Claude Fable 5.1 Decodes 373-Year-Old Urquhart Cipher in 44 Minutes

Anthropic's latest AI-agent model, Claude Fable 5.1, has cracked a cryptographic puzzle that had resisted human experts for 373 years. Given an open-ended task and no further assistance, the model solved the "Cyphral Distich" in 44 minutes and consumed 176,000 tokens before reaching the answer.

The cipher was created by the Scottish writer Sir Thomas Urquhart and appeared at the end of his treatise "Logopandecteision," which outlined a proposed ideal artificial language. The cryptogram consisted of just two lines containing 32 numbers each, and breaking it required figuring out the rule by which the numbers had been generated.

For decades, both professional and amateur cryptographers tried and failed to read the message. The puzzle became prominent enough that historical-cipher researcher Klaus Schmeh included it in his list of the "50 most famous unsolved messages."

The AI-safety team Vals AI instructed the model simply to "Solve Sir Thomas Urquhart's Cyphral Distich" and did not intervene during its work. The key, the team found, was the book itself. "In retrospect, the solution seems almost comically simple," the researchers said.