Vals AI reports Claude Fable 5.1 solving a 370-year-old cipher by using the book itself as the key

Sources: Vals AI post, HN discussion
Summary: The recovered rule is that the i-th number in a cipher line indexes a word in the i-th of Urquhart's 32 Proquiritations, and the first letter of that word is the plaintext letter. The result is largely self-checking, because each line yields exactly 32 letters and the two lines rhyme as the promised distich. The author reports 44 minutes, 176k tokens and no interjections, and states no other frontier model he tried produced a verified solve, in a post dated 2026-08-31 that is not peer reviewed.
Why it matters: The decode is checkable by anyone with the source text, which separates it from model claims that rest on the vendor's own grading.
Follow-up: Whether the larger Cyphral Octastich decode is completed, because it is partial by the author's own account, with nine letters unreadable, a one-page index shift from position 159, and no free page images available to confirm the affected positions.