diff --git a/lib/types/thread-types.js b/lib/types/thread-types.js
--- a/lib/types/thread-types.js
+++ b/lib/types/thread-types.js
@@ -338,24 +338,24 @@
   +ghostMemberIDs?: ?$ReadOnlyArray<string>,
 };
 type NewThinThreadRequest =
-  | {
+  | $ReadOnly<{
       +type: 3 | 4 | 6 | 7 | 8 | 9 | 10 | 11 | 12,
       ...BaseNewThreadRequest,
-    }
-  | {
+    }>
+  | $ReadOnly<{
       +type: 5,
       +sourceMessageID: string,
       ...BaseNewThreadRequest,
-    };
+    }>;
 
-export type ClientNewThinThreadRequest = {
+export type ClientNewThinThreadRequest = $ReadOnly<{
   ...NewThinThreadRequest,
   +calendarQuery: CalendarQuery,
-};
-export type ServerNewThinThreadRequest = {
+}>;
+export type ServerNewThinThreadRequest = $ReadOnly<{
   ...NewThinThreadRequest,
   +calendarQuery?: ?CalendarQuery,
-};
+}>;
 
 export type NewThreadResponse = {
   +updatesResult: {