langchain/callbacks/tracers/stdout.py/ConsoleCallbackHandler/_on_chain_start
class ConsoleCallbackHandler:    def _on_chain_start(self, run: Run) -> None:
        crumbs = self.get_breadcrumbs(run)
        print(
            f"{get_colored_text('[chain/start]', color='green')} "
            + get_bolded_text(f"[{crumbs}] Entering Chain run with input:\n")
            + f"{try_json_stringify(run.inputs, '[inputs]')}"
        )