diff --git a/keyserver/src/responders/activity-responders.js b/keyserver/src/responders/activity-responders.js
--- a/keyserver/src/responders/activity-responders.js
+++ b/keyserver/src/responders/activity-responders.js
@@ -3,12 +3,14 @@
 import t from 'tcomb';
 import type { TList } from 'tcomb';
 
-import type {
-  UpdateActivityResult,
-  UpdateActivityRequest,
-  SetThreadUnreadStatusRequest,
-  SetThreadUnreadStatusResult,
-  ActivityUpdate,
+import {
+  type UpdateActivityResult,
+  type UpdateActivityRequest,
+  type SetThreadUnreadStatusRequest,
+  type SetThreadUnreadStatusResult,
+  type ActivityUpdate,
+  setThreadUnreadStatusResult,
+  updateActivityResultValidator,
 } from 'lib/types/activity-types.js';
 import { tShape } from 'lib/utils/validation-utils.js';
 
@@ -17,7 +19,7 @@
   activityUpdater,
   setThreadUnreadStatus,
 } from '../updaters/activity-updaters.js';
-import { validateInput } from '../utils/validation-utils.js';
+import { validateInput, validateOutput } from '../utils/validation-utils.js';
 
 const activityUpdatesInputValidator: TList<Array<ActivityUpdate>> = t.list(
   tShape({
@@ -37,7 +39,8 @@
 ): Promise<UpdateActivityResult> {
   const request: UpdateActivityRequest = input;
   await validateInput(viewer, inputValidator, request);
-  return await activityUpdater(viewer, request);
+  const result = await activityUpdater(viewer, request);
+  return validateOutput(viewer, updateActivityResultValidator, result);
 }
 
 const setThreadUnreadStatusValidator = tShape({
@@ -52,7 +55,8 @@
   const request: SetThreadUnreadStatusRequest = input;
   await validateInput(viewer, setThreadUnreadStatusValidator, request);
 
-  return await setThreadUnreadStatus(viewer, request);
+  const result = await setThreadUnreadStatus(viewer, request);
+  return validateOutput(viewer, setThreadUnreadStatusResult, result);
 }
 
 export {
diff --git a/keyserver/src/responders/entry-responders.js b/keyserver/src/responders/entry-responders.js
--- a/keyserver/src/responders/entry-responders.js
+++ b/keyserver/src/responders/entry-responders.js
@@ -46,7 +46,7 @@
   compareNewCalendarQuery,
 } from '../updaters/entry-updaters.js';
 import { commitSessionUpdate } from '../updaters/session-updaters.js';
-import { validateInput } from '../utils/validation-utils.js';
+import { validateInput, validateOutput } from '../utils/validation-utils.js';
 
 type EntryQueryInput = {
   +startDate: string,
@@ -144,7 +144,10 @@
   await verifyCalendarQueryThreadIDs(request);
 
   const response = await fetchEntryInfos(viewer, [request]);
-  return { ...response, userInfos: {} };
+  return validateOutput(viewer, fetchEntryInfosResponseValidator, {
+    ...response,
+    userInfos: {},
+  });
 }
 
 const entryRevisionHistoryFetchInputValidator = tShape({
@@ -163,7 +166,12 @@
   const request: FetchEntryRevisionInfosRequest = input;
   await validateInput(viewer, entryRevisionHistoryFetchInputValidator, request);
   const entryHistory = await fetchEntryRevisionInfo(viewer, request.id);
-  return { result: entryHistory };
+  const response = { result: entryHistory };
+  return validateOutput(
+    viewer,
+    fetchEntryRevisionInfosResultValidator,
+    response,
+  );
 }
 
 const createEntryRequestInputValidator = tShape({
@@ -189,7 +197,8 @@
 ): Promise<SaveEntryResponse> {
   const request: CreateEntryRequest = input;
   await validateInput(viewer, createEntryRequestInputValidator, request);
-  return await createEntry(viewer, request);
+  const response = await createEntry(viewer, request);
+  return validateOutput(viewer, saveEntryResponseValidator, response);
 }
 
