Signal if the mint will return preimage for melt payment

[thesimplekid]

Currently the preimage in the bolt11 response is optional, however for L402 payments the preimage is needed to complete the flow so using cashu wallets is not possible as they do not know if they will get the preimage back. Not all backends give us the preimage so we probably cannot make it mandatory however we could at least signal if paying a melt will return a preimage, so a user knows it can be used with L402 without trying a payment first.

[DrVelvetFog]

+1 — came to this from the paid-API side (I operate an x402 facilitator and just reviewed the L402 discovery spec in lightninglabs/L402#27), and this optionality is exactly what blocks composing cashu into any 402-style flow: the preimage is the proof-of-payment the higher layer consumes, so "maybe you get it back" makes a wallet unusable for L402 even against a backend that would have returned it.

Two thoughts on shape:

• NUT-05's MeltMethodSetting already has the method-specific options object, which looks like the natural home — {"method": "bolt11", "unit": "sat", "options": {"preimage_guaranteed": true}}, with the field defined in NUT-23 since it's bolt11-specific (the flag describes NUT-23's payment_preimage melt-response member). Per method-unit pair matters since a mint can run different backends per unit.

• Worth pinning the semantics as a hard guarantee rather than best-effort: a client deciding before payment whether an L402 flow can complete needs "true ⇒ a PAID melt response MUST carry payment_preimage," and a backend that can't promise that advertises false or omits the flag. From the x402 side, that MUST-vs-usually distinction is exactly what separates fields you can build a protocol on from fields you can only log — optional-in-practice composes the same as absent. It also makes a flagged-true mint returning a settled melt without a preimage unambiguously a bug rather than a judgment call.

Happy to put up the small spec PR (the NUT-23 option definition + the NUT-06 settings example line) if useful.