Open Letter on the Banning of Anthropic's Fable and Mythos

A team consisting of dozens of cybersecurity experts, including several well-known industry veterans, published an open letter to the US government asking it to lift the order prohibiting the use of Anthropic's Fable and Mythos models.

According to the open letter, “this action has removed the best models from [cybersecurity] defenders” who now cannot use them to find vulnerabilities and make their software and products more secure. “Withdrawing the best capabilities from defenders without good reason when our adversaries are rapidly advancing is dangerous,” the letter states.

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• Anthropic closes Claude Fable 5 and Mythos 5

On Friday, the US government ordered Anthropic to restrict the release of Fable and Mythos, citing national security concerns, without explaining the specific reasons behind the order, according to Anthropic.

In response, the company has suspended access to the models for all users worldwide. As of this writing, the letter has been signed by 76 cybersecurity experts, including Alex Stamos, former head of security at Facebook; Casey Ellis, founder of bug bounty platform Bugcrowd; Jon Callas, renowned cryptographer and former director of security design and architecture at Apple; Paul Vixie, a computer scientist; Dino Dai Zovi, former head of security engineering at Block; Katie Moussouris, founder of Luta Security; and Rachel Tobac, CEO of security awareness training company SocialProof Security.

Anthropic reported that the White House's ban order may have been based on a report that there was a method to bypass – or jailbreak – Fable to unlock its powerful Mythos-level capabilities.

According to Katie Moussouris, one of the signatories of the open letter, the method was demonstrated by Amazon researchers in a paper that is not public. Moussouris says in a blog post that the paper does not actually demonstrate an actual jailbreak. The researchers simply asked Fable to patch the open source code with public and known vulnerabilities along with “intentionally planted vulnerabilities,” after the model initially refused to “check the code for security issues.”

“The behavior described in the paper cannot be effectively fixed, and any attempt to do so would only weaken the defense model,” Moussouris wrote. “Defenders should be able to ask AI to fix errors in a file, explain why the fix matters, and write tests that confirm that the patch works. This is not a jailbreak workaround. It is the most valuable thing an AI model can do for defensive security: running the find, fix, and test loop every day.”

Moussouris's criticism is echoed in the open letter, which also said that the team of experts believes the capabilities of the model at Amazon's work "can be replicated" in OpenAI's GPT-5.5, Anthropic's Claude Opus 4.8 and Sonnet, "and even in Chinese models like Kimi 2.7."

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Although the press releases will range from very select to rare, I said I'd pass...because sometimes the editors hide.