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I'm new to btc and have some experience on blockchains like ethereum. I understand that btc is based on utxo not balance model like other blockchains, but I have trouble finding which input address paid the tx fee, for example, this tx has two input addresses but which one of them paid the fee. This kind of confused me since on other chains finding fee payer is relateively simple and obivous.

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This question simply makes no sense in the UTXO model. All that happened is that two UTXOs were spent, two new UTXO were created, and a fee was paid by the difference in value between them.

If you're asking which UTXO paid for it, I think the answer is "none", because UTXOs are not actors, or identities. They're just coins. Someone certainly paid the fee, but that's a question of identity or ownership, or perhaps even intent. And that information simply does not exist on chain; for your specific transaction, possibilities include:

  • Perhaps both input UTXOs were owned by the same person, who created the transaction, and paid a receiver, and sent change back to themselves. In that case, this sender paid the fee.
  • Perhaps the input UTXOs were owned by distinct people, who jointly paid a bill, and sent some coins back to one of them. The question of who paid the fee is now a function of what portion of the bill paid was allocated to each sender - not information that exists on chain.
  • Perhaps the receiving party of the transaction offered a service that involves paying the on-chain fees, by allowing the senders to deduct that value from the transferred amount. In this case, the receiver paid the fee.
  • Perhaps the transaction was created by the miner that constructed the block. As the miner is the one collecting the fee, arguably no fee was paid by anyone in this case.
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    Thanks for you answer. It seems I didn't fully understand the utxo model before. Your explanation is very detailed and useful.
    – Xkimnt
    Commented Oct 21, 2022 at 10:43

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