diff --git a/crates/ogar-vocab/src/lib.rs b/crates/ogar-vocab/src/lib.rs
index fb7abd5..4186ffa 100644
--- a/crates/ogar-vocab/src/lib.rs
+++ b/crates/ogar-vocab/src/lib.rs
@@ -1173,6 +1173,8 @@ impl Class {
 ///   0x17XX  reserved: Blocks  (visual block-programming vocabulary; consumed
 ///                              by blockly-rs / scratch-rs)
 ///   0x18XX+ unassigned
+///   0xFFXX  reserved: lance-graph hardcoded behaviour (never minted here;
+///           see [`HARDCODED_BEHAVIOUR`])
 /// ```
 ///
 /// **Anatomy vs Health (the firewall split).** `0x0AXX` Anatomy is the
@@ -1836,6 +1838,23 @@ pub enum ConceptDomain {
     Unassigned,
 }
 
+/// The concept half of lance-graph's hardcoded behaviour classids,
+/// `0xFFFF :: 0xF###` (lance-graph plan D-DT64-7, decisions O2 and O8).
+///
+/// Those lenses are registered by lance-graph's DT64 store from compiled
+/// tables, which is the authority for them; this codebook never resolves
+/// them. The whole `0xFFXX` block is therefore reserved and stays empty: no
+/// [`CODEBOOK`] row and no [`class_ids::ALL`] row may land in it
+/// (`no_codebook_row_lands_in_the_hardcoded_behaviour_block`).
+///
+/// [`canonical_concept_domain`] keeps answering
+/// [`ConceptDomain::Unassigned`] for the block, so the domain enum and its
+/// lance-graph mirror are unchanged.
+pub const HARDCODED_BEHAVIOUR: u16 = 0xFFFF;
+
+/// The high byte of [`HARDCODED_BEHAVIOUR`]: the reserved block `0xFFXX`.
+pub const HARDCODED_BEHAVIOUR_DOMAIN: u16 = HARDCODED_BEHAVIOUR >> 8;
+
 /// Resolve a canonical id's [`ConceptDomain`] from its high byte. Pure +
 /// deterministic + O(1) — no table lookup needed.
 #[must_use]
@@ -5993,6 +6012,27 @@ mod tests {
         }
     }
 
+    #[test]
+    fn no_codebook_row_lands_in_the_hardcoded_behaviour_block() {
+        // lance-graph DT64 is the authority for 0xFFFF :: 0xF### (D-DT64-7
+        // O2/O8); a row here would make two authorities answer for it.
+        for (name, id) in CODEBOOK.iter().chain(class_ids::ALL.iter()) {
+            assert_ne!(
+                *id >> 8,
+                HARDCODED_BEHAVIOUR_DOMAIN,
+                "{name} ({id:#06x}) landed in the reserved 0xFFXX block"
+            );
+        }
+        assert_eq!(canonical_concept_name(HARDCODED_BEHAVIOUR), None);
+        assert_eq!(canonical_concept_id("hardcoded_behaviour"), None);
+        // No domain variant: the enum and its lance-graph mirror stay as
+        // they are.
+        assert_eq!(
+            canonical_concept_domain(HARDCODED_BEHAVIOUR),
+            ConceptDomain::Unassigned
+        );
+    }
+
     #[test]
     fn canonical_concept_name_known_ids_and_none_for_unknown() {
         assert_eq!(canonical_concept_name(0x0202), Some("commercial_document"));