Anthropic suspends Fable 5 and Mythos 5 after U.S. order

Anthropic disabled access to its Fable 5 and Mythos 5 models for all users on June 12 after receiving a U.S. export-control directive that invoked national security authorities. The company reported it received the directive at 5:21 p.m. ET and that the order targeted access by foreign nationals, including foreign-national employees in the United States, requiring immediate compliance.

Anthropic said it turned off the two models for every customer to ensure full compliance. Access to other Claude models remains available. New sessions will run on a user’s selected default model or on Opus 4.8, the company added. The federal letter did not include a written technical explanation of the national security concern, according to Anthropic.

Anthropic disputed the government’s assessment while noting officials identified a narrow method for bypassing safeguards. The company reviewed a demonstration of the technique and described it as limited. Anthropic’s engineers said no tester had found a single exploit capable of reliably defeating Fable 5’s protections across a broad set of cyber capabilities.

Before the release, Anthropic reported red-teaming Fable 5’s safeguards for thousands of hours in cooperation with U.S. officials, the U.K.’s AISI and multiple third-party teams. The company described a defense-in-depth approach that includes targeted safeguards, continuous monitoring and 30-day data retention for Mythos-class models.

The alleged bypass involves asking the model to read a codebase and fix software flaws, which Anthropic described as a routine developer use case. The company also noted that other publicly available models can surface similar flaws without specialized bypass techniques.

Anthropic wrote: “We are complying with the government’s legal directive and are removing access to Fable 5 and Mythos 5 for all users. However, we disagree that the finding of a narrow potential jailbreak should be cause for recalling a commercial model deployed to hundreds of millions of people.” The firm said it will share further technical details within 24 hours and is working to restore access as soon as it can while following the directive.

Users reacted online after the suspension. One user posted on social media: “Fable, my beloved, I will miss you so. Our three days together were magical.” Fable 5 and Mythos 5 were released as Anthropic’s advanced models with enhanced safety features intended to limit misuse while enabling developer tasks. The export-control directive explicitly covered foreign-national access inside and outside the United States.