 const saveEntryRequestInputValidator = tShape({
@@ -207,7 +216,8 @@
 ): Promise<SaveEntryResponse> {
   const request: SaveEntryRequest = input;
   await validateInput(viewer, saveEntryRequestInputValidator, request);
-  return await updateEntry(viewer, request);
+  const response = await updateEntry(viewer, request);
+  return validateOutput(viewer, saveEntryResponseValidator, response);
 }
 
 const deleteEntryRequestInputValidator = tShape({
@@ -231,7 +241,8 @@
 ): Promise<DeleteEntryResponse> {
   const request: DeleteEntryRequest = input;
   await validateInput(viewer, deleteEntryRequestInputValidator, request);
-  return await deleteEntry(viewer, request);
+  const response = await deleteEntry(viewer, request);
+  return validateOutput(viewer, deleteEntryResponseValidator, response);
 }
 
 const restoreEntryRequestInputValidator = tShape({
@@ -253,7 +264,8 @@
 ): Promise<RestoreEntryResponse> {
   const request: RestoreEntryRequest = input;
   await validateInput(viewer, restoreEntryRequestInputValidator, request);
-  return await restoreEntry(viewer, request);
+  const response = await restoreEntry(viewer, request);
+  return validateOutput(viewer, restoreEntryResponseValidator, response);
 }
 
 export const deltaEntryInfosResultValidator: TInterface<DeltaEntryInfosResult> =
@@ -283,12 +295,12 @@
     commitSessionUpdate(viewer, sessionUpdate),
   ]);
 
-  return {
+  return validateOutput(viewer, deltaEntryInfosResultValidator, {
     rawEntryInfos: response.rawEntryInfos,
     deletedEntryIDs: response.deletedEntryIDs,
     // Old clients expect userInfos object
     userInfos: [],
-  };
+  });
 }
 
 export {
diff --git a/keyserver/src/responders/keys-responders.js b/keyserver/src/responders/keys-responders.js
--- a/keyserver/src/responders/keys-responders.js
+++ b/keyserver/src/responders/keys-responders.js
@@ -11,7 +11,7 @@
 
 import { fetchSessionPublicKeys } from '../fetchers/key-fetchers.js';
 import type { Viewer } from '../session/viewer.js';
-import { validateInput } from '../utils/validation-utils.js';
+import { validateInput, validateOutput } from '../utils/validation-utils.js';
 
 const getSessionPublicKeysInputValidator = tShape({
   session: t.String,
@@ -30,7 +30,12 @@
   }
   const request: GetSessionPublicKeysArgs = input;
   await validateInput(viewer, getSessionPublicKeysInputValidator, request);
-  return await fetchSessionPublicKeys(request.session);
+  const response = await fetchSessionPublicKeys(request.session);
+  return validateOutput(
+    viewer,
+    getSessionPublicKeysResponseValidator,
+    response,
+  );
 }
 
 export { getSessionPublicKeysResponder };
diff --git a/keyserver/src/responders/message-report-responder.js b/keyserver/src/responders/message-report-responder.js
--- a/keyserver/src/responders/message-report-responder.js
+++ b/keyserver/src/responders/message-report-responder.js
@@ -11,7 +11,7 @@
 
 import createMessageReport from '../creators/message-report-creator.js';
 import type { Viewer } from '../session/viewer.js';
-import { validateInput } from '../utils/validation-utils.js';
+import { validateInput, validateOutput } from '../utils/validation-utils.js';
 
 const messageReportCreationRequestInputValidator = tShape({
   messageID: t.String,
@@ -32,7 +32,8 @@
   const request: MessageReportCreationRequest = input;
 
   const rawMessageInfos = await createMessageReport(viewer, request);
-  return { messageInfo: rawMessageInfos[0] };
+  const result = { messageInfo: rawMessageInfos[0] };
+  return validateOutput(viewer, messageReportCreationResultValidator, result);
 }
 
 export { messageReportCreationResponder };
diff --git a/keyserver/src/responders/message-responders.js b/keyserver/src/responders/message-responders.js
--- a/keyserver/src/responders/message-responders.js
+++ b/keyserver/src/responders/message-responders.js
@@ -60,7 +60,7 @@
   assignImages,
   assignMessageContainerToMedia,
 } from '../updaters/upload-updaters.js';
-import { validateInput } from '../utils/validation-utils.js';
+import { validateInput, validateOutput } from '../utils/validation-utils.js';
 
 const sendTextMessageRequestInputValidator = tShape({
   threadID: t.String,
@@ -118,7 +118,8 @@
   }
   const rawMessageInfos = await createMessages(viewer, [messageData]);
 
