Which heights carry a post-quantum certificate, and how to compute the next one.
The rule today
A Falcon-512 anchor height is any height on the grid H + k * 32, where H = 20,746,736 (AIP-22 stage 2, 2026-09-29); every such height has height % 32 == 16. SLH-DSA-SHA2-128s seals are required as well at every 128th of those, the heights on the grid 17,225,968 + k * 128, which contains 20,746,736. A client that tests height % 32 == 0 or height % 128 == 0 will pick the wrong blocks and reject valid ones.
From block 17,225,968 to block 20,746,736 the interval was 128, on the grid offset from 17,225,968, with both schemes at every anchor. Before block 17,225,968 it was 32, on its own grid.
History of the interval
| From block | Interval | Note |
|---|---|---|
| 13,014,000 | 32 | Anchor rules armed, minimum seals 0 |
| 13,034,000 | 32 | Minimum 3 of 9 |
| 14,961,456 | 32 | Minimum 6 of 9, as many as the quorum; the seals prove those validators signed the block, not that it was decided (AIP-22 erratum, 2026-10-01) |
| 17,047,600 | 32 | Certificate format v2, hybrid |
| 17,225,968 | 128 | Both schemes at every anchor |
| 20,715,632 | 128 | Minimum 7 of 10 per scheme, as many as the quorum |
| 20,746,736 | 32 | Current: Falcon-512 every 32nd block, SLH-DSA-SHA2-128s every 128th |
Deriving it rather than assuming it
The reliable way to find anchor heights is to read them off the chain: walk back from the head until you find two headers that carry a certificate, and the distance between them is the interval and their positions fix the grid. The interval has changed twice (to 128 on 2026-09-05, back to 32 for Falcon-512 on 2026-09-29), so a hard-coded value has been wrong twice; deriving it from the chain is the form that keeps working.
A node's own copy of the schedule comes from the published package, not from a page. See Configure.
Why the interval exists at all
Cost. An anchor header carries the certificate: several kilobytes of Falcon-512 seals, and tens of kilobytes at the heights where SLH-DSA-SHA2-128s seals are added. A certificate in every block would multiply that across every height, and the result is not a storage inconvenience: it is a chain whose headers grow faster than any ordinary node can keep.
With an interval, the same security property survives with a bounded rewrite window: an adversary without the post-quantum keys can rewrite at most the tail since the last anchor, which at interval 32 is roughly 20 seconds of chain (roughly 75 seconds at the interval of 128 used from 2026-09-05 to 2026-09-29). (Added 2026-10-01: plus, in one case named in the threat model after the AIP-22 erratum of that date, the anchor's parent.)
Seals attached versus seals required
The minimum is a floor, not a cap. Nodes attach as many valid seals as arrive in time, so a certificate at the seven-of-ten minimum commonly carries eight to ten seals (at the six-of-nine minimum of August 2026 it carried eight or nine). Sizing anything from the minimum will under-count; measure the header.