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Remove BIP 174's claim that Combine is commutative #2075

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murchandamus merged 3 commits into from
Jul 16, 2026
Merged

Remove BIP 174's claim that Combine is commutative #2075

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6699d99
Remove BIP 174's claim that Combine is commutative
shesek Jan 6, 2026
c4be017
BIP174: clarify commutativity claim
murchandamus Jul 16, 2026
2d3dc4b
bip174: Touch up commutative clarification
murchandamus Jul 16, 2026
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10 changes: 8 additions & 2 deletions bip-0174.mediawiki
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Expand Up @@ -907,8 +907,14 @@ For every type that a Combiner understands, it may refuse to combine PSBTs if it

The Combiner does not need to know how to interpret scripts in order to combine PSBTs. It can do so without understanding scripts or the network serialization format.

In general, the result of a Combiner combining two PSBTs from independent participants A and B should be functionally equivalent to a result obtained from processing the original PSBT by A and then B in a sequence.
Or, for participants performing fA(psbt) and fB(psbt): Combine(fA(psbt), fB(psbt)) == fA(fB(psbt)) == fB(fA(psbt))
In general, the result of a Combiner combining two PSBTs from independent participants A and B will be functionally equivalent to a result obtained from processing the original PSBT by A and then B in a sequence.
Or, for participants performing fA(psbt) and fB(psbt) where fA() and fB() only add unique fields to the PSBT: Combine(fA(psbt), fB(psbt)) == fA(fB(psbt)) == fB(fA(psbt))
Combining two PSBTs is commutative unless the PSBTs have conflicting fields, i.e., fields with duplicated keys but differing values.

As discussed in [[Handling Duplicated Keys]], when a combiner is asked to combine PSBTs with conflicting fields,

@jonatack jonatack Jul 16, 2026

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I think this link should be

[[#handling-duplicated-keys|Handling Duplicated Keys]]

Added a fixup in #2203.

@murchandamus murchandamus Jul 16, 2026

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Thanks!

the combiner may arbitrarily choose the value for the combined PSBT from any of the original PSBTs.
Alternatively, the combiner may refuse to combine PSBTs with conflicting content.
In this case, the previously discussed commutative property no longer holds.

===Input Finalizer===

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