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 DECODERSep 3, 2026Sep 4, 2026, 01:00 JST

Claude Fable 5.1 cracks 1653 royalist cipher

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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 test is whether the 44-minute solve reflects a genuine leap in AI reasoning or merely persistence, since humans could have solved it too. The model’s ability to flag the 1653 puzzle as promising is what sets it apart from other frontier models that failed.

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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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