Fork & edit from a checkpoint

Forking is expressed as "submit from the parent checkpoint of a specific message". Pick a message, read its parent checkpoint, and submit new input from there.

Learn more: For branching-chat and time-travel UX, see the branching chat and time travel documentation.

Mental model

Two pieces work together:

1. injectMessageMetadata(stream, msgId) — returns { parentCheckpointId } for the given message, or undefined until it loads. See Selectors.
2. submit(input, { forkFrom }) — dispatches a new run whose initial checkpoint is forkFrom, replacing anything that happened after it on the thread.

You pick a message, read its parent checkpoint, and submit from there with new input. The new turn becomes the canonical continuation of the thread — old messages after the fork point are superseded.

Example: edit a user turn

import { injectStream, injectMessageMetadata, type AnyStream } from "@langchain/angular";
import { HumanMessage, type BaseMessage } from "@langchain/core/messages";

function EditButton({
  stream,
  message,
  newContent,
}: {
  stream: AnyStream;
  message: BaseMessage;
  newContent: string;
}) {
  const metadata = injectMessageMetadata(stream, message.id);

  return (
    <button
      disabled={!metadata?.parentCheckpointId}
      onClick={() => {
        const forkFrom = metadata?.parentCheckpointId;
        if (!forkFrom) return;
        void stream.submit({ messages: [new HumanMessage(newContent)] }, { forkFrom });
      }}
    >
      Save edit
    </button>
  );
}

Example: retry an AI turn

To retry the last AI turn, fork from the parent checkpoint of the preceding human message and re-submit the same input:

function Retry({ stream }: { stream: AnyStream }) {
  const lastHuman = [...stream.messages].reverse().find((m) => m.type === "human");
  const metadata = injectMessageMetadata(stream, lastHuman?.id);

  return (
    <button
      disabled={!metadata?.parentCheckpointId || !lastHuman}
      onClick={() => {
        const forkFrom = metadata?.parentCheckpointId;
        if (!forkFrom || !lastHuman) return;
        void stream.submit({ messages: [lastHuman] }, { forkFrom });
      }}
    >
      Retry
    </button>
  );
}

API reference