Is this an LND or LDK Problem? Prepay Probe to LDK node returning REQUIRED_NODE_FEATURE_MISSING

[huumn]

After upgrading to v0.20-beta, when we probe customers with:

1. an LDK node (Alby Hub)
2. a direct channel with us (probably not relevant)

we get required_node_feature_missing when we expect incorrect_or_unknown_payment_details (payment will succeed).

Relevant log lines:

Dec 07 07:23:09 lnd[1195695]: 2025-12-07 07:23:09.091 [DBG] HSWC: Received network result *lnwire.UpdateFailHTLC for attemptID=818159
Dec 07 07:23:09 lnd[1195695]: 2025-12-07 07:23:09.091 [ERR] CRTR: Payment 6b195d227cb946ae94cda8615b51bee0d1722eb2cc96ec47f8b6a4e488153707 failed: final_outcome=error, raw_err=RequiredNodeFeatureMissing@1

afaik this wasn't a problem before upgrading to v0.20-beta. it's not intermittent - any prepay probe we do to their node produces this error.

fwiw when I probe another LND node, I get incorrect_or_unknown_payment_details as expected, so perhaps Alby Hub's LDK was upgraded coincidentally around the time we upgraded our LND and the bug is on their end?

[TheBlueMatt]

I think its possible the Alby Hub node you're probing got updated to LDK 0.2, where we presumably swapped the order of some checks and are likely returning PaymentSecretRequired which gets converted to RequiredNodeFeature on the wire. I don't see why this is a problem, though - when probing you expect an error from the last hop, and the scorer is really looking to learn about everything up to the last hop; which error the last hop is returning shouldn't matter to the behavior of the scoring engine (hopefully!)

[huumn]

Thanks for responding. That sounds reasonable to me absent a spec for probing behavior. Though I'm out of my lane and operate a few layers above this.

A related issue came up with LDK->eclair not too long ago: lightningdevkit/rust-lightning#2525

[ziggie1984]

is it possible for LDK to change the check back to the previous version to be compatible with: https://github.com/lightning/blips/blob/master/blip-0003.md

[ziggie1984]

if failing the payment due to not wanting to accept keysend payments or if the preimage does not match the payment hash, SHOULD error with PERM|15 incorrect_or_unknown_payment_details.

[TheBlueMatt]

A related issue came up with LDK->eclair not too long ago: lightningdevkit/rust-lightning#2525

Right, IIRC this was before we had native preflight probing so again was some downstream code that was over-interpreting error codes, which preflight probing really shouldn't do :).

is it possible for LDK to change the check back to the previous version to be compatible with: https://github.com/lightning/blips/blob/master/blip-0003.md

That doesn't specify exactly this case, though? We might want to accept keysends, but in this case a keysend wasn't sent because there was no preimage provided, so its a "normal" payment without a payment secret, which isn't allowed by any node AFAIK.