/errors/experiment_arm_error.proto 

--- v25/errors/experiment_arm_error.proto   2026-10-09 20:57:58.000000000 +0000
+++ v26/errors/experiment_arm_error.proto   2026-10-09 20:58:06.000000000 +0000
@@ -111,5 +111,9 @@
     // An asset group asset cannot be used multiple times in one arm.
     // Use different asset group assets in one arm of the experiment.
     DUPLICATE_ASSET_GROUP_ASSETS_IN_ONE_ARM = 23;
+
+    // Campaign already has AI Max enabled and can't participate in
+    // ADOPT_AI_MAX experiment.
+    ADOPT_AI_MAX_CAMPAIGN_ALREADY_ENABLED = 26;
   }
 }