diff --git a/api/core/agent/base_agent_runner.py b/api/core/agent/base_agent_runner.py
index 13c4e4c3d1..48c92ea2db 100644
--- a/api/core/agent/base_agent_runner.py
+++ b/api/core/agent/base_agent_runner.py
@@ -332,7 +332,7 @@ class BaseAgentRunner(AppRunner):
         agent_thought = updated_agent_thought
 
         if thought:
-            agent_thought.thought = thought
+            agent_thought.thought += thought
 
         if tool_name:
             agent_thought.tool = tool_name