diff --git a/endpoints/api/trigger.py b/endpoints/api/trigger.py
index 8df544ea2..9b0b1de64 100644
--- a/endpoints/api/trigger.py
+++ b/endpoints/api/trigger.py
@@ -392,6 +392,10 @@ class ActivateBuildTrigger(RepositoryParamResource):
         'branch_name': {
           'type': 'string',
           'description': '(GitHub Only) If specified, the name of the GitHub branch to build.'
+        },
+        'commit_sha': {
+          'type': 'string',
+          'description': '(Custom Only) If specified, the ref/SHA1 used to checkout a git repository.'
         }
       }
     }