langchain/callbacks/streamlit.py/StreamlitCallbackHandler/on_tool_end
class StreamlitCallbackHandler:    def on_tool_end(
        self,
        output: str,
        observation_prefix: Optional[str] = None,
        llm_prefix: Optional[str] = None,
        **kwargs: Any,
    ) -> None:
        """If not the final action, print out observation."""
        st.write(f"{observation_prefix}{output}")
        st.write(llm_prefix)