Subject: Re: [bitcoindev] Proposal: Unlocking Dust UTXOs as Miner Transaction Fees

From: Pieter Wuille <[email-masked]>
Date: Sat, 08 Mar 2025 23:48:54 +0000

Hello,

This is not a soft fork, for two reasons:

* Coinbase transactions can only have exactly one input. I don't think there is a particularly good reason for this besides simplicity, but that is the current rule. Allowing a coinbase transaction to additionally also spend certain outputs would require a hardfork.

* The outputs being marked as dust are not allowed to be spent by miners. Changing this requires a hardfork as well. Think about it: if this was possible with a softfork, it must mean that doing what you're proposing would *already be legal* today, and thus not need this proposed change in the first place. Softforks can only outlaw formerly legal behavior.

Furthermore, I don't really see the point. The proposal requires both a coinbase txin to spend the coin, plus a signature in a separate transaction, in the same block. To pay the miner for the opportunity cost of not including normal transactions with these bytes, the fee for this OP_RETURN output should economically be priced at the block's feerate for the size of the OP_RETURN output *plus* the cost of the coinbase transaction input. Together, they are no smaller (and with witness discount, I suspect larger) than the user just spending their "dust" output, and thus the fee for using this OP_RETURN-based mechanism would be larger than the value of the dust output.

-- 
Pieter

On Saturday, March 8th, 2025 at 1:23 PM, Nighttime Satoshi <[email-masked]> wrote:

> Dear fellow Bitcoin developers,
> 
> I'm excited to share a proposal addressing a long-standing Bitcoin challenge: economically unviable dust UTXOs.

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