Claude Fable 5.1 solves a 370-year-old cipher in a Vals AI test

The AI-evaluation firm Vals AI reported that Claude Fable 5.1 — the Anthropic model released the following day — had deciphered Sir Thomas Urquhart’s Cyphral Distich, a cipher printed in his Logopandecteision (1653) that had gone unsolved for centuries and appears on Klaus Schmeh’s list of the fifty most notable unsolved encrypted messages. Given only the puzzle, the model “arrived at a solution after 44 minutes, 176k tokens, and zero interjections”.

The break turned on recognising that the cipher used the book itself as its key: each of its 64 numbers indexes a word in the corresponding numbered passage of the surrounding text, and the first letters of those words spell out a royalist couplet — “O God uphold King Charls the second and / Make him the supreme ruler of this land.” Vals AI noted that the result is largely self-verifying, since each line resolves to exactly 32 letters and forms a rhyming couplet; the model also tackled the longer related Cyphral Octastich, though one stretch came out unreadable, which Vals attributed to either a transcription error or a mistake by Urquhart himself.

The claim rests on a single company’s write-up rather than independent replication, but the puzzle’s structure makes a correct solution hard to fake. It is a narrow demonstration of the kind of long-horizon, self-checking reasoning that vendors increasingly cite when describing frontier models — here applied not to a benchmark but to a genuinely open historical problem.