trimMessages

import { trimMessages, AIMessage, BaseMessage, HumanMessage, SystemMessage } from "@langchain/core/messages";

const messages = [
  new SystemMessage("This is a 4 token text. The full message is 10 tokens."),
  new HumanMessage({
    content: "This is a 4 token text. The full message is 10 tokens.",
    id: "first",
  }),
  new AIMessage({
    content: [
      { type: "text", text: "This is the FIRST 4 token block." },
      { type: "text", text: "This is the SECOND 4 token block." },
    ],
    id: "second",
  }),
  new HumanMessage({
    content: "This is a 4 token text. The full message is 10 tokens.",
    id: "third",
  }),
  new AIMessage({
    content: "This is a 4 token text. The full message is 10 tokens.",
    id: "fourth",
  }),
];

function dummyTokenCounter(messages: BaseMessage[]): number {
  // treat each message like it adds 3 default tokens at the beginning
  // of the message and at the end of the message. 3 + 4 + 3 = 10 tokens
  // per message.

  const defaultContentLen = 4;
  const defaultMsgPrefixLen = 3;
  const defaultMsgSuffixLen = 3;

  let count = 0;
  for (const msg of messages) {
    if (typeof msg.content === "string") {
      count += defaultMsgPrefixLen + defaultContentLen + defaultMsgSuffixLen;
    }
    if (Array.isArray(msg.content)) {
      count +=
        defaultMsgPrefixLen +
        msg.content.length * defaultContentLen +
        defaultMsgSuffixLen;
    }
  }
  return count;
}