Claude Fable 5.1 cracks a 370-year-old cipher in 44 minutes, no human help
bcherny · x · 2026-09-14
vals.ai gave Claude Fable 5.1 an open task: solve Sir Thomas Urquhart's Cyphral Distich, a 64-number cryptogram unsolved for 370 years and listed among the Top 50 unsolved encrypted messages. The model solved it in 44 minutes using 176k tokens with zero human intervention.
The key insight humans had missed: the cryptogram appears right after Urquhart's 32 'Proquiritations,' and the text deliberately emphasizes the number 32. Earlier attempts with frequency analysis and substitution had all failed. The model's solution reportedly looks embarrassingly obvious in hindsight — a landmark demo of long-horizon autonomous reasoning on open problems.
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