diff --git a/src/routers/deep_agent_chat.py b/src/routers/deep_agent_chat.py
index d77896b..1f84054 100755
--- a/src/routers/deep_agent_chat.py
+++ b/src/routers/deep_agent_chat.py
@@ -130,6 +130,7 @@ async def chat_stream(request: DeepAgentChatRequest):
 
     # 5. 初始化系统消息
     initial_messages = []
+    design_backend = ""
     if not source_thread_id or is_branching:
         cp = request.config_params
         if cp: