diff --git a/backend/src/controllers/editUsername.controller.ts b/backend/src/controllers/editUsername.controller.ts
index 3a04fb9..ccdf39d 100644
--- a/backend/src/controllers/editUsername.controller.ts
+++ b/backend/src/controllers/editUsername.controller.ts
@@ -22,6 +22,14 @@ const EditUsernameController = async (
         .status(400)
         .json({ success: false, message: "Please enter your new username" });
       
+    // --- ĐOẠN CODE BẠN ĐÓNG GÓP THÊM VÀO ---
+    if (newUsername.length < 3 || newUsername.length > 20) {
+      return res
+        .status(400)
+        .json({ success: false, message: "Username must be between 3 and 20 characters" });
+    }
+    // --------------------------------------
+
     const existingUser = await User.findOne({ clientId });
 
     if (!existingUser)
@@ -39,4 +47,4 @@ const EditUsernameController = async (
   }
 };
 
-export default EditUsernameController;
\ No newline at end of file
+export default EditUsernameController;