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
@@ -394,9 +394,15 @@
   memberIDs: $ReadOnlyArray<string>,
   sourceMessageID: ?string,
 ): string {
-  const pendingThreadKey = sourceMessageID
-    ? `sidebar/${sourceMessageID}`
-    : [...memberIDs].sort().join('+');
+  let pendingThreadKey;
+  if (sourceMessageID && threadTypeIsThick(threadType)) {
+    pendingThreadKey = `thick_sidebar/${sourceMessageID}`;
+  } else if (sourceMessageID) {
+    pendingThreadKey = `sidebar/${sourceMessageID}`;
+  } else {
+    pendingThreadKey = [...memberIDs].sort().join('+');
+  }
+
   const pendingThreadTypeString = sourceMessageID ? '' : `type${threadType}/`;
   return `pending/${pendingThreadTypeString}${pendingThreadKey}`;
 }
@@ -415,13 +421,21 @@
   if (!pendingThreadIDMatches) {
     return null;
   }
+
   const [threadTypeString, threadKey] = pendingThreadIDMatches[1].split('/');
-  const threadType =
-    threadTypeString === 'sidebar'
-      ? threadTypes.SIDEBAR
-      : assertThreadType(Number(threadTypeString.replace('type', '')));
-  const memberIDs = threadTypeString === 'sidebar' ? [] : threadKey.split('+');
-  const sourceMessageID = threadTypeString === 'sidebar' ? threadKey : null;
+  let threadType;
+  if (threadTypeString === 'thick_sidebar') {
+    threadType = threadTypes.THICK_SIDEBAR;
+  } else if (threadTypeString === 'sidebar') {
+    threadType = threadTypes.SIDEBAR;
+  } else {
+    threadType = assertThreadType(Number(threadTypeString.replace('type', '')));
+  }
+
+  const threadTypeStringIsSidebar =
+    threadTypeString === 'sidebar' || threadTypeString === 'thick_sidebar';
+  const memberIDs = threadTypeStringIsSidebar ? [] : threadKey.split('+');
+  const sourceMessageID = threadTypeStringIsSidebar ? threadKey : null;
   return {
     threadType,
     memberIDs,
diff --git a/lib/shared/thread-utils.test.js b/lib/shared/thread-utils.test.js
--- a/lib/shared/thread-utils.test.js
+++ b/lib/shared/thread-utils.test.js
@@ -3,6 +3,7 @@
 import {
   parsePendingThreadID,
   threadInfoFromRawThreadInfo,
+  getPendingThreadID,
 } from './thread-utils.js';
 import { threadInfoValidator } from '../permissions/minimally-encoded-thread-permissions-validators.js';
 import type { RawThreadInfo } from '../types/minimally-encoded-thread-permissions-types.js';
@@ -30,6 +31,15 @@
     );
   });
 
+  const thickSidebarResult = {
+    threadType: threadTypes.THICK_SIDEBAR,
+    memberIDs: [],
+    sourceMessageID: '12345',
+  };
+  expect(parsePendingThreadID('pending/thick_sidebar/12345')).toStrictEqual(
+    thickSidebarResult,
+  );
+
   it('should return correct data for real pending sidebar ID', () => {
     const pendingPersonalResult = {
       threadType: threadTypes.GENESIS_PERSONAL,
@@ -195,3 +205,11 @@
     expect(threadInfoValidator.is(threadInfo)).toBe(true);
   });
 });
+
+describe('getPendingThreadID', () => {
+  it('should return correct ID from thick sidebar input', () => {
+    expect(
+      getPendingThreadID(threadTypes.THICK_SIDEBAR, [], '12345'),
+    ).toStrictEqual('pending/thick_sidebar/12345');
+  });
+});
diff --git a/lib/utils/validation-utils.js b/lib/utils/validation-utils.js
--- a/lib/utils/validation-utils.js
+++ b/lib/utils/validation-utils.js
@@ -112,7 +112,7 @@
   '[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}';
 const idSchemaRegex = `(?:(?:[0-9]+|${uuidRegex})\\|)?(?:[0-9]+|${uuidRegex})`;
 
-const pendingThreadIDRegex = `pending/(type[0-9]+/[0-9]+(\\+[0-9]+)*|sidebar/${idSchemaRegex})`;
+const pendingThreadIDRegex = `pending/(type[0-9]+/[0-9]+(\\+[0-9]+)*|(sidebar|thick_sidebar)/${idSchemaRegex})`;
 const thickThreadIDRegex: RegExp = new RegExp(`^${uuidRegex}$`);
 
 const chatNameMaxLength = 191;