diff --git a/rest-api-spec/src/main/resources/rest-api-spec/api/reindex.json b/rest-api-spec/src/main/resources/rest-api-spec/api/reindex.json
index 0ebfddd9413e1..2fbaf86cab616 100644
--- a/rest-api-spec/src/main/resources/rest-api-spec/api/reindex.json
+++ b/rest-api-spec/src/main/resources/rest-api-spec/api/reindex.json
@@ -45,9 +45,9 @@
         "default":"5m"
       },
       "slices":{
-        "type":"number",
+        "type":"number|string",
         "default":1,
-        "description":"The number of slices this task should be divided into. Defaults to 1 meaning the task isn't sliced into subtasks."
+        "description":"The number of slices this task should be divided into. Defaults to 1, meaning the task isn't sliced into subtasks. Can be set to `auto`."
       },
       "max_docs":{
         "type":"number",
diff --git a/rest-api-spec/src/main/resources/rest-api-spec/api/update_by_query.json b/rest-api-spec/src/main/resources/rest-api-spec/api/update_by_query.json
index 066bd8856a9c2..591371798d440 100644
--- a/rest-api-spec/src/main/resources/rest-api-spec/api/update_by_query.json
+++ b/rest-api-spec/src/main/resources/rest-api-spec/api/update_by_query.json
@@ -181,9 +181,9 @@
         "description":"The throttle to set on this request in sub-requests per second. -1 means no throttle."
       },
       "slices":{
-        "type":"number",
+        "type":"number|string",
         "default":1,
-        "description":"The number of slices this task should be divided into. Defaults to 1 meaning the task isn't sliced into subtasks."
+        "description":"The number of slices this task should be divided into. Defaults to 1, meaning the task isn't sliced into subtasks. Can be set to `auto`."
       }
     },
     "body":{