diff --git a/src/api/friends/MessengerThreadChat.test.ts b/src/api/friends/MessengerThreadChat.test.ts
new file mode 100644
index 0000000..53320c4
--- /dev/null
+++ b/src/api/friends/MessengerThreadChat.test.ts
@@ -0,0 +1,29 @@
+import { describe, expect, it } from 'vitest';
+import { MessengerThreadChat } from './MessengerThreadChat';
+
+describe('MessengerThreadChat.offlineDelivered', () =>
+{
+    it('is true for a CHAT message with extraData "offline"', () =>
+    {
+        const chat = new MessengerThreadChat(5, 'hello', 60, 'offline', MessengerThreadChat.CHAT);
+        expect(chat.offlineDelivered).toBe(true);
+    });
+
+    it('is false for a normal CHAT message with no extraData', () =>
+    {
+        const chat = new MessengerThreadChat(5, 'hello', 0, null, MessengerThreadChat.CHAT);
+        expect(chat.offlineDelivered).toBe(false);
+    });
+
+    it('is false when extraData is some other value (e.g. group chat data)', () =>
+    {
+        const chat = new MessengerThreadChat(5, 'hi', 0, 'Bob/figurestr/5', MessengerThreadChat.CHAT);
+        expect(chat.offlineDelivered).toBe(false);
+    });
+
+    it('is false for a non-CHAT type even if extraData is "offline"', () =>
+    {
+        const chat = new MessengerThreadChat(5, 'hi', 0, 'offline', MessengerThreadChat.ROOM_INVITE);
+        expect(chat.offlineDelivered).toBe(false);
+    });
+});
diff --git a/src/api/friends/MessengerThreadChat.ts b/src/api/friends/MessengerThreadChat.ts
index 5e37167..c58b366 100644
--- a/src/api/friends/MessengerThreadChat.ts
+++ b/src/api/friends/MessengerThreadChat.ts
@@ -74,6 +74,11 @@ export class MessengerThreadChat
         return this._extraData;
     }
 
+    public get offlineDelivered(): boolean
+    {
+        return (this._type === MessengerThreadChat.CHAT) && (this._extraData === 'offline');
+    }
+
     public get date(): Date
     {
         return this._date;