-  return { newMessageInfo: rawMessageInfos[0] };
+  const response = { newMessageInfo: rawMessageInfos[0] };
+  return validateOutput(viewer, sendMessageResponseValidator, response);
 }
 
 const fetchMessageInfosRequestInputValidator = tShape({
@@ -144,7 +145,10 @@
     { threadCursors: request.cursors },
     request.numberPerThread ? request.numberPerThread : defaultNumberPerThread,
   );
-  return { ...response, userInfos: {} };
+  return validateOutput(viewer, fetchMessageInfosResponseValidator, {
+    ...response,
+    userInfos: {},
+  });
 }
 
 const sendMultimediaMessageRequestInputValidator = t.union([
@@ -239,7 +243,8 @@
     );
   }
 
-  return { newMessageInfo };
+  const response = { newMessageInfo };
+  return validateOutput(viewer, sendMessageResponseValidator, response);
 }
 
 const sendReactionMessageRequestInputValidator = tShape({
@@ -313,7 +318,8 @@
 
   const rawMessageInfos = await createMessages(viewer, [messageData]);
 
-  return { newMessageInfo: rawMessageInfos[0] };
+  const response = { newMessageInfo: rawMessageInfos[0] };
+  return validateOutput(viewer, sendMessageResponseValidator, response);
 }
 
 const editMessageRequestInputValidator = tShape({
@@ -403,7 +409,8 @@
 
   const newMessageInfos = await createMessages(viewer, messagesData);
 
-  return { newMessageInfos };
+  const response = { newMessageInfos };
+  return validateOutput(viewer, sendEditMessageResponseValidator, response);
 }
 
 const fetchPinnedMessagesResponderInputValidator = tShape({
@@ -425,7 +432,8 @@
     fetchPinnedMessagesResponderInputValidator,
     input,
   );
-  return await fetchPinnedMessageInfos(viewer, request);
+  const response = await fetchPinnedMessageInfos(viewer, request);
+  return validateOutput(viewer, fetchPinnedMessagesResultValidator, response);
 }
 
 export {
diff --git a/keyserver/src/responders/relationship-responders.js b/keyserver/src/responders/relationship-responders.js
--- a/keyserver/src/responders/relationship-responders.js
+++ b/keyserver/src/responders/relationship-responders.js
@@ -11,7 +11,7 @@
 
 import type { Viewer } from '../session/viewer.js';
 import { updateRelationships } from '../updaters/relationship-updaters.js';
-import { validateInput } from '../utils/validation-utils.js';
+import { validateInput, validateOutput } from '../utils/validation-utils.js';
 
 const updateRelationshipInputValidator = tShape({
   action: t.enums.of(relationshipActionsList, 'relationship action'),
@@ -31,7 +31,8 @@
 ): Promise<RelationshipErrors> {
   const request: RelationshipRequest = input;
   await validateInput(viewer, updateRelationshipInputValidator, request);
-  return await updateRelationships(viewer, request);
+  const response = await updateRelationships(viewer, request);
+  return validateOutput(viewer, relationshipErrorsValidator, response);
 }
 
 export { updateRelationshipsResponder };
diff --git a/keyserver/src/responders/report-responders.js b/keyserver/src/responders/report-responders.js
--- a/keyserver/src/responders/report-responders.js
+++ b/keyserver/src/responders/report-responders.js
@@ -29,7 +29,7 @@
   fetchReduxToolsImport,
 } from '../fetchers/report-fetchers.js';
 import type { Viewer } from '../session/viewer.js';
-import { validateInput } from '../utils/validation-utils.js';
+import { validateInput, validateOutput } from '../utils/validation-utils.js';
 
 const tActionSummary = tShape({
   type: t.String,
@@ -156,7 +156,7 @@
   if (!response) {
     throw new ServerError('ignored_report');
   }
-  return response;
+  return validateOutput(viewer, reportCreationResponseValidator, response);
 }
 
