Unable to set a transaction fee of <1 sat/vB when using "send"

[FOSSil-Freedom]

When trying to use the "send" option in Jam, it only seems to accept transaction fee rates between 1.001 and 350 sats/vB, even when updating my bitcoin node config to allow <1 sat/vB transaction fees.

[bota87]

Same for me

[itsadityaaaaa]

@theborakompanioni Assign me this take, I am done with the initial setup , also can you please provide me the ideal behaviour context, is the transaction fee rates must be between different limits or the user must be free to enter whatever he wanted

[kunal-595]

Hi @theborakompanioni,
I’d like to work on this issue. Could you please assign it to me?

[theborakompanioni]

So, the issue is that one cannot go below 1sat/vbyte from the fee settings dialog when setting raw sat/vbyte value.
Afaik, this is because of a limitation in the JM API: Values between 0 - 1000 are interpreted as unit "block target", value >= 1000 are interpreted as unit "sats/kvbyte". This means that you cannot set the value to something below 1 sat/vbyte as the value would be be interpreted as "block target".
The real improvement would be to change the RPC API to allow adding a unit, either "block target" or "sat/kvbyte", instead of deriving it from the provided value alone.
Does this make sense?

Workaround is currently to set the "block target" and let the fee estimation provide the sub-1sat/vbyte value for a transaction.

[itsadityaaaaa]

@theborakompanioni thanks for the clarification

[sagar-h007]

Hi @theborakompanioni, I’d like to work on this issue. From what I understand, the limitation comes from how the API infers the fee unit based on the value, which ends up treating anything below 1000 as a block target. Because of that, it’s not possible to set fees below 1 sat/vB directly. I’m thinking of addressing this by allowing the RPC API to take an explicit fee unit instead of guessing it from the value. That way we can support sat/vB properly, including sub-1 values. I’d then update the backend to respect this unit and make a small adjustment on the UI side so the correct unit gets passed through. I’ll also make sure the current behavior still works if no unit is provided, to avoid breaking anything.

[theborakompanioni]

Hey @sagar-h007. Sounds good. This requires changes in https://github.com/JoinMarket-Org/joinmarket-clientserver - currently not fully maintained, so unsure if a PR will be reviewed and merged any time soon. Support for https://github.com/joinmarket-ng/joinmarket-ng hopefully added soon - maybe first check if the problem exists there are well?