How did Claude Fable 5.1 solve the puzzle?

It used systematic trial and error and persistence, not advanced cryptanalysis.

What does the decrypted message say?

The message reads: 'O God uphold King Charls the Second and make him the supreme ruler of this land.'

Large Language ModelsTHE DECODER1h agoSep 4, 2026, 01:00 JST1 min read

Claude Fable 5.1 cracks 1653 royalist cipher

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Key takeaway

Anthropic's Claude Fable 5.1 solved a 1653 cipher in 44 minutes.
The solution supports King Charles II.
Other AI models had failed.

3 Key Points

What happened
Anthropic's Claude Fable 5.1 deciphered the Cyphral Distich, a 1653 puzzle by Sir Thomas Urquhart, in 44 minutes without human help. The solution spells a royalist message supporting King Charles II.
Why it matters
The puzzle was considered unsolved, and other frontier models failed to produce a verifiable solution. Fable 5.1 succeeded through systematic trial and error, not advanced cryptanalysis.
What to watch
The model flagged the puzzle itself as promising, suggesting AI can identify and solve long-standing problems through persistence.

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Context & Analysis

The Cyphral Distich consists of two lines of 32 numbers each, pointing to words in Urquhart's publication. Researchers had spent months testing other models on unsolved puzzles, but none yielded results. Fable 5.1's success highlights the role of persistence in AI problem-solving, potentially inspiring more attempts at historical puzzles.

FAQ

How did Claude Fable 5.1 solve the puzzle?: It used systematic trial and error and persistence, not advanced cryptanalysis.
What does the decrypted message say?: The message reads: 'O God uphold King Charls the Second and make him the supreme ruler of this land.'

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