diff --git a/src/lib.rs b/src/lib.rs
index b6a1dca..01b8146 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -404,6 +404,43 @@ mod tests {
         assert_eq!(plan.status, "planned");
     }
 
+
+    #[test]
+    fn session_idempotency_is_scoped_per_tenant_user_session() {
+        let mut store = TestMemoryStore::new();
+
+        append_session_message(
+            &mut store,
+            AppendSessionMessageRequest {
+                tenant_id: "tenant_1".to_string(),
+                user_id: "usr_1".to_string(),
+                session_id: "sess_1".to_string(),
+                role: "user".to_string(),
+                content: "first".to_string(),
+                idempotency_key: Some("same-key".to_string()),
+            },
+            20,
+        )
+        .unwrap();
+
+        let appended = append_session_message(
+            &mut store,
+            AppendSessionMessageRequest {
+                tenant_id: "tenant_2".to_string(),
+                user_id: "usr_2".to_string(),
+                session_id: "sess_2".to_string(),
+                role: "user".to_string(),
+                content: "second".to_string(),
+                idempotency_key: Some("same-key".to_string()),
+            },
+            20,
+        )
+        .unwrap();
+
+        assert_eq!(appended.hot_tail_count, 1);
+        assert_eq!(store.session_messages.len(), 2);
+    }
+
     #[test]
     fn retrieval_telemetry_contains_contract_fields() {
         let mut store = TestMemoryStore::new();
diff --git a/src/testkit/mod.rs b/src/testkit/mod.rs
index 85eb963..c5d94c4 100644
--- a/src/testkit/mod.rs
+++ b/src/testkit/mod.rs
@@ -213,11 +213,12 @@ impl AuditSink for TestMemoryStore {
 
 impl SessionStore for TestMemoryStore {
     fn append_session_message(&mut self, message: SessionMessage) -> CoreResult<()> {
-        if self
-            .session_messages
-            .iter()
-            .any(|existing| existing.idempotency_key == message.idempotency_key)
-        {
+        if self.session_messages.iter().any(|existing| {
+            existing.tenant_id == message.tenant_id
+                && existing.user_id == message.user_id
+                && existing.session_id == message.session_id
+                && existing.idempotency_key == message.idempotency_key
+        }) {
             return Ok(());
         }
         self.session_messages.push(message);