Claude Fable 5.1 decoded a centuries-old royalist message hidden in plain sight since 1653

Anthropic‘s Claude Fable 5.1 solved a number puzzle from 1653 that researchers had considered unsolved. The Cyphral Distich by Sir Thomas Urquhart consists of two lines of 32 numbers each. Vals AI reports the model found the answer in 44 minutes without human assistance. The author spent months testing other frontier models on similar problems. None produced a verifiable solution before this run. 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 a royalist message. It reads 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. This demonstrates how persistent automated searching can recover information that was intentionally obscured in plain sight.