 const reportMultiCreationRequestInputValidator = tShape({
@@ -224,7 +224,12 @@
     fetchErrorReportInfosRequestInputValidator,
     request,
   );
-  return await fetchErrorReportInfos(viewer, request);
+  const response = await fetchErrorReportInfos(viewer, request);
+  return validateOutput(
+    viewer,
+    fetchErrorReportInfosResponseValidator,
+    response,
+  );
 }
 
 async function errorReportDownloadResponder(
diff --git a/keyserver/src/responders/search-responders.js b/keyserver/src/responders/search-responders.js
--- a/keyserver/src/responders/search-responders.js
+++ b/keyserver/src/responders/search-responders.js
@@ -11,7 +11,7 @@
 
 import { searchForUsers } from '../search/users.js';
 import type { Viewer } from '../session/viewer.js';
-import { validateInput } from '../utils/validation-utils.js';
+import { validateInput, validateOutput } from '../utils/validation-utils.js';
 
 const userSearchRequestInputValidator = tShape({
   prefix: t.maybe(t.String),
@@ -29,7 +29,8 @@
   const request: UserSearchRequest = input;
   await validateInput(viewer, userSearchRequestInputValidator, request);
   const searchResults = await searchForUsers(request);
-  return { userInfos: searchResults };
+  const result = { userInfos: searchResults };
+  return validateOutput(viewer, userSearchResultValidator, result);
 }
 
 export { userSearchResponder };
diff --git a/keyserver/src/responders/thread-responders.js b/keyserver/src/responders/thread-responders.js
--- a/keyserver/src/responders/thread-responders.js
+++ b/keyserver/src/responders/thread-responders.js
@@ -56,7 +56,7 @@
   joinThread,
   toggleMessagePinForThread,
 } from '../updaters/thread-updaters.js';
-import { validateInput } from '../utils/validation-utils.js';
+import { validateInput, validateOutput } from '../utils/validation-utils.js';
 
 const threadDeletionRequestInputValidator = tShape({
   threadID: t.String,
@@ -77,7 +77,8 @@
 ): Promise<LeaveThreadResult> {
   const request: ThreadDeletionRequest = input;
   await validateInput(viewer, threadDeletionRequestInputValidator, request);
-  return await deleteThread(viewer, request);
+  const result = await deleteThread(viewer, request);
+  return validateOutput(viewer, leaveThreadResultValidator, result);
 }
 
 const roleChangeRequestInputValidator = tShape({
@@ -105,7 +106,8 @@
 ): Promise<ChangeThreadSettingsResult> {
   const request: RoleChangeRequest = input;
   await validateInput(viewer, roleChangeRequestInputValidator, request);
-  return await updateRole(viewer, request);
+  const result = await updateRole(viewer, request);
+  return validateOutput(viewer, changeThreadSettingsResultValidator, result);
 }
 
 const removeMembersRequestInputValidator = tShape({
@@ -119,7 +121,8 @@
 ): Promise<ChangeThreadSettingsResult> {
   const request: RemoveMembersRequest = input;
   await validateInput(viewer, removeMembersRequestInputValidator, request);
-  return await removeMembers(viewer, request);
+  const result = await removeMembers(viewer, request);
+  return validateOutput(viewer, changeThreadSettingsResultValidator, result);
 }
 
 const leaveThreadRequestInputValidator = tShape({
@@ -132,7 +135,8 @@
 ): Promise<LeaveThreadResult> {
   const request: LeaveThreadRequest = input;
   await validateInput(viewer, leaveThreadRequestInputValidator, request);
-  return await leaveThread(viewer, request);
+  const result = await leaveThread(viewer, request);
+  return validateOutput(viewer, leaveThreadResultValidator, result);
 }
 
 const updateThreadRequestInputValidator = tShape({
@@ -155,7 +159,8 @@
 ): Promise<ChangeThreadSettingsResult> {
   const request: UpdateThreadRequest = input;
   await validateInput(viewer, updateThreadRequestInputValidator, request);
-  return await updateThread(viewer, request);
+  const result = await updateThread(viewer, request);
+  return validateOutput(viewer, changeThreadSettingsResultValidator, result);
 }
 
 const threadRequestValidationShape = {
@@ -201,9 +206,10 @@
   const request: ServerNewThreadRequest = input;
   await validateInput(viewer, newThreadRequestInputValidator, request);
 
