before_model() — langchain Function Reference

Source Code

libs/langchain_v1/langchain/agents/middleware/summarization.py lines 293–328

    def before_model(
        self, state: AgentState[Any], runtime: Runtime[ContextT]
    ) -> dict[str, Any] | None:
        """Process messages before model invocation, potentially triggering summarization.

        Args:
            state: The agent state.
            runtime: The runtime environment.

        Returns:
            An updated state with summarized messages if summarization was performed.
        """
        messages = state["messages"]
        self._ensure_message_ids(messages)

        total_tokens = self.token_counter(messages)
        if not self._should_summarize(messages, total_tokens):
            return None

        cutoff_index = self._determine_cutoff_index(messages)

        if cutoff_index <= 0:
            return None

        messages_to_summarize, preserved_messages = self._partition_messages(messages, cutoff_index)

        summary = self._create_summary(messages_to_summarize)
        new_messages = self._build_new_messages(summary)

        return {
            "messages": [
                RemoveMessage(id=REMOVE_ALL_MESSAGES),
                *new_messages,
                *preserved_messages,
            ]
        }