injectMessageMetadata

Read metadata recorded for a specific message id — today exposes parentCheckpointId, the checkpoint the message was first seen on. Designed for fork / edit flows:

readonly meta = injectMessageMetadata(this.stream, () => this.msg().id);
// meta()?.parentCheckpointId

messageId accepts a raw string, a Signal<string | undefined>, or a plain getter — the binding re-evaluates whenever the id changes.

Returns undefined when the id isn't known yet (e.g. the server hasn't emitted parent_checkpoint for that message, or the message arrived via messages-channel deltas only and no values snapshot has landed for it yet).