_find_safe_cutoff_point() — langchain Function Reference

Source Code

libs/langchain_v1/langchain/agents/middleware/summarization.py lines 553–586

    def _find_safe_cutoff_point(messages: list[AnyMessage], cutoff_index: int) -> int:
        """Find a safe cutoff point that doesn't split AI/Tool message pairs.

        If the message at `cutoff_index` is a `ToolMessage`, search backward for the
        `AIMessage` containing the corresponding `tool_calls` and adjust the cutoff to
        include it. This ensures tool call requests and responses stay together.

        Falls back to advancing forward past `ToolMessage` objects only if no matching
        `AIMessage` is found (edge case).
        """
        if cutoff_index >= len(messages) or not isinstance(messages[cutoff_index], ToolMessage):
            return cutoff_index

        # Collect tool_call_ids from consecutive ToolMessages at/after cutoff
        tool_call_ids: set[str] = set()
        idx = cutoff_index
        while idx < len(messages) and isinstance(messages[idx], ToolMessage):
            tool_msg = cast("ToolMessage", messages[idx])
            if tool_msg.tool_call_id:
                tool_call_ids.add(tool_msg.tool_call_id)
            idx += 1

        # Search backward for AIMessage with matching tool_calls
        for i in range(cutoff_index - 1, -1, -1):
            msg = messages[i]
            if isinstance(msg, AIMessage) and msg.tool_calls:
                ai_tool_call_ids = {tc.get("id") for tc in msg.tool_calls if tc.get("id")}
                if tool_call_ids & ai_tool_call_ids:
                    # Found the AIMessage - move cutoff to include it
                    return i

        # Fallback: no matching AIMessage found, advance past ToolMessages to avoid
        # orphaned tool responses
        return idx