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
@@ -12,7 +12,6 @@
 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,
@@ -54,6 +53,7 @@
 } from '../updaters/thread-permission-updaters.js';
 import { joinThread } from '../updaters/thread-updaters.js';
 import RelationshipChangeset from '../utils/relationship-changeset.js';
+import { isInvalidSidebarSource } from 'lib/types/message-types.js';
 
 const { commbot } = bots;
 
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
@@ -48,10 +48,10 @@
 import SentencePrefixSearchIndex from '../shared/sentence-prefix-search-index.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,
+  isInvalidSidebarSource,
 } from '../types/message-types.js';
 import { userRelationshipStatus } from '../types/relationship-types.js';
 import {
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
@@ -140,6 +140,7 @@
 import type { CallServerEndpointResultInfoInterface } from '../utils/call-server-endpoint.js';
 import { values } from '../utils/objects.js';
 import { tID, tNumber, tShape } from '../utils/validation-utils.js';
+import { messageSpecs } from '../shared/messages/message-specs.js';
 
 const composableMessageTypes = new Set([
   messageTypes.TEXT,
@@ -199,10 +200,11 @@
   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
+    (message.type === messageTypes.REACTION ||
+      message.type === messageTypes.EDIT_MESSAGE ||
+      message.type === messageTypes.SIDEBAR_SOURCE ||
+      message.type === messageTypes.TOGGLE_PIN) &&
+    messageSpecs[message.type].canBeSidebarSource
   );
 }