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 { messageSpecs } from 'lib/shared/messages/message-specs.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,
@@ -195,8 +195,11 @@
     validateMembers: { initialMemberIDs, ghostMemberIDs },
   } = await promiseAll(checkPromises);
 
-  if (sourceMessage && isInvalidSidebarSource(sourceMessage)) {
-    throw new ServerError('invalid_parameters');
+  if (sourceMessage) {
+    const messageSpec = messageSpecs[sourceMessage.type];
+    if (!messageSpec.canBeSidebarSource(sourceMessage)) {
+      throw new ServerError('invalid_parameters');
+    }
   }
 
   let { id } = request;
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
@@ -10,6 +10,7 @@
 import { type ParserRules } from './markdown.js';
 import { extractMentionsFromText } from './mention-utils.js';
 import { getMessageTitle } from './message-utils.js';
+import { messageSpecs } from './messages/message-specs.js';
 import { relationshipBlockedInEitherDirection } from './relationship-utils.js';
 import threadWatcher from './thread-watcher.js';
 import {
@@ -46,7 +47,6 @@
 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,
@@ -1275,11 +1275,12 @@
   const messageCreatorUserInfo = useSelector(
     state => state.userStore.userInfos[messageInfo.creator.id],
   );
+  const messageSpec = messageSpecs[messageInfo.type];
 
   if (
     !messageInfo.id ||
     threadInfo.sourceMessageID === messageInfo.id ||
-    isInvalidSidebarSource(messageInfo)
+    !messageSpec.canBeSidebarSource(messageInfo)
   ) {
     return false;
   }