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
@@ -101,6 +101,7 @@
 import { useSelector } from '../utils/redux-utils.js';
 import { firstLine } from '../utils/string-utils.js';
 import { trimText } from '../utils/text-utils.js';
+import { idSchemaRegex } from '../utils/validation-utils.js';
 
 const chatNameMaxLength = 191;
 const chatNameMinLength = 0;
@@ -272,8 +273,7 @@
   return `pending/${pendingThreadTypeString}${pendingThreadKey}`;
 }
 
-const pendingThreadIDRegex =
-  'pending/(type[0-9]+/[0-9]+(\\+[0-9]+)*|sidebar/[0-9]+)';
+const pendingThreadIDRegex = `pending/(type[0-9]+/[0-9]+(\\+[0-9]+)*|sidebar/${idSchemaRegex})`;
 
 type PendingThreadIDContents = {
   +threadType: ThreadType,
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
@@ -13,6 +13,15 @@
     expect(parsePendingThreadID('pending/sidebar/12345')).toStrictEqual(
       sidebarResult,
     );
+
+    const sidebarResultWithNewSchema = {
+      threadType: threadTypes.SIDEBAR,
+      memberIDs: [],
+      sourceMessageID: '789|12345',
+    };
+    expect(parsePendingThreadID('pending/sidebar/789|12345')).toStrictEqual(
+      sidebarResultWithNewSchema,
+    );
   });
 
   it('should return correct data for real pending sidebar ID', () => {
diff --git a/lib/utils/url-utils.js b/lib/utils/url-utils.js
--- a/lib/utils/url-utils.js
+++ b/lib/utils/url-utils.js
@@ -1,5 +1,6 @@
 // @flow
 
+import { idSchemaRegex } from './validation-utils.js';
 import { pendingThreadIDRegex } from '../shared/thread-utils.js';
 
 export type URLInfo = {
@@ -20,7 +21,7 @@
 // to regexes must be reflected in infoFromURL.
 const yearRegex = new RegExp('(/|^)year/([0-9]+)(/|$)', 'i');
 const monthRegex = new RegExp('(/|^)month/([0-9]+)(/|$)', 'i');
-const threadRegex = new RegExp('(/|^)thread/([0-9]+)(/|$)', 'i');
+const threadRegex = new RegExp(`(/|^)thread/(${idSchemaRegex})(/|$)`, 'i');
 const verifyRegex = new RegExp('(/|^)verify/([a-f0-9]+)(/|$)', 'i');
 const calendarRegex = new RegExp('(/|^)calendar(/|$)', 'i');
 const chatRegex = new RegExp('(/|^)chat(/|$)', 'i');
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
@@ -219,6 +219,8 @@
   return input;
 }
 
+const idSchemaRegex = '(?:[0-9]+\\|)?[0-9]+';
+
 export {
   tBool,
   tString,
@@ -245,4 +247,5 @@
   convertClientIDsToServerIDs,
   convertServerIDsToClientIDs,
   convertObject,
+  idSchemaRegex,
 };
diff --git a/lib/utils/validation-utils.test.js b/lib/utils/validation-utils.test.js
--- a/lib/utils/validation-utils.test.js
+++ b/lib/utils/validation-utils.test.js
@@ -1,5 +1,6 @@
 // @flow
 
+import invariant from 'invariant';
 import t from 'tcomb';
 
 import {
@@ -10,6 +11,7 @@
   tID,
   convertServerIDsToClientIDs,
   convertClientIDsToServerIDs,
+  idSchemaRegex,
 } from './validation-utils.js';
 import { threadTypes } from '../types/thread-types-enum.js';
 import { values } from '../utils/objects.js';
@@ -189,3 +191,19 @@
     ).toStrictEqual(serverData);
   });
 });
+
+describe('idSchemaRegex tests', () => {
+  it('should capture ids', () => {
+    const regex = new RegExp(`^(${idSchemaRegex})$`);
+    const ids = ['123|123', '0|0', '123', '0'];
+
+    for (const id of ids) {
+      const result = regex.exec(id);
+      expect(result).not.toBeNull();
+      invariant(result, 'result is not null');
+      const matches = [...result];
+      expect(matches).toHaveLength(2);
+      expect(matches[1]).toBe(id);
+    }
+  });
+});