-  return await createThread(viewer, request, {
+  const result = await createThread(viewer, request, {
     silentlyFailMembers: request.type === threadTypes.SIDEBAR,
   });
+  return validateOutput(viewer, newThreadResponseValidator, result);
 }
 
 const joinThreadRequestInputValidator = tShape({
@@ -235,7 +241,8 @@
     await verifyCalendarQueryThreadIDs(request.calendarQuery);
   }
 
-  return await joinThread(viewer, request);
+  const result = await joinThread(viewer, request);
+  return validateOutput(viewer, threadJoinResultValidator, result);
 }
 
 const threadFetchMediaRequestInputValidator = tShape({
@@ -253,7 +260,8 @@
 ): Promise<ThreadFetchMediaResult> {
   const request: ThreadFetchMediaRequest = input;
   await validateInput(viewer, threadFetchMediaRequestInputValidator, request);
-  return await fetchMediaForThread(viewer, request);
+  const result = await fetchMediaForThread(viewer, request);
+  return validateOutput(viewer, threadFetchMediaResultValidator, result);
 }
 
 const toggleMessagePinRequestInputValidator = tShape({
@@ -273,7 +281,8 @@
 ): Promise<ToggleMessagePinResult> {
   const request: ToggleMessagePinRequest = input;
   await validateInput(viewer, toggleMessagePinRequestInputValidator, request);
-  return await toggleMessagePinForThread(viewer, request);
+  const result = await toggleMessagePinForThread(viewer, request);
+  return validateOutput(viewer, toggleMessagePinResultValidator, result);
 }
 
 export {
diff --git a/keyserver/src/responders/user-responders.js b/keyserver/src/responders/user-responders.js
--- a/keyserver/src/responders/user-responders.js
+++ b/keyserver/src/responders/user-responders.js
@@ -29,10 +29,11 @@
   notificationTypeValues,
   logInActionSources,
 } from 'lib/types/account-types.js';
-import type {
-  ClientAvatar,
-  UpdateUserAvatarRequest,
-  UpdateUserAvatarResponse,
+import {
+  type ClientAvatar,
+  clientAvatarValidator,
+  type UpdateUserAvatarRequest,
+  type UpdateUserAvatarResponse,
 } from 'lib/types/avatar-types.js';
 import type {
   IdentityKeysBlob,
@@ -58,6 +59,7 @@
   threadSubscriptionValidator,
 } from 'lib/types/subscription-types.js';
 import { rawThreadInfoValidator } from 'lib/types/thread-types.js';
+import { createUpdatesResultValidator } from 'lib/types/update-types.js';
 import {
   type PasswordUpdate,
   loggedOutUserInfoValidator,
@@ -129,7 +131,7 @@
 import { userSubscriptionUpdater } from '../updaters/user-subscription-updaters.js';
 import { viewerAcknowledgmentUpdater } from '../updaters/viewer-acknowledgment-updater.js';
 import { getOlmUtility } from '../utils/olm-utils.js';
-import { validateInput } from '../utils/validation-utils.js';
+import { validateInput, validateOutput } from '../utils/validation-utils.js';
 
 const subscriptionUpdateRequestInputValidator = tShape({
   threadID: t.String,
@@ -151,7 +153,9 @@
   const request: SubscriptionUpdateRequest = input;
   await validateInput(viewer, subscriptionUpdateRequestInputValidator, request);
   const threadSubscription = await userSubscriptionUpdater(viewer, request);
-  return { threadSubscription };
+  return validateOutput(viewer, subscriptionUpdateResponseValidator, {
+    threadSubscription,
+  });
 }
 
 const accountUpdateInputValidator = tShape({
@@ -206,12 +210,13 @@
     ]);
     viewer.setNewCookie(anonymousViewerData);
   }
-  return {
+  const response = {
     currentUserInfo: {
       id: viewer.id,
       anonymous: true,
     },
   };
+  return validateOutput(viewer, logOutResponseValidator, response);
 }
 
