The Claude Fable 5.1 model from Anthropic independently deciphered the "Cyphral Distich" cryptogram by Scottish writer Sir Thomas Urquhart — a text that had remained unsolved for over 370 years since its publication in 1653. The solution was announced on social network X by Rayan Krishnan, head of the company Vals AI: the analysis took 44 minutes and 176,000 tokens, and the model received no hints from the operator.
The cryptogram consists of two lines of 32 numbers each and concludes Urquhart's treatise "Logopandecteision." The model was given an open-ended task: it was instructed to independently find an unsolved historical cryptogram and propose a solution. From many options, Fable 5.1 chose precisely the "Cyphral Distich."
The solution was not hidden in an external cipher alphabet but within the book's text itself. As explained in the experiment's description, the cryptogram immediately follows 32 numbered "Proquiritations"—wishes that Urquhart included in the treatise. Each number in the cipher line indicates the ordinal position of a word within the corresponding Proquiritation, and the first letters of these words form a coherent text.
A Royalist's Prayer
The obtained hidden message reads: "O GOD UPHOLD KING CHARLS THE SECOND AND / MAKE HIM THE SUPREME RULER OF THIS LAND" — a royalist prayer supporting King Charles II and affirming his supreme rule. Each of the two lines contains exactly 32 letters, the lines rhyme with each other and form a distich—exactly the format promised by the author in the 17th century:
O GOD UPHOLD KING CHARLS THE SECOND AND
MAKE HIM THE SUPREME RULER OF THIS LAND
The historical logic also fits: Urquhart was a staunch monarchist, and the hidden prayer for Charles II fully aligns with his political views. Prior to Fable 5.1's solution, the cryptogram was considered one of the most famous unsolved historical puzzles—attempts to crack it had been made since the late 19th century, but frequency analysis and substitution methods had yielded no results.
A Second Cipher and Model Release
Using the same method, the model tackled Urquhart's larger cryptogram—the "Cyphral Octastich" from his 1652 treatise "The Jewel," which encodes 285 numbers. This time, the key was not wishes but the page numbering of another book by the author. Fable 5.1 proposed a solution in the form of a royalist prayer structured as an ottava rima—an Italian eight-line stanza.
Both decryptions were performed by the Vals AI research team and have not yet undergone independent verification by the cryptographic community. Anthropic officially presented the Claude Fable 5.1 model alongside Claude Mythos 5.1 on September 1, 2026.
The AI Perspective
From a machine data analysis perspective, the most intriguing aspect of this story is not the result but the method of obtaining it. The model received an open-ended task and independently selected Urquhart's cryptogram from among many unsolved historical ciphers: Krishnan named the ability to autonomously find a worthy problem a distinct sign of the model's development. This approach changes the very role of AI—from a substitution tool, it becomes an investigator posing questions.
A historical parallel suggests itself with the deciphering of Enigma: the breakthrough back then also occurred not through brute force but thanks to understanding the structure of the language and the habits of the cipher's author. The demonstration with Cyphral Distich comes against the backdrop of broader results—Fable 5.1 took first place in the Vals Index among all tested models. Will this decipherment remain a curiosity or the first step toward AI's mass work with historical archives?
