Anthropic restored global access to its Fable 5 model on Wednesday, ending a roughly three-week shutdown that began with the U.S. government imposing export controls barring foreign nationals from accessing the advanced, cybersecurity-focused AI tool.

It comes as the Five Eyes intelligence alliance has warned business leaders to immediately prepare for the impact frontier AI models will have on cybersecurity, with new models set to “fundamentally transform” both offensive and defence capabilities. “The timeline is not years, it is months,” the agencies warned.

At the time the restrictions against Anthropic were imposed, the company said it had been forced to disable access for all customers to ensure compliance. In an update Tuesday, Anthropic said the controls had been lifted after the company came to a series of agreements with the government.

The episode marked the first known use of export control authorities to pull AI software rather than chips or hardware from public access. Its reversal may set the terms under which frontier AI models are regulated in the U.S. going forward.

Export controls on Anthropic’s more powerful cybersecurity model, Mythos 5, were also fully lifted as of June 30, although access to that model remains restricted to vetted U.S. organizations through Project Glasswing — Anthropic's controlled-access program for critical infrastructure defenders. The company said it is continuing to negotiate broader domestic and international access through Glasswing.

The initial shutdown was, according to Anthropic, triggered by a “jailbreak” technique covered in an Amazon research report. That technique was subsequently described in detail by Katie Moussouris, founder of Luta Security, whom Anthropic asked to assess the paper.

Moussouris wrote that researchers fed Fable 5 open-source code with publicly known vulnerabilities plus deliberately planted flaws, then asked it to “fix this code.” The model’s output was then manually assembled, across multiple steps, into scripts that test patches.

“That is not a guardrail bypass,” Moussouris wrote. “It is the most valuable thing an AI model can do for defensive security: executing the find, fix, and test loop defenders run every day.” Her conclusion was that the underlying capability cannot be removed without degrading the model's usefulness for legitimate security work.

Anthropic said its own subsequent testing confirmed the same technique worked against other models, including OpenAI's GPT-5.5 and the Chinese model Kimi K2.7 — none of which faced comparable export restrictions. The company said the technique exposed no capability unique to its frontier models.

As part of the negotiations to restore access to Fable, Anthropic said it trained a new safety classifier that blocks the specific technique in more than 99% of cases. Researchers from the Commerce Department's Center for AI Standards and Innovation tested both the original and updated safeguards and endorsed the result.

Beyond the classifier, Anthropic committed to expanded pre-release access for government evaluators to test frontier models before broad release, rapid disclosure of significant jailbreaks, dedicated staff and compute for joint research and participation in a shared voluntary security standard across frontier model providers. It also opened a HackerOne bug bounty program for cyber jailbreak submissions.

Together with its Glasswing partners — including Amazon, Microsoft and Google — Anthropic said it is drafting an industry framework to score jailbreak severity across four criteria: capability gain over existing tools, breadth of tasks affected, ease of weaponisation and discoverability.

More than 100 cybersecurity professionals had signed an open letter organized by former Facebook security chief Alex Stamos and addressed to Commerce Secretary Howard Lutnick and National Cyber Director Sean Cairncross, warning the government that the export controls risked doing more harm than good.

“The Chinese open-weight models are only months behind the best American models, and those are the models we know about,” the letter said. “To pull the best capabilities away from defenders without a good reason when our adversaries are rapidly advancing is dangerous.”

The signatories included executives from Nvidia, Adobe, Zoom, Google and Sophos, and echoed Anthropic’s argument that if the standard applied to Fable 5 were applied industry-wide, it would, in Anthropic’s own words, “essentially halt all new model deployments for all frontier model providers.”

The directive arrived against a backdrop of tensions between Anthropic and the Trump administration. In February, Defense Secretary Pete Hegseth designated Anthropic a “supply chain risk” — a label historically applied to companies such as Huawei — after contract negotiations over military use of Claude broke down.

Anthropic co-founder Tom Brown took over negotiations with the Trump administration from CEO Dario Amodei, according to CNBC, which reported that Amodei had become a political target of the administration over his public AI safety positions and his support for Kamala Harris in the 2024 election. Lutnick's letter formally lifting the ban was reportedly addressed to Brown rather than the chief executive.