diff --git a/keyserver/src/socket/socket.js b/keyserver/src/socket/socket.js
--- a/keyserver/src/socket/socket.js
+++ b/keyserver/src/socket/socket.js
@@ -17,10 +17,7 @@
 import { mostRecentUpdateTimestamp } from 'lib/shared/update-utils.js';
 import { hasMinCodeVersion } from 'lib/shared/version-utils.js';
 import { endpointIsSocketSafe } from 'lib/types/endpoints.js';
-import {
-  type RawEntryInfo,
-  calendarQueryValidator,
-} from 'lib/types/entry-types.js';
+import type { RawEntryInfo } from 'lib/types/entry-types.js';
 import { defaultNumberPerThread } from 'lib/types/message-types.js';
 import { redisMessageTypes, type RedisMessage } from 'lib/types/redis-types.js';
 import {
@@ -30,6 +27,7 @@
 import {
   sessionCheckFrequency,
   stateCheckInactivityActivationInterval,
+  sessionStateValidator,
 } from 'lib/types/session-types.js';
 import {
   type ClientSocketMessage,
@@ -106,12 +104,7 @@
         cookie: t.maybe(tCookie),
         sessionID: t.maybe(t.String),
       }),
-      sessionState: tShape({
-        calendarQuery: calendarQueryValidator,
-        messagesCurrentAsOf: t.Number,
-        updatesCurrentAsOf: t.Number,
-        watchedIDs: t.list(t.String),
-      }),
+      sessionState: sessionStateValidator,
       clientResponses: t.list(clientResponseInputValidator),
     }),
   }),
diff --git a/lib/types/session-types.js b/lib/types/session-types.js
--- a/lib/types/session-types.js
+++ b/lib/types/session-types.js
@@ -1,13 +1,16 @@
 // @flow
 
+import t, { type TInterface } from 'tcomb';
+
 import type { AuthActionSource } from './account-types.js';
-import type { CalendarQuery } from './entry-types.js';
+import { type CalendarQuery, calendarQueryValidator } from './entry-types.js';
 import type { MixedRawThreadInfos } from './thread-types.js';
 import {
   type UserInfo,
   type CurrentUserInfo,
   type LoggedOutUserInfo,
 } from './user-types.js';
+import { tShape } from '../utils/validation-utils.js';
 
 export const cookieLifetime = 30 * 24 * 60 * 60 * 1000; // in milliseconds
 // Interval the server waits after a state check before starting a new one
@@ -95,12 +98,19 @@
 };
 
 export type SessionState = {
-  calendarQuery: CalendarQuery,
-  messagesCurrentAsOf: number,
-  updatesCurrentAsOf: number,
-  watchedIDs: $ReadOnlyArray<string>,
+  +calendarQuery: CalendarQuery,
+  +messagesCurrentAsOf: number,
+  +updatesCurrentAsOf: number,
+  +watchedIDs: $ReadOnlyArray<string>,
 };
 
+export const sessionStateValidator: TInterface<SessionState> = tShape({
+  calendarQuery: calendarQueryValidator,
+  messagesCurrentAsOf: t.Number,
+  updatesCurrentAsOf: t.Number,
+  watchedIDs: t.list(t.String),
+});
+
 export type SessionIdentification = Partial<{
   cookie: ?string,
   sessionID: ?string,