Claude Fable 5.1 Cracks a 370-Year-Old Cipher No Human Could Solve
Claude Fable 5.1 solved the Cyphral Distich, a 370-year-old cryptogram, in 44 minutes and 176k tokens with no human hints. Researchers say the key was hidden in the book itself.
What happened
Anthropic's Claude Fable 5.1 has solved the Cyphral Distich — a 370-year-old cryptogram left by Scottish writer Sir Thomas Urquhart — in a single open-ended session, according to a new analysis from the evaluation firm vals.ai.
The puzzle is just two lines of 32 numbers printed at the end of Urquhart's 1652 book Logopandecteision. It was posed as an open problem in Notes and Queries in 1899 and later listed among the Top 50 unsolved encrypted messages by cipher researcher Klaus Schmeh.
How it was cracked
The model worked for 44 minutes and roughly 176,000 tokens without a single human hint. The breakthrough: the key was the book itself. The cipher is followed by 32 "Proquiritations" — a number Urquhart stresses as uniquely important. For the i-th number in a line, take the i-th Proquiritation, use its number as a word index and read the first letter.
The plaintext reads: "O GOD UPHOLD KING CHARLS THE SECOND AND MAKE HIM THE SUPREME RULER OF THIS LAND." Each line yields exactly 32 letters and the two lines rhyme.
Why it matters
Decades of cryptographers missed a clue hidden in plain sight. For AI watchers this is another data point on long-horizon reasoning: a frontier model running unsupervised for hours on a problem humans never solved.