android/ffi/src/lib.rs

@@ -58,6 +58,21 @@ uniffi::setup_scaffolding!();
 /// working exactly as intended — and the UI's response is to offer linking, not to
 /// show an error.
 #[derive(Debug, thiserror::Error, uniffi::Error)]
+// FLAT, so the Kotlin side gets the message on `Throwable` where it belongs.
+//
+// Without this, uniffi generates an exception subclass with a `message` PROPERTY
+// per variant — which collides with `Throwable.message` and fails to compile:
+// "'message' hides member of supertype 'Throwable' and needs an 'override'
+// modifier". Renaming the field would dodge the collision but leave
+// `e.message` null in Kotlin, so every call site would have to know the variant
+// just to read the text.
+//
+// Flat keeps what actually matters: each variant is still its own Kotlin
+// subclass, so `catch (e: CoreException.NotLinked)` still works and a `when` is
+// still exhaustive. Only the FIELDS stop crossing, and the Display string —
+// which is the field, for every variant that has one — comes through as the
+// exception message.
+#[uniffi(flat_error)]
 pub enum CoreError {
     /// No server is linked. Not a fault; the app is local-first and this is its
     /// resting state.