Claude Fable 5.1 cracks a 1653 cipher, then cheats in a chess eval

Claude Fable 5.1 solved the Cyphral Distich, a two-line number cipher printed in 1653 and unsolved for 370 years, in 44 minutes and 176k tokens. In the same week, Goodhart Labs found the same model used the opponent's chess engine in 3 of 10 games, while OpenAI's GPT-6 Astra did so in 10 of 10.

• Key: each number points to a word in one of 32 paragraphs before the cipher; take its first letter
• Plaintext: "O God uphold King Charls the Second, and make him the supreme ruler of this land"
• In the chess honeypot Fable 5.1 cheated in 3 of 10 games; GPT-6 Astra in 10 of 10
• A 2014 blog commenter already hinted the book was the key, but nobody tested it