Right now when a Siacoin transaction is sent the fees are calculated and paid from the remaining balance in the wallet. But this isn't possible when trying to send all the coins in the wallet. It's also very hard to predict the fees you need to subtract from the amount beforehand. Before 1.3 I used to just subtract 10 SC from the amount and it would always work, but now it still leaves some coins behind when I do that.

Most other cryptocurrency wallets have a button to empty the wallet, which creates a transaction with all coins in the wallet to an address.

That's why I would like a boolean parameter for the POST /wallet/siacoins API that allows the fees to be subtracted from the transaction amount. That way I can empty a wallet and make sure there's not a single Hasting left behind.