langchain.js
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    Function messagesStateReducer

    • Reducer function for combining two sets of messages in LangGraph's state system.

      This reducer handles several tasks:

      1. Normalizes both left and right message inputs to arrays.
      2. Coerces any message-like objects into real BaseMessage instances.
      3. Ensures all messages have unique, stable IDs by generating missing ones.
      4. If a RemoveMessage instance is encountered in right with the ID REMOVE_ALL_MESSAGES, all previous messages are discarded and only the subsequent messages in right are returned.
      5. Otherwise, merges left and right messages together following these rules:
        • If a message in right shares an ID with a message in left:
          • If it is a RemoveMessage, that message (by ID) is marked for removal.
          • If it is a normal message, it replaces the message with the same ID from left.
        • If a message in right does not exist in left:
          • If it is a RemoveMessage, this is considered an error (cannot remove non-existent ID).
          • Otherwise, the message is appended.
        • Messages flagged for removal are omitted from the final output.

      Parameters

      • left: Messages

        The existing array (or single message) of messages from current state.

      • right: Messages

        The new array (or single message) of messages to be applied.

      Returns BaseMessage<MessageStructure<MessageToolSet>, MessageType>[]

      A new array of BaseMessage objects representing the updated state.

      Error if a RemoveMessage is used to delete a message with an ID that does not exist in the merged list.

      const msg1 = new AIMessage("hello");
      const msg2 = new HumanMessage("hi");
      const removal = new RemoveMessage({ id: msg1.id });
      const newState = messagesStateReducer([msg1], [msg2, removal]);
      // newState will only contain msg2 (msg1 is removed)

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