Last month, Anthropic introduced its Claude Fable 5 and Mythos 5 models. They both share the same underlying model, but Fable 5 was released with strong safeguards to make it safer for general use. Mythos 5, which has fewer safeguards, was only released to a small number of trusted Project Glasswing partners for use in defensive cybersecurity.
But just a few days after the launch of these models, the US government applied export controls to them, and Anthropic suspended access to both models for all users. The export control directive came after the government became aware of a report in which Amazon researchers had found a method of bypassing Fable 5’s safeguards, prompting it so that it identified a number of software vulnerabilities.
Since then, Anthropic has worked closely with the government and other partners, including Amazon, to review the report and evidence. The testing revealed that many other models like Claude Opus 4.8, GPT 5,5, and Kimi K2.7 could identify the same vulnerabilities as Fable 5 did, while the demonstration produced by Fable 5 on how to exploit the vulnerability was also done by Claude Haiku 4.5, Sonnet 4.6, Opud 4.6, Opus 4.7, Opus 4.8, GPT-5.4, GPT-5.5 and Kimi K2.7.
The company has addressed the issue and trained an improved safety classifier that targets and blocks the behaviour described in the report. Yesterday, finally, the export controls on Fable 5 and Mythos 5 have been lifted.
Claude Fable 5 will be available again globally tomorrow.
After a series of productive conversations with the US government, we're redeploying the model with a new set of classifiers to target and block more cybersecurity tasks. In the near term, some routine tasks like coding…
— Anthropic (@AnthropicAI) July 1, 2026
Claude Fable 5 and Mythos 5 to make a comeback
Starting today, Fable 5 is available to users globally on the Claude platform, Claude.ai, Claude Code, and Claude Cowork. For Pro, Max, Team, and select Enterprise plans, Fable 5 will be included for up to 50% of weekly usage limits through July 7, after which it will be available via usage credits. Anthropic will re-enable access on AWS, Google Cloud, and Microsoft Foundry as quickly as possible.
The company has also restored access to Mythos 5 for a set of US organisations, following the US government’s approval on June 26. Anthropic continues to coordinate with the government to expand access to the broader set of domestic and international partners in the Glasswing program.
Users will be notified if a request to Fable 5 is blocked, and the request will instead be sent to Opus 4.8. The new classifier means that the specific technique described in the Amazon report is blocked in over 99% of cases. In a very small fraction of cases, the model may provide information that isn’t detailed enough to help a cyberattacker. The new classifier also comes at the cost of flagging benign requests more often during routine coding and debugging tasks.
Though Anthropic has warned that, like all safety mechanisms, classifiers can make mistakes and sometimes fail to notice potentially dangerous content, and in some cases they can be deliberately “jailbroken,” users can prompt the model in unusual ways to trick the classifiers and get the model to produce some harmful outputs.
Therefore, the company has deliberately set the safety classifiers to trigger on a set of requests that are likely benign.
For Fable 5, the safety margin is made much larger than any prior launch, meaning that many more benign requests would be blocked. The company will also continue to update its classifiers as they learn more about novel jailbreak techniques.
Anthropic is also partnering with Amazon, Microsoft, Google and Glasswing partners to draft a consensus framework for assessing the severity of AI jailbreaks and how AI developers should respond to them.
The company is scaling up its collaboration with the US government on model testing and safeguards. This will include pre-release access to models and safeguards for evaluation, information sharing on jailbreaks and misuse, and dedicated resources for joint research.
