What happens if payment settles and your agent crashes before saving it?

So I ran into a pretty annoying problem while building RoundWatch, and I’m curious how others deal with this.

Catching the moment a blockchain payment happens? Honestly, that’s the easy part.

The real headache starts when something breaks mid-flight:
Your agent finishes the job -> the payment goes through on-chain -> but the system crashes right before updating the local state.

When everything boots back up, how does your system know what actually went down? If you don't handle this right, you risk double-spending, losing track of progress, or getting stuck trying to resolve a payment that’s already settled.

That’s exactly the edge case I ended up solving with RoundWatch. Instead of just assuming 'hey, we found the transaction, we're good,' the system keeps a durable watch on the expected settlement. It only moves the state forward once it has verifiable proof on the chain. And yes, the state survives sudden restarts.

How do you guys handle this in your own AI agents or crypto-payment workflows?

Do you just keep everything in your backend using idempotency keys and DB states? Webhooks? Polling the chain? Event sourcing?

And honestly, at what point does using an external settlement observer actually make sense, rather than just adding yet another dependency to your stack?