BIP370: add Input Finalizer section; BIP371: remove finalizer clause from an output field #2277

1. 
   fametrano commented at 4:01 PM on September 7, 2026: contributor

   Two independent commits.

   BIP370: add an Input Finalizer section

   In a version 2 PSBT, the fields that define an input live in the input map: PSBT_IN_PREVIOUS_TXID, PSBT_IN_OUTPUT_INDEX, PSBT_IN_SEQUENCE, and the two required-locktime fields.

   BIP174 tells the Input Finalizer to clear every field of a finalized input except the UTXO and unknown fields. Followed literally on a version 2 input, this deletes the fields above. Without PSBT_IN_PREVIOUS_TXID or PSBT_IN_OUTPUT_INDEX the PSBT is invalid; without the sequence and locktime fields the Transaction Extractor builds a different transaction.

   BIP370 defines every other role for version 2 but has no Input Finalizer section. This adds one, stating that the finalizer keeps those five fields. Bitcoin Core already does this: its input serializer writes them whether or not the input is finalized.

   BIP371: remove a finalizer clause from an output field

   PSBT_OUT_TAP_BIP32_DERIVATION carries the note "Finalizers should remove this field after PSBT_IN_FINAL_SCRIPTWITNESS is constructed".

   This is an output field. An output has no PSBT_IN_FINAL_SCRIPTWITNESS, and the clause names no input to act on. The two neighbouring output fields carry no such note. This removes it.

   Both are minor amendments to Deployed BIPs.

2. fametrano renamed this:
   BIP370: add the Input Finalizer role; BIP371: drop the finalizer clause from PSBT_OUT_TAP_BIP32_DERIVATION
   BIP370: add Input Finalizer section; BIP371: remove finalizer clause from an output field
   on Sep 7, 2026
3. murchandamus added the label Proposed BIP modification on Sep 9, 2026
4. murchandamus added the label Pending acceptance on Sep 9, 2026
5. murchandamus requested review from achow101 on Sep 9, 2026
6. fametrano force-pushed on Sep 12, 2026
7. fametrano force-pushed on Sep 19, 2026
8. bip-0370: say what the Input Finalizer keeps in a version 2 input

   BIP 174 tells the Input Finalizer to clear all other data except the
   UTXO and unknown fields from a finalized input. It was written when the
   unsigned transaction was a global field. PSBTv2 moves the data that
   defines the input into the input map, and a finalizer that follows the
   sentence literally removes it: without PSBT_IN_PREVIOUS_TXID or
   PSBT_IN_OUTPUT_INDEX the result is invalid, and without
   PSBT_IN_SEQUENCE and the locktime fields the Transaction Extractor
   builds a different transaction.
   
   BIP 370 amends Creator, Updater, Signer and Transaction Extractor for
   PSBTv2, adds the Constructor, and says nothing about the Input
   Finalizer. Add the section: the five fields are kept. Bitcoin Core
   does this today: at 19b0ff2fa0, PSBTInput::Serialize in src/psbt.h
   writes them for every PSBTv2 input, finalized or not.

   1e431879af
9. bip-0371: drop the finalizer clause from PSBT_OUT_TAP_BIP32_DERIVATION

   The six taproot input fields say "Finalizers should remove this field
   after PSBT_IN_FINAL_SCRIPTWITNESS is constructed", and so does the
   output field PSBT_OUT_TAP_BIP32_DERIVATION. An output has no
   PSBT_IN_FINAL_SCRIPTWITNESS, the clause names no input, and its two
   neighbours in the output table carry no such clause. Drop it; Bitcoin
   Core does not clear output fields on finalization.

   8629462a94
10. fametrano force-pushed on Sep 23, 2026
11. 
    achow101 commented at 11:39 PM on September 23, 2026: member

    ACK 8629462a94d6b7fcfe8ef6251a5304da4c794cec

12. jonatack removed the label Pending acceptance on Sep 23, 2026
13. jonatack merged this on Sep 23, 2026
14. jonatack closed this on Sep 23, 2026

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