langchain/chat_models/anthropic.py/ChatAnthropic/_convert_one_message_to_text
class ChatAnthropic:    def _convert_one_message_to_text(self, message: BaseMessage) -> str:
        if isinstance(message, ChatMessage):
            message_text = f"\n\n{message.role.capitalize()}: {message.content}"
        elif isinstance(message, HumanMessage):
            message_text = f"{self.HUMAN_PROMPT} {message.content}"
        elif isinstance(message, AIMessage):
            message_text = f"{self.AI_PROMPT} {message.content}"
        elif isinstance(message, SystemMessage):
            message_text = f"{self.HUMAN_PROMPT} <admin>{message.content}</admin>"
        else:
            raise ValueError(f"Got unknown type {message}")
        return message_text