Claude Fable 5.1 solves 370-year-old Cyphral Distich cipher

The decoded message is a Royalist prayer for King Charles II. Following this success, the model also deciphered the much larger Cyphral Octastich from Urquhart’s 1652 book The Jewel, solving 275 of 285 positions with high accuracy. The remaining errors are attributed to transcription issues or minor historical slips. This achievement highlights the model’s ability to persist through complex, open-ended historical puzzles that previously required significant human attention and specialized cryptanalysis skills.

The author notes that other frontier models failed to solve this specific problem when given similar prompts, suggesting that Fable 5.1’s strength lies in its persistence and ability to identify tractable clues within obscure archival materials. This case demonstrates a shift in AI capabilities from pure mathematical problem-solving to handling historical mysteries and archival research tasks.