I **think** I have diagnosed the issue. Here is a callback URL that does not work: https://btcpay989117.lndyn.com/BTC/UILNURL/pay/lnaddress/pedromvpg?amount=1000&nostr=%7B%22id%22%3A%22da4036f7220a39ac48bb6497b1090048f49ee078d8677bec4aef6490f2b95a25%22%2C%22sig%22%3A%22d11193e5e90ce9975447678c43dbc470f5113688b2d008f8b5c539a7afbb9c895c41a30c097c44ccfc6ce26ddae0fed527837c62be8c20b0905cabe17ac9486d%22%2C%22pubkey%22%3A%2297f848adcc4c6276685fe48426de5614887c8a51ada0468cec71fba938272911%22%2C%22created_at%22%3A1743989620%2C%22kind%22%3A9734%2C%22tags%22%3A%5B%5B%22relays%22%2C%22wss%3A%2F%2Fbostr.lightningspore.com%22%2C%22wss%3A%2F%2Frelay-nwc-dev.rizful.com%2Fv1%22%2C%22wss%3A%2F%2Fbrb.io%2F%22%2C%22wss%3A%2F%2Feden.nostr.land%2F%22%2C%22wss%3A%2F%2Fnos.lol%2F%22%2C%22wss%3A%2F%2Fnostr-pub.semisol.dev%2F%22%2C%22wss%3A%2F%2Fnostr.orangepill.dev%2F%22%2C%22wss%3A%2F%2Fnostr.zebedee.cloud%2F%22%2C%22wss%3A%2F%2Foffchain.pub%2F%22%2C%22wss%3A%2F%2Fpuravida.nostr.land%2F%22%2C%22wss%3A%2F%2Fpyramid.fiatjaf.com%2F%22%2C%22wss%3A%2F%2Frelay.current.fyi%2F%22%2C%22wss%3A%2F%2Frelay.damus.io%2F%22%2C%22wss%3A%2F%2Frelay.snort.social%2F%22%2C%22wss%3A%2F%2Fnostr.wine%2F%22%2C%22wss%3A%2F%2F140.f7z.io%2F%22%2C%22wss%3A%2F%2Frelay.f7z.io%2F%22%2C%22wss%3A%2F%2Frelay.primal.net%2F%22%5D%2C%5B%22amount%22%2C%221000%22%5D%2C%5B%22lnurl%22%2C%22https%3A%2F%2Fbtcpay989117.lndyn.com%2F.well-known%2Flnurlp%2Fpedromvpg%22%5D%2C%5B%22p%22%2C%228cd2d0f8310f7009e94f50231870756cb39ba68f37506044910e2f71482b1788%22%5D%2C%5B%22e%22%2C%22b8a8ca5154b2725b6810df191f9d399ede245c0e79fd5bae94a19456711ddc66%22%5D%5D%2C%22content%22%3A%22%E2%9C%A8%22%7D&lnurl=LNURL1DP68GURN8GHJ7CN5VDCXZ7FE8QUNZVFH9EKXUEREDCHXXMMD9UH8WETVDSKKKMN0WAHZ7MRWW4EXCUP0WPJKGUN0D4M8QECJMRCWV ........ and here is a callback URL that DOES work... https://btcpay989117.lndyn.com/BTC/UILNURL/pay/lnaddress/pedromvpg?amount=1000&nostr=%7B%22id%22%3A%226ffa1ef877d86d62c1c7813c42e699e916fe53ccfc277740524a14a8bd25c719%22%2C%22sig%22%3A%2208c4924fc86875a7384a62bbe3eee72165b82789a63a8dacb9bce722a8c23fce638b8339075a4b184f347ec5992dc1f77ca74b649c31613c39f636a7c02bae93%22%2C%22pubkey%22%3A%2297f848adcc4c6276685fe48426de5614887c8a51ada0468cec71fba938272911%22%2C%22created_at%22%3A1743989580%2C%22kind%22%3A9734%2C%22tags%22%3A%5B%5B%22relays%22%2C%22wss%3A%2F%2Fbostr.lightningspore.com%22%2C%22wss%3A%2F%2Frelay-nwc-dev.rizful.com%2Fv1%22%2C%22wss%3A%2F%2Fbrb.io%2F%22%2C%22wss%3A%2F%2Feden.nostr.land%2F%22%2C%22wss%3A%2F%2Fnos.lol%2F%22%5D%2C%5B%22amount%22%2C%221000%22%5D%2C%5B%22lnurl%22%2C%22https%3A%2F%2Fbtcpay989117.lndyn.com%2F.well-known%2Flnurlp%2Fpedromvpg%22%5D%2C%5B%22p%22%2C%228cd2d0f8310f7009e94f50231870756cb39ba68f37506044910e2f71482b1788%22%5D%2C%5B%22e%22%2C%22b8a8ca5154b2725b6810df191f9d399ede245c0e79fd5bae94a19456711ddc66%22%5D%5D%2C%22content%22%3A%22%E2%9C%A8%22%7D&lnurl=LNURL1DP68GURN8GHJ7CN5VDCXZ7FE8QUNZVFH9EKXUEREDCHXXMMD9UH8WETVDSKKKMN0WAHZ7MRWW4EXCUP0WPJKGUN0D4M8QECJMRCWV ..... you can see that the callback URL that DOES work is SHORTER...

I think your "callback url" implementation is rejecting encoded nostr events which are too long ... these events can get too long (in characters) if there are a lot of relays.... the relevant part of the spec is here:

GitHub

nips/57.md at 3e8c33ee79e3d2eb46ca498eddadcc6ef1ef7329 · nostr-protocol/nips

Nostr Implementation Possibilities. Contribute to nostr-protocol/nips development by creating an account on GitHub.

This is why it "works in the browser" -- because using, for example, Yakihonne, it only adds like 3 to 5 relay URLs... but in my URL, I am adding like 20 relay URLs.....

I actually ran into this error myself previously! I think this part of the spec is not well-worded:

GitHub

nips/57.md at 3e8c33ee79e3d2eb46ca498eddadcc6ef1ef7329 · nostr-protocol/nips

Nostr Implementation Possibilities. Contribute to nostr-protocol/nips development by creating an account on GitHub.

"the generation of the description hash invoice" .... I think that your implementation is actually putting the entire Nostr event (as a string) into the the description of the invoice... and for example LND won't allow a description over a certain number of characters! (It's a DOS vulnerability if its too long)...... So your implementation just needs to put the HASH of the nostr event into the description, not the full NOSTR event!

The place for you to start is this step in NIP-57 ..

GitHub

nips/57.md at 3e8c33ee79e3d2eb46ca498eddadcc6ef1ef7329 · nostr-protocol/nips

Nostr Implementation Possibilities. Contribute to nostr-protocol/nips development by creating an account on GitHub.

.. anyway you can test it with the LONG & FAILING url I provided previously....