tl;dr: Claude Fable 5.1 solved Sir Thomas Urquhart's 370-year-old Cyphral Distich cipher by realizing the key was the book itself: each of the 64 numbers indexes a word in one of Urquhart's 32 Proquiritations, with the first letter of that word yielding a royalist prayer for Charles II. The model then applied the same technique (using page numbers instead of paragraphs) to largely decipher Urquhart's longer Cyphral Octastich from The Jewel (1652). The author frames this as evidence that LLMs can crack historical puzzles bottlenecked mainly by sustained human attention rather than raw cryptanalytic skill.