diff --git a/agent/tool_output_length_middleware.py b/agent/tool_output_length_middleware.py
index 34a939f..96b6c1e 100644
--- a/agent/tool_output_length_middleware.py
+++ b/agent/tool_output_length_middleware.py
@@ -314,7 +314,15 @@ class ToolOutputLengthMiddleware(AgentMiddleware):
         if isinstance(result, ToolMessage):
             # Process ToolMessage
             content = result.text
-            if content and len(content) > self.max_length:
+            # 如果内容为空字符串，填充为 "empty"
+            if content is None or content == '':
+                logger.info(f"Tool output for '{tool_name}' is empty, filling with 'empty'")
+                return ToolMessage(
+                    content="empty",
+                    tool_call_id=result.tool_call_id,
+                    name=result.name
+                )
+            if len(content) > self.max_length:
                 # Truncate the content
                 truncated_content, metadata = self.truncate_content(content, tool_name)
 
@@ -355,7 +363,16 @@ class ToolOutputLengthMiddleware(AgentMiddleware):
                 for msg in messages:
                     if isinstance(msg, ToolMessage):
                         tool_msg_tool_name = getattr(msg, 'name', tool_name)
-                        if self.should_process_tool(tool_msg_tool_name) and len(msg.content) > self.max_length:
+                        msg_content = msg.content if isinstance(msg.content, str) else str(msg.content) if msg.content else ''
+                        # 如果内容为空字符串，填充为 "empty"
+                        if msg_content == '':
+                            logger.info(f"Tool output for '{tool_msg_tool_name}' in Command is empty, filling with 'empty'")
+                            updated_messages.append(ToolMessage(
+                                content="empty",
+                                tool_call_id=msg.tool_call_id,
+                                name=msg.name
+                            ))
+                        elif self.should_process_tool(tool_msg_tool_name) and len(msg_content) > self.max_length:
                             # Truncate the ToolMessage content
                             truncated_content, metadata = self.truncate_content(msg.content, tool_msg_tool_name)
 
@@ -424,7 +441,15 @@ class ToolOutputLengthMiddleware(AgentMiddleware):
         # Handle different return types (same logic as sync version)
         if isinstance(result, ToolMessage):
             content = result.text
-            if content and len(content) > self.max_length:
+            # 如果内容为空字符串，填充为 "empty"
+            if content is None or content == '':
+                logger.info(f"Tool output for '{tool_name}' is empty, filling with 'empty'")
+                return ToolMessage(
+                    content="empty",
+                    tool_call_id=result.tool_call_id,
+                    name=result.name
+                )
+            if len(content) > self.max_length:
                 truncated_content, metadata = self.truncate_content(content, tool_name)
 
                 truncated_message = ToolMessage(
@@ -458,7 +483,16 @@ class ToolOutputLengthMiddleware(AgentMiddleware):
             for msg in messages:
                 if isinstance(msg, ToolMessage):
                     tool_msg_tool_name = getattr(msg, 'name', tool_name)
-                    if self.should_process_tool(tool_msg_tool_name) and len(msg.content) > self.max_length:
+                    msg_content = msg.content if isinstance(msg.content, str) else str(msg.content) if msg.content else ''
+                    # 如果内容为空字符串，填充为 "empty"
+                    if msg_content == '':
+                        logger.info(f"Tool output for '{tool_msg_tool_name}' in Command is empty, filling with 'empty'")
+                        updated_messages.append(ToolMessage(
+                            content="empty",
+                            tool_call_id=msg.tool_call_id,
+                            name=msg.name
+                        ))
+                    elif self.should_process_tool(tool_msg_tool_name) and len(msg_content) > self.max_length:
                         truncated_content, metadata = self.truncate_content(msg.content, tool_msg_tool_name)
 
                         truncated_msg = ToolMessage(