Confused by automatic addition of tx fees by sendmany

[pc]

So, now that the Mac version of 0.3.21 came out, I decided to play around with the new "sendmany" feature, and I was rather surprised.

First, I did Transaction 1, although all the addresses were addresses from the same wallet, and it seemed to work fine, and did not add a fee.

Then, I did Transaction 2, to 5 outside addresses, and it added a 0.01 transaction fee without any prompting. The transaction is hardly any bigger, so I'm rather confused.

I don't necessarily object to the fee itself, but the fact that it was deducted from my balance without me realizing it or being asked about it. I thought the point of "sendmany" was that bundling transactions saved resources, so I'm confused that I'm paying a fee now, whereas I never needed a pay a fee just sending to a single address before. And I don't see what's different about my two transactions to the network that would cause them to be treated differently.

Perhaps it's something to do with my unfamiliarity with using bitcoin via the command line, as I usually have been using the GUI.

I'd appreciate any enlightenment that somebody could give me. Thank you.

[Gavin Andresen]

bitcoind doesn't prompt before adding fees because it is meant to be used by websites and other services where there may not be a person available to push an "Ok, pay a fee" button.  There might not even be a place to put the button, either.

Your Transaction 2 paid a fee because it ran into the "small, low-priority transactions must pay" code that is new with version 0.3.21.  It was low priority because its outputs were small and its input was fairly new.

[pc]

Thank you very much for your response.

Is there some bitcoin.conf option that lets one override the behavior, to always set the fee to a particular value or always reject the transaction attempt if it would need a fee? Or something in the sendmany command itself? Is anything like it planned? In this particular case, I would have been fine with no fee and just waiting forever for it to finally get high enough priority to get into a block, so it seems like I ought to have been able to say so.

If not, maybe that's the perfect project for me to finally get my hands dirty with the Bitcoin coinbase.

[DrKenobi]

You can specify how much fees you want to sent per 1KB with the -paytx cmdline argument.

For my site I'm using a fixed fee of 0.01 to help support the network. (Using bitcoind [...] -paytxfee=0.01)