langchain/callbacks/stdout.py/StdOutCallbackHandler/on_tool_end
class StdOutCallbackHandler:    def on_tool_end(
        self,
        output: str,
        color: Optional[str] = None,
        observation_prefix: Optional[str] = None,
        llm_prefix: Optional[str] = None,
        **kwargs: Any,
    ) -> None:
        """If not the final action, print out observation."""
        if observation_prefix is not None:
            print_text(f"\n{observation_prefix}")
        print_text(output, color=color if color else self.color)
        if llm_prefix is not None:
            print_text(f"\n{llm_prefix}")