Anthropic’s Claude Fable 5.1 appears to have cracked a centuries-old number puzzle that researchers considered unsolved. The “Cyphral Distich” by Sir Thomas Urquhart, published in 1653, consists of two lines of 32 numbers each. According to Vals AI, Fable 5.1 solved it in 44 minutes with no human help.

The author had spent months testing other frontier models on unsolved puzzles. None produced a verifiable solution. Fable 5.1 was tasked with finding a solvable puzzle on its own. It sifted through various problems and flagged the Cyphral Distich as promising.

The key was hiding in the book itself. Each number points to a word in one of the 32 sections of Urquhart’s publication. The first letters of those words spell out: “O God uphold King Charls the Second and make him the supreme ruler of this land.” In hindsight, the solution is simple, and humans could have solved it too. Fable 5.1 succeeded not through better cryptanalysis but through systematic trial and error and sheer persistence, according to Vals AI.