[ajaysehwal]

RouteParametersConfig::max_total_routing_fee_msat is documented to default to 1% of the payment amount plus 50 sats. When no route parameters are given at all, ldk-node keeps LDK's default limit. But ldk-node's spontaneous send applies provided route parameters as a whole: it overwrites the limit with the given value, which is None if the field is unset, and None means no limit. A spontaneous send with route parameters but without an explicit fee limit could therefore pay arbitrary routing fees.

This affects every client that sends route parameters without a fee limit. Notably, ldk-server-cli spontaneous-send always sends route parameters (filling the other fields with their defaults), so it did not limit routing fees unless --max-total-routing-fee was passed.

Here, we fill in the documented default (1% of the amount plus 50 sats) in the spontaneous send handler whenever route parameters are provided without a fee limit. Explicit limits, including 0, are left untouched, and nothing changes if no route parameters are sent.

Notes for reviewers

• BOLT11 and regular BOLT12 payments are not affected: LDK only overrides its default limit if one is given.
• Payments to BOLT12 async static invoices do not get the default limit at all, even without route parameters (send_payment_for_static_invoice in rust-lightning assigns the config's None). That is outside of this PR and I think it needs a fix in LDK/ldk-node.
• A related gap that I did not change here: the other three RouteParametersConfig fields are plain uint32s, so a client that sends only a fee limit gets max_path_count = 0 etc. Making them optional would be a small follow-up.
• I did not touch the proto or the CLI: the proto comment is accurate after this change, and the CLI behaves correctly with the server-side default.

Testing

• New unit tests for with_default_max_routing_fee: the default is applied for an unset limit (including amount 0 and u64::MAX), explicit limits (0, 1,000, u64::MAX) are kept, other parameters are untouched, and absent parameters stay absent.
• cargo test -p ldk-server passes (143 tests); cargo fmt is clean. I did not run the e2e tests.

[ldk-reviews-bot]

👋 Thanks for assigning @benthecarman as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[benthecarman]

this seems more like a bug in ldk-node than ldk-server

[ajaysehwal]

this seems more like a bug in ldk-node than ldk-server

Agreed, this appears to be an ldk-node bug. Both SpontaneousPayment::send_inner and the BOLT11 probing methods overwrite the default fee limit with None when no fee limit is explicitly configured. I've opened a fix in ldk-node: lightningdevkit/ldk-node#1221. Please review it when you get a chance.

I'll close this PR once the fix is merged and we've bumped the ldk-node dependency.