fix get_tool_return (#11346)

Author: hwchase17

libs/langchain/langchain/agents/agent.py

@@ -1229,11 +1229,14 @@ def _get_tool_return(
         """Check if the tool is a returning tool."""
         agent_action, observation = next_step_output
         name_to_tool_map = {tool.name: tool for tool in self.tools}
+        return_value_key = "output"
+        if len(self.agent.return_values) > 0:
+            return_value_key = self.agent.return_values[0]
         # Invalid tools won't be in the map, so we return False.
         if agent_action.tool in name_to_tool_map:
             if name_to_tool_map[agent_action.tool].return_direct:
                 return AgentFinish(
-                    {self.agent.return_values[0]: observation},
+                    {return_value_key: observation},
                     "",
                 )
         return None