diff --git a/lib/shared/updates/join-thread-spec.js b/lib/shared/updates/join-thread-spec.js
--- a/lib/shared/updates/join-thread-spec.js
+++ b/lib/shared/updates/join-thread-spec.js
@@ -178,4 +178,5 @@
     truncationStatus: messageTruncationStatusValidator,
     rawEntryInfos: t.list(rawEntryInfoValidator),
   }),
+  getUpdatedThreadInfo: (update: ThreadJoinUpdateInfo) => update.threadInfo,
 });
diff --git a/lib/shared/updates/update-spec.js b/lib/shared/updates/update-spec.js
--- a/lib/shared/updates/update-spec.js
+++ b/lib/shared/updates/update-spec.js
@@ -14,9 +14,11 @@
   MessageTruncationStatuses,
   FetchMessageInfosResult,
 } from '../../types/message-types.js';
+import type { RawThreadInfo } from '../../types/minimally-encoded-thread-permissions-types.js';
 import type {
   RawThreadInfos,
   MixedRawThreadInfos,
+  LegacyRawThreadInfo,
 } from '../../types/thread-types.js';
 import type { UpdateType } from '../../types/update-types-enum.js';
 import type {
@@ -103,4 +105,7 @@
   +generateOpsForUserInfoUpdates?: (
     update: UpdateInfo,
   ) => ?$ReadOnlyArray<UserStoreOperation>,
+  +getUpdatedThreadInfo?: (
+    info: UpdateInfo,
+  ) => LegacyRawThreadInfo | RawThreadInfo,
 };
diff --git a/lib/shared/updates/update-thread-spec.js b/lib/shared/updates/update-thread-spec.js
--- a/lib/shared/updates/update-thread-spec.js
+++ b/lib/shared/updates/update-thread-spec.js
@@ -119,4 +119,5 @@
     time: t.Number,
     threadInfo: mixedRawThreadInfoValidator,
   }),
+  getUpdatedThreadInfo: (update: ThreadUpdateInfo) => update.threadInfo,
 });