diff --git a/lib/types/socket-types.js b/lib/types/socket-types.js
--- a/lib/types/socket-types.js
+++ b/lib/types/socket-types.js
@@ -228,12 +228,12 @@
   currentUserInfo: currentUserInfoValidator,
 });
 
-export type ServerStateSyncFullSocketPayload = {
+export type ServerStateSyncFullSocketPayload = $ReadOnly<{
   ...ServerFullStateSync,
   +type: 0,
   // Included iff client is using sessionIdentifierTypes.BODY_SESSION_ID
   +sessionID?: string,
-};
+}>;
 const serverStateSyncFullSocketPayloadValidator =
   tShape<ServerStateSyncFullSocketPayload>({
     ...serverFullStateSyncValidator.meta.props,
@@ -255,33 +255,33 @@
   userInfos: t.list(userInfoValidator),
 });
 
-export type ClientIncrementalStateSync = {
+export type ClientIncrementalStateSync = $ReadOnly<{
   ...BaseIncrementalStateSync,
   +updatesResult: ClientUpdatesResult,
-};
-export type StateSyncIncrementalActionPayload = {
+}>;
+export type StateSyncIncrementalActionPayload = $ReadOnly<{
   ...ClientIncrementalStateSync,
   +calendarQuery: CalendarQuery,
   +keyserverID: string,
-};
-type ClientStateSyncIncrementalSocketPayload = {
+}>;
+export type ClientStateSyncIncrementalSocketPayload = $ReadOnly<{
   +type: 1,
   ...ClientIncrementalStateSync,
-};
+}>;
 
-export type ServerIncrementalStateSync = {
+export type ServerIncrementalStateSync = $ReadOnly<{
   ...BaseIncrementalStateSync,
   +updatesResult: ServerUpdatesResult,
-};
+}>;
 const serverIncrementalStateSyncValidator = tShape<ServerIncrementalStateSync>({
   ...baseIncrementalStateSyncValidator.meta.props,
   updatesResult: serverUpdatesResultValidator,
 });
 
-type ServerStateSyncIncrementalSocketPayload = {
+type ServerStateSyncIncrementalSocketPayload = $ReadOnly<{
   +type: 1,
   ...ServerIncrementalStateSync,
-};
+}>;
 const serverStateSyncIncrementalSocketPayloadValidator =
   tShape<ServerStateSyncIncrementalSocketPayload>({
     type: tNumber(stateSyncPayloadTypes.INCREMENTAL),