diff --git a/keyserver/src/socket/fetch-data.js b/keyserver/src/socket/fetch-data.js
--- a/keyserver/src/socket/fetch-data.js
+++ b/keyserver/src/socket/fetch-data.js
@@ -82,7 +82,7 @@
       +userInfos: Promise<$ReadOnlyArray<UserInfo>>,
     } = (promises: any);
     const results = await promiseAll(castPromises);
-    const payload: ServerStateSyncFullSocketPayload = {
+    let payload: ServerStateSyncFullSocketPayload = {
       type: stateSyncPayloadTypes.FULL,
       messagesResult,
       threadInfos: results.threadInfos,
@@ -97,7 +97,7 @@
       // and specify viewer.sessionChanged
       const { sessionID } = viewer;
       invariant(sessionID !== null && sessionID !== undefined, 'should be set');
-      payload.sessionID = sessionID;
+      payload = { ...payload, sessionID };
       viewer.sessionChanged = false;
     }
     return payload;
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),