diff --git a/crates/idfoto-core/src/item_types/mod.rs b/crates/idfoto-core/src/item_types/mod.rs
index eb6d2d1..f8a6e22 100644
--- a/crates/idfoto-core/src/item_types/mod.rs
+++ b/crates/idfoto-core/src/item_types/mod.rs
@@ -35,6 +35,9 @@ pub enum ItemType {
     Totp,
 }
 
+// INVARIANT: no *Core struct may have a field serialized as "type" —
+// that key is reserved for serde's internal tag. Use "kind" for
+// type-discriminant fields within core structs (CardKind, TotpKind).
 #[derive(Debug, Clone, Serialize, Deserialize)]
 #[serde(tag = "type", rename_all = "snake_case")]
 pub enum ItemCore {