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
@@ -60,6 +60,7 @@
   threadTypes,
   threadPermissions,
   threadTypeIsCommunityRoot,
+  assertThreadType,
 } from '../types/thread-types';
 import { type ClientUpdateInfo, updateTypes } from '../types/update-types';
 import type {
@@ -272,6 +273,34 @@
 const locallyUniqueThreadIDRegex =
   'pending/(type[0-9]+/[0-9]+(\\+[0-9]+)*|sidebar/[0-9]+)';
 
+type LocallyUniqueThreadIDContents = {
+  +threadType: ThreadType,
+  +memberIDs: $ReadOnlyArray<string>,
+  +sourceMessageID: ?string,
+};
+
+function parseLocallyUniqueThreadID(
+  pendingThreadID: string,
+): ?LocallyUniqueThreadIDContents {
+  const locallyUniqueRegex = new RegExp(`^${locallyUniqueThreadIDRegex}$`);
+  const pendingThreadIDMatches = locallyUniqueRegex.exec(pendingThreadID);
+  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;
+  return {
+    threadType,
+    memberIDs,
+    sourceMessageID,
+  };
+}
+
 type CreatePendingThreadArgs = {
   +viewerID: string,
   +threadType: ThreadType,
@@ -1385,6 +1414,7 @@
   getSingleOtherUser,
   getLocallyUniqueThreadID,
   locallyUniqueThreadIDRegex,
+  parseLocallyUniqueThreadID,
   createPendingThread,
   createPendingThreadItem,
   createPendingSidebar,
diff --git a/lib/shared/thread-utils.test.js b/lib/shared/thread-utils.test.js
new file mode 100644
--- /dev/null
+++ b/lib/shared/thread-utils.test.js
@@ -0,0 +1,60 @@
+// @flow
+
+import { threadTypes } from '../types/thread-types';
+import { parseLocallyUniqueThreadID } from './thread-utils';
+
+describe('parseLocallyUniqueThreadID(pendingThreadID: string)', () => {
+  it('should return correct data for real pending sidebar ID', () => {
+    const sidebarResult = {
+      threadType: threadTypes.SIDEBAR,
+      memberIDs: [],
+      sourceMessageID: '12345',
+    };
+    expect(parseLocallyUniqueThreadID('pending/sidebar/12345')).toStrictEqual(
+      sidebarResult,
+    );
+  });
+
+  it('should return correct data for real pending sidebar ID', () => {
+    const pendingPersonalResult = {
+      threadType: threadTypes.PERSONAL,
+      memberIDs: ['83810', '86622'],
+      sourceMessageID: null,
+    };
+    expect(
+      parseLocallyUniqueThreadID('pending/type6/83810+86622'),
+    ).toStrictEqual(pendingPersonalResult);
+
+    const pendingCommunityOpenResult = {
+      threadType: threadTypes.COMMUNITY_OPEN_SUBTHREAD,
+      memberIDs: ['83810', '86622', '83889'],
+      sourceMessageID: null,
+    };
+    expect(
+      parseLocallyUniqueThreadID('pending/type3/83810+86622+83889'),
+    ).toStrictEqual(pendingCommunityOpenResult);
+  });
+
+  it('should return null when there are missing information in ID', () => {
+    expect(parseLocallyUniqueThreadID('pending/type4/')).toBeNull();
+    expect(parseLocallyUniqueThreadID('type12/83810+86622')).toBeNull();
+    expect(parseLocallyUniqueThreadID('pending/83810')).toBeNull();
+    expect(parseLocallyUniqueThreadID('pending')).toBeNull();
+    expect(parseLocallyUniqueThreadID('')).toBeNull();
+    expect(parseLocallyUniqueThreadID('pending/something/12345')).toBeNull();
+  });
+
+  it('should return null when the format is invalid', () => {
+    expect(parseLocallyUniqueThreadID('someothertext/type1/12345')).toBeNull();
+    expect(
+      parseLocallyUniqueThreadID('pending/type6/12312+++11+12'),
+    ).toBeNull();
+    expect(parseLocallyUniqueThreadID('pending/type3/83810+')).toBeNull();
+  });
+
+  it('should throw invariant violation when thread type is invalid ', () => {
+    expect(() =>
+      parseLocallyUniqueThreadID('pending/type123/12345'),
+    ).toThrowError('number is not ThreadType enum');
+  });
+});