageEntity.swift, is: @UnionValue enum MessageReaction: Sendable { case customReaction(CustomReaction) } @AppEnum(schema: .messages.customReaction) enum CustomReaction: String, AppEnum { case sticker static let caseDisplayRepresentations: [Self: DisplayRepresentation] = [ .sticker: "Sticker" ] } Two suggestions: Update the schema-template comment in the docs to either name MessageReaction explicitly with a @UnionValue annotation, or include a one-line note: "The reaction field is a @UnionValue enum wrapping the customReaction schema enum — see UnicornChat sample." The current placeholder <#ReadReaction#> plus the prose suggesting "you can generate the properties... with the @AppEntity(.messages.message) Swift macro" reads as if the developer just supplies any AppEnum-conformer. Improve the diagnostic. Property type does not match required AppSchemaEntity property type 'ReadReactionCases:(Schema<Tapback> | AttributedString)' is unhelpful because ReadReactionCases doesn't appear in any reachable type. Either rename to something developer-facing, or include a fix-it that suggests @UnionValue. iOS 27.0 beta, Xcode 27 beta, macOS Tahoe 26.4.