Fable 5.1 claims a 373-year-old cipher solved in 44 minutes. Nobody has checked it

On September 1, a r/singularity post by u/RusselTheBrickLayer announced that Claude Fable 5.1 had solved a "373 year old cipher", drawing roughly 490 upvotes and more than 150 comments. The claim underneath is Vals AI's — an evaluation and elicitation firm whose blog, dated August 31, the eve of Fable 5.1's launch, opens with a hedge: "We gave Claude Fable 5.1 an open task: solve Sir Thomas Urquhart's Cyphral Distich. It appears to have actually solved it."

If it holds, the solve closes a cryptogram open since 1653. No historical-cipher expert has vouched for it, Anthropic has not claimed it, and the mechanism may matter more than the headline: persistence, not cryptanalysis.

The Cipher That Outlasted Three Centuries of Attempts

Sir Thomas Urquhart was a Scottish poet, Rabelais' first English translator, and a Royalist — tradition says he died of laughter on hearing of Charles II's restoration. At the end of his Logopandecteision (London, 1653) he printed two rows of numbers, 32 per line, 64 in all, now known as the Cyphral Distich. (Vals' own materials disagree on the age: the blog says 370 years, its X post 373.) The puzzle was posed as an open problem in Notes and Queries in 1899 and later made #28 on Klaus Schmeh's "Top 50 unsolved encrypted messages" list; the HCPortal registry tracks both of Urquhart's cryptograms as entry #8.

Every recorded attempt assumed an external key — frequency analysis, substitution alphabets, homophonic substitution, nomenclator mappings. Nothing worked. Schmeh's statistical look found a clean size-frequency correlation, which hinted at word substitution, but the text stayed out of reach.

The Claimed Key Was the Book Itself

Vals' write-up is by Geby Jaff, the elicitor who ran the session. His account: Fable 5.1 realized the key was internal — not an alphabet to reconstruct from outside, but the book itself. The cryptogram sits immediately after Urquhart's 32 "Proquiritations", a section whose size the author insists on:

"there can no number like that of two and thirty ... be pitched upon"

The rule the model converged on: for position i in a cipher line, open the i-th Proquiritation, use the cipher number as a word index, and take the first letter of that word. Out came:

O GOD UPHOLD KING CHARLS THE SECOND AND MAKE HIM THE SUPREME RULER OF THIS LAND

Vals points to the self-checking structure: each line resolves to exactly 32 letters, the lines rhyme (and / land), and the politics fit a committed Royalist; the spelling "CHARLS" is period-accurate, not a reconstruction error. The solve took 44 minutes and 176k tokens with "zero interjections" from Jaff, after months in which no other frontier model Vals tried produced a verified solve. Jaff steered it away from the hardest targets — "Kryptos K4 might be a little too hard and convoluted for current models" — and told it to skip ciphers with known solutions or many plausible answers, since a cipher whose author has been dead for centuries may never be verifiable at all.

A Second Solve, With Its Caveats Attached

The same session reportedly produced a second plaintext: the Cyphral Octastick in The Jewel (1652), 285 numbers across eight lines plus a closing Decagram. Vals claims it decodes — the same internal-key rule, with the book's numbered pages standing in for the Proquiritations — as a royalist prayer in ottava rima (ABABABCC) dated London, March 1652, opening "GREAT LORD, MANTAINE THAT REGAL FAMILIE". The report flags its rough edges rather than hiding them: eight of nine unresolved letters in line 5 spell C-O-N-E-R-T-H-T-O, no English under any hypothesis tried — "either Urquhart slipped here or numbers of this line were misprinted"; from position 159 on, every number keys to page k-1, suggesting one page used twice or a misprinted "5.5"; line 4 resolves to "IRSH", plausibly a slip for IRISH that keeps the line at ten syllables. Confirming those positions, Vals writes, needs a physical copy or the 1983 Jack & Lyall edition. The post lists verification scripts (verify_octastick.py, SOLUTION.md) and cites the Dictionary of the Scots Language for "swey" — scaffolding a fabricator rarely bothers with.

What Kind of Solve Is This?

The blog keeps the claim at arm's length — the model "didn't perform some extraordinary feat of cryptanalysis. It's actually the opposite. The answer was simple in hindsight. It just kept looking until it found it." That is either refreshing honesty or careful repositioning of a modest result — and Vals AI is not a neutral observer. It sells evaluations and elicitation of frontier models; a demo of one model's persistence is its marketing, and the solve is dated to the eve of Fable 5.1's public release, implying early access that Anthropic has not confirmed.

The mechanism decides what gets credited. Classical cryptanalysis recovers a message from ciphertext under an adversarial model; here the "cipher" is a word-index scheme whose key sat in plain sight in the same book. The hard part was noticing the key inside the text, then grinding out 64 lookups. If the account is accurate, Fable 5.1 solved an attention problem, not a cryptanalytic one — which is exactly the capability Vals is pitching. The top comment on its X post makes the point: "how many unsolved ciphers did fable 5.1 attempt before this one yielded? hard to judge the result without that denominator."

The case for the solve, on the other hand, is genuinely strong. A plaintext that lines up letter counts, a rhyme, and a known political biography is not something a model hallucinates into place, and the Octastick decode carries honest failure modes. What is missing is an outside check: Klaus Schmeh, who has covered Urquhart's poems since 2014, is the obvious reviewer — and has said nothing yet.

What Would Settle It

A historical-cipher specialist works the Distich rule against the digitized 1834 Maitland Club edition on archive.org and confirms all 64 letters; the Octastick positions get checked against a physical Jewel or the 1983 Jack & Lyall edition; a third party runs a fresh Fable 5.1 session from scratch and sees whether the puzzle still yields. Vals' footnote — one coordinate depends on counting the Latin "hinc inde" as a single unit — is exactly the kind of detail an independent pass would vindicate or sink. Launch context is in our Fable 5.1 release coverage.

Conclusions

The Cyphral Distich episode is the historical-cipher cousin of the GPT-5.6 prime-gap claim: a vendor-side announcement of a solved hard problem, self-verifying on inspection, still awaiting anyone external to check it. If Vals' rule is right, the puzzle was never cryptographically deep — it was a 373-year test of whether anyone would think to look inside the book. That is the lesson Vals wants drawn about persistence as a model capability, worth taking seriously even while this solve stays unverified.

The plaintext is checkable in an afternoon against the archive.org scan, and the vendor's caveats read like good faith. Expect Schmeh or another Cipherbrain regular to test the rule publicly — the verdict should arrive far faster than 373 years. Until then, the honest summary is Vals' own: "It appears to have actually solved it."