diff --git a/keyserver/src/creators/thread-creator.js b/keyserver/src/creators/thread-creator.js
--- a/keyserver/src/creators/thread-creator.js
+++ b/keyserver/src/creators/thread-creator.js
@@ -9,10 +9,10 @@
   generatePendingThreadColor,
   generateRandomColor,
 } from 'lib/shared/color-utils.js';
-import { isInvalidSidebarSource } from 'lib/shared/message-utils.js';
 import { getThreadTypeParentRequirement } from 'lib/shared/thread-utils.js';
 import type { Shape } from 'lib/types/core.js';
 import { messageTypes } from 'lib/types/message-types-enum.js';
+import { isInvalidSidebarSource } from 'lib/types/message-types.js';
 import { threadPermissions } from 'lib/types/thread-permission-types.js';
 import {
   threadTypes,
diff --git a/keyserver/src/fetchers/message-fetchers.js b/keyserver/src/fetchers/message-fetchers.js
--- a/keyserver/src/fetchers/message-fetchers.js
+++ b/keyserver/src/fetchers/message-fetchers.js
@@ -5,7 +5,6 @@
 import {
   sortMessageInfoList,
   shimUnsupportedRawMessageInfos,
-  isInvalidSidebarSource,
 } from 'lib/shared/message-utils.js';
 import { messageSpecs } from 'lib/shared/messages/message-specs.js';
 import { getNotifCollapseKey } from 'lib/shared/notif-utils.js';
@@ -15,6 +14,10 @@
   type MessageType,
   assertMessageType,
 } from 'lib/types/message-types-enum.js';
+import {
+  isInvalidSidebarSource,
+  defaultNumberPerThread,
+} from 'lib/types/message-types.js';
 import {
   type RawMessageInfo,
   type RawComposableMessageInfo,
@@ -29,7 +32,6 @@
   type FetchPinnedMessagesResult,
   type SearchMessagesResponse,
 } from 'lib/types/message-types.js';
-import { defaultNumberPerThread } from 'lib/types/message-types.js';
 import { threadPermissions } from 'lib/types/thread-permission-types.js';
 import { ServerError } from 'lib/utils/errors.js';
 
diff --git a/lib/shared/message-utils.js b/lib/shared/message-utils.js
--- a/lib/shared/message-utils.js
+++ b/lib/shared/message-utils.js
@@ -655,17 +655,6 @@
   return `${localIDPrefix}${nextLocalID}`;
 }
 
-function isInvalidSidebarSource(
-  message: RawMessageInfo | MessageInfo,
-): boolean %checks {
-  return (
-    message.type === messageTypes.REACTION ||
-    message.type === messageTypes.EDIT_MESSAGE ||
-    message.type === messageTypes.SIDEBAR_SOURCE ||
-    message.type === messageTypes.TOGGLE_PIN
-  );
-}
-
 export {
   localIDPrefix,
   messageKey,
@@ -695,5 +684,4 @@
   modifyItemForResultScreen,
   constructChangeRoleEntityText,
   useNextLocalID,
-  isInvalidSidebarSource,
 };
diff --git a/lib/shared/messages/sidebar-source-message-spec.js b/lib/shared/messages/sidebar-source-message-spec.js
--- a/lib/shared/messages/sidebar-source-message-spec.js
+++ b/lib/shared/messages/sidebar-source-message-spec.js
@@ -16,12 +16,12 @@
   type SidebarSourceMessageData,
   type SidebarSourceMessageInfo,
   type ClientDBMessageInfo,
+  isInvalidSidebarSource,
   rawSidebarSourceMessageInfoValidator,
 } from '../../types/message-types.js';
 import type { RawUnsupportedMessageInfo } from '../../types/messages/unsupported.js';
 import type { NotifTexts } from '../../types/notif-types.js';
 import type { RelativeUserInfo } from '../../types/user-types.js';
-import { isInvalidSidebarSource } from '../message-utils.js';
 import { hasMinCodeVersion } from '../version-utils.js';
 
 export const sidebarSourceMessageSpec: MessageSpec<
diff --git a/lib/shared/thread-utils.js b/lib/shared/thread-utils.js
--- a/lib/shared/thread-utils.js
+++ b/lib/shared/thread-utils.js
@@ -9,7 +9,7 @@
 import { generatePendingThreadColor } from './color-utils.js';
 import { type ParserRules } from './markdown.js';
 import { extractMentionsFromText } from './mention-utils.js';
-import { getMessageTitle, isInvalidSidebarSource } from './message-utils.js';
+import { getMessageTitle } from './message-utils.js';
 import { relationshipBlockedInEitherDirection } from './relationship-utils.js';
 import threadWatcher from './thread-watcher.js';
 import {
@@ -46,6 +46,7 @@
 import { getUserAvatarForThread } from '../shared/avatar-utils.js';
 import type { CalendarQuery } from '../types/entry-types.js';
 import { messageTypes } from '../types/message-types-enum.js';
+import { isInvalidSidebarSource } from '../types/message-types.js';
 import {
   type RobotextMessageInfo,
   type ComposableMessageInfo,
diff --git a/lib/types/message-types.js b/lib/types/message-types.js
--- a/lib/types/message-types.js
+++ b/lib/types/message-types.js
@@ -195,6 +195,17 @@
   return ourMessageType;
 }
 
