US government bans Anthropic’s Fable 5 and Mythos 5 over jailbreak concerns

The US government has ordered Anthropic to restrict access to its two most advanced AI models, Fable 5 and Mythos 5, to US citizens only. Anthropic, unable to verify the citizenship of its users, responded by taking both models offline entirely. The move marks the first time the US government has issued an export control directive targeting a large language model.

Anthropic received the directive on Friday at 5:21 PM Eastern Time. The letter cited national security authorities but provided no specific details. The company published a statement saying its understanding was that the government believed a method to bypass, or “jailbreak,” Fable 5’s safety guardrails had been discovered.

A dispute over how serious the threat really is

Fable 5 is the publicly available version of Mythos 5, a model Anthropic itself has described as potentially dangerous due to its ability to find and exploit vulnerabilities in computer code. Anthropic designed Fable 5 with guardrails specifically to prevent misuse for cybersecurity attacks. A jailbreak is a technique that can disable those guardrails.

David Sacks, an adviser to President Trump, wrote on X (Twitter) that a “highly credible trusted partner” had discovered a jailbreak and reported it to the administration. According to Sacks, the government asked Anthropic CEO Dario Amodei to fix the issue or take the model down. Amodei refused, Sacks claimed, arguing the jailbreak posed no serious risk.

Anthropic disputes this characterisation. In its public statement, the company said it had reviewed a demonstration of the jailbreak technique and found it produced only minor, previously known vulnerabilities. Anthropic added that other publicly available models, including OpenAI’s GPT-5.5, could achieve the same results without any bypass. “We have not even received a disclosure of a concerning non-universal potential jailbreak that led to a harmful result,” the company wrote.

Semafor reports that Amazon informed the government about the jailbreak and that Amazon CEO Andy Jassy had been in direct contact with members of the administration. An Amazon spokesperson confirmed that the company sometimes advises governments on potential security risks but declined to share details.

Broader context: a strained relationship

Anthropic and the Trump administration have clashed before. The company lobbied against the administration’s attempts to override state-level AI regulation and is currently suing the Pentagon over the use of its models for autonomous weapons. Sacks denied these conflicts played any role in the decision, writing that “the admin values Anthropic’s technical capabilities” and that the ball is now in Anthropic’s court to fix the issue.

Semafor also reports that a China-linked group may have accessed Mythos, which partly motivated the export controls. Anthropic said the government did not raise Chinese access in its conversations about the jailbreak. The company prohibits access to its products from within China.

Commentators and industry observers have raised questions about the timing and motivation of the directive. The announcement came on a Friday evening, a pattern critics associate with attempts to limit immediate market reaction. Some observers note that Anthropic’s competitors have closer ties to the current administration, and that Fable 5 was widely considered the strongest model available to consumers at the time it was pulled.

Anthropic said it is complying with the directive while disagreeing with it. The company argued that applying this standard across the industry “would essentially halt all new model deployments for all frontier model providers.” It said it expects to share more technical details within 24 hours and is working to restore access as quickly as possible.

For users and businesses access ended with almost no warning. Australian legal AI firm Isaacus noted in a blog post that the incident highlights the risks of depending on externally hosted AI models, and said it was doubling down on offering self-hostable versions of its own models as a result.

Sources

• Exclusive: White House’s export limits on Anthropic linked to concerns about Chinese access – Semafor
• David Sacks (@DavidSacks) on X – X (Twitter)
• Statement on the US government directive to suspend access to Fable 5 and Mythos 5 – Anthropic
• Tech Things: There is a massive shadow hanging over this Fable thing – 12 Grams of Carbon
• Our response to the US ban on Fable 5 and Mythos 5 – Isaacus