(Admittedly it was buried pretty deep in that 300+ page PDF, but they did at least disclose it. If they hadn't I imagine it would have taken quite some time for the research community to figure out what was going on.)
And to be clear, this isn't the safeguard where the model is explicitly downgraded to Opus, but rather where the Fable/Mythos model's "effectiveness" is transparently "limited" via "prompt modification, steering vectors, or parameter-efficient fine-tuning (PEFT)".
[0]: https://web.archive.org/web/20260609173222/https://www.anthr...
[1]: https://simonwillison.net/2026/Jun/10/if-claude-fable-stops-...
They could have simply told people "we do not permit using Claude models to perform frontier AI research," which is defensible from a policy point of view. This particular usage of their products requires no deception, nor hiding information prevent abuse.
However, instead, they chose for some reason to publicly display a morally poor way to execute a reasonable business decision (preventing abuse, defending your business interests, etc.)