+export function isInvalidSidebarSource(
+  message: RawMessageInfo | MessageInfo,
+): boolean %checks {
+  return (
+    message.type === messageTypes.REACTION ||
+    message.type === messageTypes.EDIT_MESSAGE ||
+    message.type === messageTypes.SIDEBAR_SOURCE ||
+    message.type === messageTypes.TOGGLE_PIN
+  );
+}
+
 // *MessageData = passed to createMessages function to insert into database
 // Raw*MessageInfo = used by server, and contained in client's local store
 // *MessageInfo = used by client in UI code
diff --git a/lib/shared/message-utils.test.js b/lib/types/message-types.test.js
rename from lib/shared/message-utils.test.js
rename to lib/types/message-types.test.js
--- a/lib/shared/message-utils.test.js
+++ b/lib/types/message-types.test.js
@@ -1,29 +1,29 @@
 // @flow
 
-import { isInvalidSidebarSource } from './message-utils.js';
-import { messageTypes } from '../types/message-types-enum.js';
-import type { RawSidebarSourceMessageInfo } from '../types/message-types.js';
-import type { RawAddMembersMessageInfo } from '../types/messages/add-members.js';
-import type { RawChangeRoleMessageInfo } from '../types/messages/change-role.js';
-import type { RawChangeSettingsMessageInfo } from '../types/messages/change-settings.js';
-import type { RawCreateEntryMessageInfo } from '../types/messages/create-entry.js';
-import type { RawCreateSidebarMessageInfo } from '../types/messages/create-sidebar.js';
-import type { RawCreateSubthreadMessageInfo } from '../types/messages/create-subthread.js';
-import type { RawCreateThreadMessageInfo } from '../types/messages/create-thread.js';
-import type { RawDeleteEntryMessageInfo } from '../types/messages/delete-entry.js';
-import type { RawEditEntryMessageInfo } from '../types/messages/edit-entry.js';
-import type { RawEditMessageInfo } from '../types/messages/edit.js';
-import type { RawImagesMessageInfo } from '../types/messages/images.js';
-import type { RawJoinThreadMessageInfo } from '../types/messages/join-thread.js';
-import type { RawLeaveThreadMessageInfo } from '../types/messages/leave-thread.js';
-import type { RawMediaMessageInfo } from '../types/messages/media.js';
-import type { RawReactionMessageInfo } from '../types/messages/reaction.js';
-import type { RawRemoveMembersMessageInfo } from '../types/messages/remove-members.js';
-import type { RawRestoreEntryMessageInfo } from '../types/messages/restore-entry.js';
-import type { RawTextMessageInfo } from '../types/messages/text.js';
-import type { RawTogglePinMessageInfo } from '../types/messages/toggle-pin.js';
-import type { RawUpdateRelationshipMessageInfo } from '../types/messages/update-relationship.js';
-import { threadTypes } from '../types/thread-types-enum.js';
+import { messageTypes } from './message-types-enum.js';
+import { isInvalidSidebarSource } from './message-types.js';
+import type { RawSidebarSourceMessageInfo } from './message-types.js';
+import type { RawAddMembersMessageInfo } from './messages/add-members.js';
+import type { RawChangeRoleMessageInfo } from './messages/change-role.js';
+import type { RawChangeSettingsMessageInfo } from './messages/change-settings.js';
+import type { RawCreateEntryMessageInfo } from './messages/create-entry.js';
+import type { RawCreateSidebarMessageInfo } from './messages/create-sidebar.js';
+import type { RawCreateSubthreadMessageInfo } from './messages/create-subthread.js';
+import type { RawCreateThreadMessageInfo } from './messages/create-thread.js';
+import type { RawDeleteEntryMessageInfo } from './messages/delete-entry.js';
+import type { RawEditEntryMessageInfo } from './messages/edit-entry.js';
+import type { RawEditMessageInfo } from './messages/edit.js';
+import type { RawImagesMessageInfo } from './messages/images.js';
+import type { RawJoinThreadMessageInfo } from './messages/join-thread.js';
+import type { RawLeaveThreadMessageInfo } from './messages/leave-thread.js';
+import type { RawMediaMessageInfo } from './messages/media.js';
+import type { RawReactionMessageInfo } from './messages/reaction.js';
+import type { RawRemoveMembersMessageInfo } from './messages/remove-members.js';
+import type { RawRestoreEntryMessageInfo } from './messages/restore-entry.js';
+import type { RawTextMessageInfo } from './messages/text.js';
+import type { RawTogglePinMessageInfo } from './messages/toggle-pin.js';
+import type { RawUpdateRelationshipMessageInfo } from './messages/update-relationship.js';
+import { threadTypes } from './thread-types-enum.js';
 
 describe('isInvalidSidebarSource', () => {
   it('should return false for RawTextMessageInfo', () => {