/errors/mutate_error.proto 

--- v25/errors/mutate_error.proto   2026-07-21 22:09:38.000000000 +0000
+++ v26/errors/mutate_error.proto   2026-11-09 16:22:49.000000000 +0000
@@ -68,5 +68,9 @@
     // Mutates are generally not allowed if the customer contains non-exempt
     // campaigns without the EU political advertising declaration.
     EU_POLITICAL_ADVERTISING_DECLARATION_REQUIRED = 17;
+
+    // The combination of resources in this mutate is not supported for
+    // multi-party approval.
+    MULTI_PARTY_APPROVAL_UNSUPPORTED_RESOURCE_COMBINATION = 18;
   }
 }