What are you considering spent? When the proof is sent from one person to the other. Or when the receiver actually claims it from the mint.
Discussion
Right now, I have it receiving and sending Lightning payments only. My initial motivation is to first build a persistent back end for my custodial Lightning payment service.
The next step is to receive tokens directly, likely through a DM. Once I get all of the basic mechanics worked out, I will build in the multi-mint capability. I was happy to get all of the swapping and consolidation working. That was a big job.
Have you looked into NIP-60 yet? Your idea sounds more general purpose, but we are working on a cashu-specific one: https://github.com/nostr-protocol/nips/pull/1369
Yep. NIP-60 was my starting point, but I realized I could do more general stuff. My primary motivation is to swap out an internal custodial wallet component I built that relies on a Postgres database. I realized I could use a relay, and with better security (removing my database as a honeypot of data). I implemented the proof storage as per the NIP and added in some lightning capabilities.