Anthropic's new Claude Fable 5.1 model produced a solution to a 373-year-old cipher in 44 minutes, resolving a puzzle that had defeated cryptographers since 1653.
Key Points:
- A 17th-century cryptogram by Sir Thomas Urquhart, unsolved since 1653, was decoded in a single sitting.
- The hidden text is a royalist prayer for King Charles II, built from an indexing trick inside Urquhart's own book.
- The result comes from one evaluation firm's testing and has not been independently reviewed.
Vals AI Cipher Test
Evaluation firm Vals AI set the model loose on Sir Thomas Urquhart's Cyphral Distich, a cryptogram tucked into the closing pages of the Scottish writer's 1653 treatise Logopandecteision.
Researcher Geby Jaff documented the run in a company blog post, and said the model burned through 176,000 tokens without receiving a single hint from him along the way. The entire puzzle input amounted to two lines of 32 numbers.
Cryptographers had circled that sequence for well over a century, and a reader first asked for a solution in the British journal Notes and Queries in 1899. Historical cipher specialist Klaus Schmeh later ranked the distich 28th on a widely cited list of the 50 most famous messages nobody has read.
Earlier attempts leaned on frequency analysis and substitution, and none of them worked.
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Urquhart Cipher Key Revealed
The key turned out to be the book itself rather than any external alphabet. Urquhart printed 32 numbered wishes, which he called Proquiritations, directly before the cipher, and each number simply points to one word inside the matching wish.
Taking the first letter of every selected word yields a rhyming couplet, a royalist prayer asking God to uphold King Charles II and to make him supreme ruler of the land. Jaff cautioned that one coordinate depends on whether a Latin phrase counts as a single unit, and that verifying a second, longer Urquhart cryptogram would need a physical copy of a 1652 edition.
The solve rests on his own checks rather than on independent peer review.
Anthropic Fable 5.1 Research Push
The experiment ran on a model that Anthropic shipped on Sept. 1 alongside its restricted sibling Mythos 5.1. Both versions share the same weights and differ only in their safeguards.
Fable 5.1 reached 52.6% on an agentic science benchmark, against 24.7% for the version it replaces, and cache reads now cost 75% less at $0.25 per million tokens.
That release capped a stretch in which the company leaned hard on research results to defend the price of a tier that costs twice what its cheaper Opus line does. It also reported that Fable 5.1 trained a neural network to remap a third of Venus from old NASA radar images, and that Mythos 5.1 designed proteins that bound to their targets in nearly half of 12 laboratory tests.
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