Claude Fable 5.1 Cracked a 373‑Year‑Old Unsolvable Cipher in Just 44 Minutes

Anthropic unveiled Claude Fable 5.1 — and the new version took just 44 minutes to crack the Cyphral Distich, Thomas Urquhart's cipher that had been considered an unsolvable riddle for 373 years. In this experiment, it's a rare case: the answer came not after years of debate, but almost instantly.

The Cyphral Distich had long been ranked among the most famous unsolved cryptographic puzzles out there. On the list of humanity's most mysterious ciphers, it sat at number 28 — and the text itself looked deceptively modest: two lines of 32 numbers each, making a sequence of 64 digits in total.

The riddle first appeared in the treatise Logopandecteision, or An Introduction to the Universal Language. In that work, Thomas Urquhart laid out his vision for a universal tongue and included 32 numbered wishes within it. According to Fable 5.1's interpretation, the numbers weren't part of some elaborate cryptographic scheme — they were a straightforward set of instructions: each one told you which word to pick from the corresponding wish.

Once the 64 words were selected, taking their first letters spelled out a hidden message: "O God support the King Charles the 2d and make him supreme governor of this land." It was this unexpectedly simple decryption that became the headline result of the experiment — effectively wiping out 373 years of failed attempts.

The source also notes that in some tests, Fable 5.1 nearly doubled the performance of its predecessors, while in others it delivered steady, consistent gains. Against the backdrop of the Cyphral Distich story, this release stands out all the more: the model didn't just show speed — it proved it could find a way through what humans had long seen as an almost impenetrable wall.

And by the way, they also released Fable 5.1 with a note that all generated texts will now carry a watermark that can't be turned off.

So how does that land with you: 373 years of mystery against 44 minutes of model runtime?