Class MessagesPlaceholder

libs/core/langchain_core/prompts/chat.py:110–243

Prompt template that assumes variable is already list of messages. A placeholder which can be used to pass in a list of messages. Direct usage: .. code-block:: python from langchain_core.prompts import MessagesPlaceholder prompt = MessagesPlaceholder("his

class MessagesPlaceholder(BaseMessagePromptTemplate):
    """Prompt template that assumes variable is already list of messages.

    A placeholder which can be used to pass in a list of messages.

    Direct usage:

        .. code-block:: python

            from langchain_core.prompts import MessagesPlaceholder

            prompt = MessagesPlaceholder("history")
            prompt.format_messages() # raises KeyError

            prompt = MessagesPlaceholder("history", optional=True)
            prompt.format_messages() # returns empty list []

            prompt.format_messages(
                history=[
                    ("system", "You are an AI assistant."),
                    ("human", "Hello!"),
                ]
            )
            # -> [
            #     SystemMessage(content="You are an AI assistant."),
            #     HumanMessage(content="Hello!"),
            # ]

    Building a prompt with chat history:

        .. code-block:: python

            from langchain_core.prompts import ChatPromptTemplate, MessagesPlaceholder

            prompt = ChatPromptTemplate.from_messages(
                [
                    ("system", "You are a helpful assistant."),
                    MessagesPlaceholder("history"),
                    ("human", "{question}")
                ]
            )
            prompt.invoke(
               {
                   "history": [("human", "what's 5 + 2"), ("ai", "5 + 2 is 7")],
                   "question": "now multiply that by 4"
               }
            )
            # -> ChatPromptValue(messages=[
            #     SystemMessage(content="You are a helpful assistant."),
            #     HumanMessage(content="what's 5 + 2"),
            #     AIMessage(content="5 + 2 is 7"),
            #     HumanMessage(content="now multiply that by 4"),
            # ])

    Limiting the number of messages:

        .. code-block:: python