bip360: correct the claimed P2MR witness size saving #2223

pull jeanpablojp wants to merge 1 commits into bitcoin:master from jeanpablojp:bip360-fix-size-claim changing 1 files +1 −1

jeanpablojp commented at 2:23 PM on July 28, 2026: none
In Transaction Size and Fees, the BIP says a P2MR witness "will always be 32 bytes smaller than an equivalent P2TR script path spend witness". That holds at every depth except m = 7, where it is 34.

The control blocks are 1 + 32m for P2MR and 33 + 32m for P2TR, and each is serialised with a compact size prefix. At m = 7 the P2TR control block is 257 bytes, which crosses the 253-byte boundary and takes a 3-byte prefix, while the P2MR one is 225 bytes and still takes 1:
```
def cs(n): return 1 if n < 253 else 3
for m in range(1, 12):
    a, b = 1 + 32*m, 33 + 32*m
    print(m, (cs(b) + b) - (cs(a) + a))
# 1..6 -> 32, 7 -> 34, 8..11 -> 32
```
The same section already footnotes this boundary a few paragraphs earlier, for the P2MR side only ("If m >= 8, then the compact size will use 3 bytes rather than 1 byte"), so the reasoning is there, it just isn't carried into the comparison.

Noticed while measuring witness weights for a regtest implementation of the BIP.

bip360: correct the claimed P2MR witness size saving

The Transaction Size and Fees section says a P2MR witness is always 32
bytes smaller than an equivalent P2TR script path spend witness. The
control blocks are 1 + 32*m and 33 + 32*m bytes, each serialised with
a compact size prefix, so at m = 7 the P2TR control block is 257 bytes
and takes a 3-byte prefix while the P2MR one is 225 bytes and takes 1.
The saving there is 34 bytes, not 32. Every other depth up to 128
gives 32.

The section already footnotes this boundary for the P2MR side a few
paragraphs earlier.

33202403b3

jonatack added the label Proposed BIP modification on Jul 28, 2026
jonatack added the label Pending acceptance on Jul 28, 2026

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