Over 50 cybersecurity professionals have publicly requested the US government lift the ban on access to Mythos 5 and Fable 5, the latest frontier large language models (LLMs) released by AI company Anthropic.

On June 12, Anthropic announced that the US government had issued an export control directive to suspend all access to Fable 5 and Mythos 5, released just a few days earlier, by any foreign national.

This decision prompted the AI company to suspend access to both models for all customers to ensure compliance with the directive.

Fable 5 was presented by Anthropic as a general-access LLM powered by the same underlying frontier AI model as Mythos 5 – an upgrade from Claude Mythos Preview – but with additional guardrails, especially in areas like cybersecurity where the company said it “could be misused to cause serious damage.”

The US government invoked “national security concerns” to explain its directive, which Anthropic believes originated from research that allegedly found a method of bypassing Fable 5’s guardrails.

“We reviewed a demonstration of this specific technique being used to identify a small number of previously known, minor vulnerabilities. These vulnerabilities all appear relatively simple and we have found that other publicly-available models are able to discover them as well without requiring a bypass,” stated Anthropic.

The company denied the existence of a “universal jailbreak” for Fable 5.

Cybersecurity Community Criticizes Fable, Mythos Ban

Two days later, a group of 54 CISOs, cybersecurity practitioners and vendors signed an open letter addressed to Howard Lutnick, the US Secretary of Commerce, and Sean Cairncross, the US National Cyber Director.

The group asked for the export control directives on Fable and Mythos to be lifted.

They also called for the US government to “commit to an open, scientific and transparent process of handling AI risk assessments in the future.”

While the signatories acknowledged that Anthropic’s latest models are “quite good at finding flaws and weaponizing exploits,” they argued they are not the only tools that can be used for this purpose.

The ability to identify insecure code is a fundamental feature of any secure coding assistant and equivalent capabilities already exist across other models including OpenAI's GPT-5.5, Anthropic's Claude Opus and Sonnet, and Chinese models such as Kimi 2.7, the signatories noted.

Additionally, they acknowledged Anthropic’s contribution to prevent Fable from “cyber offensive uses” and said the AI company is now addressing the research that likely prompted the US government’s decision.

“To pull the best capabilities away from defenders without a good reason when our adversaries are rapidly advancing is dangerous,” they warned.

The open letter also said the US government action has created market uncertainty and risked America’s AI leadership “without any real risk to justify it.”

Signatories included Alex Stamos, chief product officer at Corridor and former chief security officer at Facebook and Yahoo, Joe Levy, CEO of Sophos, and Sandra McLeod, CISO at Zoom Communications.

Their stance has been shared by other cybersecurity experts. Despite not having signed the open letter, William Wright, CEO of Closed Door Security, said that, while the US reaction “suggests that the worries around jailbreaking these models are real,” banning access to the model is the wrong approach.

“Cutting off access to the model so abruptly will cause huge logistical problem, both within Anthropic and within any critical industry partners given access to the model. Rather than foster resilience, this move creates chaos,” he explained.

He called the US government to work “transparently and with clear guiding principles” with AI and cybersecurity experts.

Image credits: Nwz /  jackpress / Shutterstock.com