Hi Albert, thanks for the routing tip, filed as FB23203177 The full reproduction, the four type variants I tried, the actual working @UnionValue / customReaction pattern from UnicornChat, and the two doc/diagnostic suggestions from the forum post are all in the FB body so the AppIntents and Documentation teams have everything inline. The placeholder itself is in the schema-template comment generated by the @AppEntity(schema: .messages.message) macro expansion — Xcode surfaces it as the editable <#ReadReaction#> token when you accept the fix-it for the missing reaction property. It's not in the prose docs (you were right that it's missing there), only in the macro-emitted stub the IDE inserts. Happy to test any patch or doc revision once it lands. Lukas