 const deleteAccountRequestInputValidator = tShape({
@@ -226,7 +231,7 @@
   await validateInput(viewer, deleteAccountRequestInputValidator, request);
   const result = await deleteAccount(viewer, request);
   invariant(result, 'deleteAccount should return result if handed request');
-  return result;
+  return validateOutput(viewer, logOutResponseValidator, result);
 }
 
 const deviceTokenUpdateRequestInputValidator = tShape({
@@ -287,7 +292,8 @@
       throw new ServerError('invalid_signature');
     }
   }
-  return await createAccount(viewer, request);
+  const response = await createAccount(viewer, request);
+  return validateOutput(viewer, registerResponseValidator, response);
 }
 
 type ProcessSuccessfulLoginParams = {
@@ -492,13 +498,14 @@
 
   const id = userRow.id.toString();
 
-  return await processSuccessfulLogin({
+  const response = await processSuccessfulLogin({
     viewer,
     input,
     userID: id,
     calendarQuery,
     signedIdentityKeysBlob,
   });
+  return validateOutput(viewer, logInResponseValidator, response);
 }
 
 const siweAuthRequestInputValidator = tShape({
@@ -626,7 +633,7 @@
   }
 
   // 9. Complete login with call to `processSuccessfulLogin(...)`.
-  return await processSuccessfulLogin({
+  const response = await processSuccessfulLogin({
     viewer,
     input,
     userID,
@@ -634,6 +641,7 @@
     socialProof,
     signedIdentityKeysBlob,
   });
+  return validateOutput(viewer, logInResponseValidator, response);
 }
 
 const updatePasswordRequestInputValidator = tShape({
@@ -654,7 +662,8 @@
   if (request.calendarQuery) {
     request.calendarQuery = normalizeCalendarQuery(request.calendarQuery);
   }
-  return await updatePassword(viewer, request);
+  const response = await updatePassword(viewer, request);
+  return validateOutput(viewer, logInResponseValidator, response);
 }
 
 const updateUserSettingsInputValidator = tShape({
@@ -671,7 +680,7 @@
 ): Promise<void> {
   const request: UpdateUserSettingsRequest = input;
   await validateInput(viewer, updateUserSettingsInputValidator, request);
-  return await updateUserSettings(viewer, request);
+  await updateUserSettings(viewer, request);
 }
 
 const policyAcknowledgmentRequestInputValidator = tShape({
@@ -691,13 +700,24 @@
   await viewerAcknowledgmentUpdater(viewer, request.policy);
 }
 
+export const updateUserAvatarResponseValidator: TInterface<UpdateUserAvatarResponse> =
+  tShape<UpdateUserAvatarResponse>({
+    updates: createUpdatesResultValidator,
+  });
+
+const updateUserAvatarResponderValidator = t.union([
+  t.maybe(clientAvatarValidator),
+  updateUserAvatarResponseValidator,
+]);
+
 async function updateUserAvatarResponder(
   viewer: Viewer,
   input: any,
 ): Promise<?ClientAvatar | UpdateUserAvatarResponse> {
   const request: UpdateUserAvatarRequest = input;
   await validateInput(viewer, updateUserAvatarRequestValidator, request);
-  return await updateUserAvatar(viewer, request);
+  const result = await updateUserAvatar(viewer, request);
+  return validateOutput(viewer, updateUserAvatarResponderValidator, result);
 }
 
 export {
diff --git a/lib/types/update-types.js b/lib/types/update-types.js
--- a/lib/types/update-types.js
+++ b/lib/types/update-types.js
@@ -15,6 +15,7 @@
   type UserInfo,
   userInfoValidator,
   type UserInfos,
+  userInfosValidator,
   type LoggedInUserInfo,
   loggedInUserInfoValidator,
   type OldLoggedInUserInfo,
@@ -395,6 +396,11 @@
   +viewerUpdates: $ReadOnlyArray<ServerUpdateInfo>,
   +userInfos: UserInfos,
 };
+export const createUpdatesResultValidator: TInterface<CreateUpdatesResult> =
+  tShape<CreateUpdatesResult>({
+    viewerUpdates: t.list(serverUpdateInfoValidator),
+    userInfos: userInfosValidator,
+  });
 
 export type ServerCreateUpdatesResponse = {
   +viewerUpdates: $ReadOnlyArray<ServerUpdateInfo>,