BIP174: Specify PSBT_IN_PREVIOUS_TXID serialization order #2001

1. 
   Shadouts commented at 7:04 pm on October 9, 2025: contributor

   This change updates descriptions for the PSBT_IN_PREVIOUS_TXID field to specify serialization order.

   It was not straightforward the order with which to store the txid in a psbt. It’s not clearly specified in the spec, and there are not many resources or parsers to aid in debugging PSBTv2. I found the BIP370 test vector “Case: PSBTv0 but with PSBT_IN_PREVIOUS_TXID.” seems to imply that the order of this txid value in PSBT_IN_PREVIOUS_TXID should match network serialization order.

   I’m torn on if “little endian” is the correct phrase to use here in conjunction with “txid”. I usually think of a “txid” as the big endian, human-presented order of a txn hash, but maybe it’s okay to use interchangeably.

2. specify PSBT_IN_PREVIOUS_TXID serialization order 8b74cf054e
3. murchandamus renamed this:
   Specify PSBT_IN_PREVIOUS_TXID serialization order
   BIP174: Specify PSBT_IN_PREVIOUS_TXID serialization order
   on Oct 9, 2025
4. murchandamus added the label Proposed BIP modification on Oct 9, 2025
5. murchandamus added the label Pending acceptance on Oct 9, 2025
6. 
   murchandamus commented at 9:54 pm on October 9, 2025: contributor

   cc: @achow101
7. 
   achow101 commented at 7:10 pm on December 8, 2025: member

   “Little endian” is ambiguous; I’ve used “little endian” to describe the byte swapped form that is displayed to users. The byte order is the same as everywhere else in Bitcoin as it is the direct output from hashing.
8. 
   murchandamus commented at 7:36 pm on December 8, 2025: contributor

   It sounds like this change will not be accepted as is. Please find a way to rephrase that, @Shadouts.
9. murchandamus added the label PR Author action required on Dec 8, 2025
10. fix: remove ambiguous use of endianness language 249919cbc6
11. 
    Shadouts commented at 6:47 pm on December 18, 2025: contributor

    “Little endian” is ambiguous; I’ve used “little endian” to describe the byte swapped form that is displayed to users. The byte order is the same as everywhere else in Bitcoin as it is the direct output from hashing.

    Agreed. Endianness with regard to hash output can be ambiguous. I’ve updated the text removing mention of “endian” while still clearing up the order ambiguity.

    32 byte txid in standard byte order, not display byte order, of the previous transaction whose output at PSBT_IN_OUTPUT_INDEX is being spent.

12. Shadouts requested review from murchandamus on Dec 18, 2025
13. 
    achow101 commented at 6:41 pm on January 14, 2026: member

    ACK, but please squash
14. murchandamus merged this on Jan 14, 2026
15. murchandamus closed this on Jan 14, 2026

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16. murchandamus removed the label Pending acceptance on Jan 14, 2026
17. murchandamus removed the label PR Author action required on Jan 14, 2026

Contributors
Shadouts murchandamus achow101

Review Requested
murchandamus

Labels
Proposed BIP modification