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

pull fametrano wants to merge 2 commits into bitcoin:master from fametrano:psbt-finalizer-clearing changing 2 files +6 −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.

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.

4901287cf4

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.

f5034ac310

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

Contributors

fametrano

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

BIP370: add an Input Finalizer section

BIP371: remove a finalizer clause from an output field