On Tuesday, 9 June 2026, Anthropic presented its new engine Claude Fable 5. The following day I put it to use. But it was not long before it was ordered withdrawn by the authorities in the United States.
Here is Claude's own description of the engineWhat is it?Fable 5 is a Mythos-class model that Anthropic has made safe for general use. It is state-of-the-art on nearly all tested benchmarks, with exceptional performance in software engineering, knowledge work, vision and scientific research. The longer and more complex the task, the greater Fable 5's lead over previous models.
The relationship to Mythos 5Fable 5 and Mythos 5 are the same underlying model — it is the safety restrictions that distinguish them. Fable is the publicly available version with conservative protective filters, whilst Mythos 5 is reserved for a small number of trusted partners (cyberdefence organisations and soon certain biology researchers) with some restrictions lifted.
Restrictions and fallbackWhen Fable's classifiers detect requests related to cybersecurity, biology/chemistry or model distillation, the response is automatically handled by Claude Opus 4.8 instead. Over 95% of Fable sessions trigger no such fallback.
OBS: An update also came on 12 June stating that access to Fable 5 and Mythos 5 is temporarily suspended — apparently due to an export control directive from the US government.
The current status is therefore that the model is unavailable.
My commentFor three days, then, Fable 5 was available to me. The mathematics problem that you are shortly to see, and which it is hoped will lead to a proof that the Horizon Equation is correct, was therefore immediately put to Fable 5. With remarkable results, I must say.
You will therefore receive an additional conversation that must be referenced here — I call it "the Fable conversation." But first, here is the mathematical challenge that was given to Fable 5 — before any human beings had seen it.
