gui: Use PSBT v0 #36539

1. 
   davidgumberg commented at 8:18 AM on October 15, 2026: contributor

   This fixes: #36518

   There is no test added here for the bump fee PSBT's being v0, since there aren't any existing tests for watch only wallet feebumping in the GUI. If reviewers insist on this I can add it, but I wanted to keep the diff minimal. I have tested this manually.

   AFAIU for the external signer stuff it shouldn't matter much whether it's v0 or v2, since users are very unlikely to be using HWI software that doesn't speak v0, but out of caution I've set the external signer PSBT to v0 for the GUI.

   This PR originally took the approach of adding a checkbox, (davidgumberg/gui/2026-10-15-psbt-gui), but this was dropped to simplify backporting past string freeze.

   LLM Disclosure: The previous approach was drafted using an LLM, but this branch was written by me. I used an LLM to explore adding a fee bump watchonly GUI test, and decided against it.

2. DrahtBot added the label GUI on Oct 15, 2026
3. 
   DrahtBot commented at 8:18 AM on October 15, 2026: contributor

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   External sites

   • Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36539
   • Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36539.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36539#author-ralph

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   Conflicts

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4. fanquake added this to the milestone 32.0 on Oct 15, 2026
5. 
   davidgumberg commented at 12:53 AM on October 16, 2026: contributor

   @achow101 @polespinasa

6. 
   achow101 commented at 1:12 AM on October 16, 2026: member

   I don't think we should be adding any GUI elements for a PR targeting a backport, particularly as it adds strings for translation. We can do v0 only in the GUI I guess.

7. 
   fanquake commented at 4:45 AM on October 16, 2026: member

   cc @hebasto

8. 
   Sjors commented at 8:13 AM on October 17, 2026: member

   We can do v0 only in the GUI I guess.

   That seems fine for now if it's easier. There could be a (release) note to use the RPC for v2 when needed.

9. gui: wallet: Use v0 PSBT

   PSBTv2 is not very interesting for current GUI users, and defaulting to
   it causes issues for existing users. Because this change is a fix, it
   only changes the default back instead of adding a checkbox.

   c21f71f5e0
10. refactor: psbt: Don't set a default version for PSBT constructor f0777e6855
11. davidgumberg force-pushed on Oct 18, 2026
12. davidgumberg renamed this:
    gui: Make PSBT version toggleable: v0 by default
    gui: Use PSBT v0
    on Oct 18, 2026
13. 
    davidgumberg commented at 11:11 PM on October 18, 2026: contributor

    I've pushed so that the GUI just returns v0 PSBT's.

Milestone
32.0