/errors/asset_group_error.proto 

--- v23/errors/asset_group_error.proto  2026-06-18 21:38:47.000000000 +0000
+++ v24/errors/asset_group_error.proto  2026-06-18 21:38:54.000000000 +0000
@@ -84,5 +84,9 @@

     // Cannot mutate asset group for campaign with removed status.
     CANNOT_MUTATE_ASSET_GROUP_FOR_REMOVED_CAMPAIGN = 17;
+
+    // The asset group cannot be removed because it's the last one in the
+    // campaign.
+    CANNOT_REMOVE_ALL_ASSET_GROUPS_FROM_CAMPAIGN = 18;
   }
 }