diff --git a/server/crates/arbiter-server/src/actors/vault/mod.rs b/server/crates/arbiter-server/src/actors/vault/mod.rs
index 23b4cad..b995891 100644
--- a/server/crates/arbiter-server/src/actors/vault/mod.rs
+++ b/server/crates/arbiter-server/src/actors/vault/mod.rs
@@ -63,6 +63,9 @@ pub enum Error {
 
     #[error("Broken database")]
     BrokenDatabase,
+
+    #[error("Integrity key version mismatch: envelope uses key {envelope}, current key is {current}")]
+    KeyVersionMismatch { envelope: i32, current: i32 },
 }
 
 struct Unsealed {
@@ -393,7 +396,10 @@ impl Vault {
         } = Self::expect_unsealed(&mut self.state)?;
 
         if *root_key_history_id != key_version {
-            return Ok(false);
+            return Err(Error::KeyVersionMismatch {
+                envelope: key_version,
+                current: *root_key_history_id,
+            });
         }
 
         let mut hmac = root_key.0.read_inline(|k| {
diff --git a/server/crates/arbiter-server/src/crypto/integrity/v1.rs b/server/crates/arbiter-server/src/crypto/integrity/v1.rs
index edb2274..a88506f 100644
--- a/server/crates/arbiter-server/src/crypto/integrity/v1.rs
+++ b/server/crates/arbiter-server/src/crypto/integrity/v1.rs
@@ -192,7 +192,10 @@ pub async fn verify_entity<E: Integrable>(
         Ok(false) => Err(Error::MacMismatch {
             entity_kind: E::KIND,
         }),
-        Err(SendError::HandlerError(vault::Error::Sealed)) => Ok(AttestationStatus::Unavailable),
+        Err(SendError::HandlerError(vault::Error::Sealed))
+        | Err(SendError::HandlerError(vault::Error::KeyVersionMismatch { .. })) => {
+            Ok(AttestationStatus::Unavailable)
+        }
         Err(_) => Err(Error::VaultSend),
     }
 }
@@ -331,4 +334,47 @@ mod tests {
             .unwrap_err();
         assert!(matches!(err, Error::MacMismatch { .. }));
     }
+
+    #[tokio::test]
+    async fn key_version_mismatch_returns_unavailable_not_mac_mismatch() {
+        use crate::db::schema::integrity_envelope;
+        use super::AttestationStatus;
+
+        const ENTITY_ID: &[u8] = b"entity-id-rotation-test";
+
+        let db = db::create_test_pool().await;
+        let vault = bootstrapped_vault(&db).await;
+        let mut conn = db.get().await.unwrap();
+
+        let entity = DummyEntity {
+            payload_version: 1,
+            payload: b"payload-v1".to_vec(),
+        };
+
+        sign_entity(&mut conn, &vault, &entity, ENTITY_ID)
+            .await
+            .unwrap();
+
+        // Simulate key rotation: update the stored key_version to a stale value.
+        // After real rotation the vault's root_key_history_id would advance, but
+        // here we achieve the same mismatch by back-dating the envelope's key_version.
+        diesel::update(integrity_envelope::table)
+            .filter(integrity_envelope::entity_kind.eq("dummy_entity"))
+            .filter(integrity_envelope::entity_id.eq(ENTITY_ID))
+            .set(integrity_envelope::key_version.eq(0))
+            .execute(&mut conn)
+            .await
+            .unwrap();
+
+        // Must NOT error — version mismatch is Unavailable, not tampered.
+        let status = verify_entity(&mut conn, &vault, &entity, ENTITY_ID)
+            .await
+            .expect("key version mismatch must not be treated as an error");
+
+        assert_eq!(
+            status,
+            AttestationStatus::Unavailable,
+            "stale key_version must yield Unavailable, not MacMismatch"
+        );
+    }
 }