A Branch Is a Copied Prefix
Fork copies a conversation prefix and its independent workspace checkpoint into a new session identifier. The live flags are firebrand fork <session> [--turn N] [--edit] [--at N]. There is no --through. The new file starts its own append. Checkpoints are named folds of that prefix, not parallel histories. apply-checkpoint applies a verified git workspace checkpoint back to the origin checkout (--turn). It does not restore a conversation fold as the next starting point. None of these verbs rewrite the source notebook. If you need to 'fix' a past line, you are asking for an editor, and Firebrand will not give you one.
firebrand replay renders the record. No model call. It does not prove that the same request would decode to the same events tomorrow. That decode proof is ReplayLeg: a fixture walked through a dialect, used in conformance tests. Diff and stats read the notebook; they do not invent a second score. The laboratory track will spend a full lesson on compare-without-ranking. Here the rule is smaller: a fork that silently drops a line is not a branch. It is a rewrite with extra ceremony.
What Replay Can Prove
The CLI replay verb can show what the notebook already stored. ReplayLeg can prove that a captured wire, against the same dialect, produces the same normalized events — or that it does not. Neither proves the model would say the same words tomorrow. Tokens are not a hash. The record's job is to make the difference visible, not to pretend sampling is deterministic.