Both Claude Fable 5 and Claude Mythos 5, reportedly use the same underlying model. The key difference between the two AI models is that Claude Fable 5, which is being publicly released, has guardrails in place that will block the model from answering many user questions related to cybersecurity, biology, and chemistry.

Anthropic had initially moved to limit the release of Claude Mythos over concerns that the model’s capabilities could be exploited by bad actors to develop hacking tools that could catch defenders off guard. The company’s claims of Mythos’ ability to find and exploit hidden flaws in software that are used by banks, power grids, and other critical infrastructure set off alarm bells globally, with governments and organisations around the world scrambling to prepare for how attackers might eventually use AI to carry out cyber attacks.

Now, with the public release of a Mythos-class AI model, Anthropic’s idea of giving researchers and policymakers a headstart to prepare for advanced AI systems, will be put to the test. The two month-interval also underscores Anthropic’s business tension of wanting to release a Mythos-class AI model for general use before the tech industry has resolved the cybersecurity concerns of these models.

Anthropic rival OpenAI also privately launched a model with advanced cybersecurity capabilities in April. Both AI giants have also confidentially filed for IPOs and are looking to go public later this year.

“We’re working as quickly as we can to safely release Mythos-level capabilities in general access. To do so, we’ll need highly robust safeguards that prevent the model’s cyber capabilities from being misused—safeguards that we (and, to our knowledge, all other AI developers) have yet to develop,” Anthropic wrote in an update about Project Glasswing last week.

“We’re trying to make improvements in a way that’s beneficial, even if we don’t have the perfect [solution] for every use case to start. Out of all the different approaches, this emerged as the most viable and the best one. We just ended up feeling like this was the best product choice for users to get the maximum value out of Fable 5,” Anthropic’s head of product management, Diane Penn, was quoted as saying by Wired. Penn added that testing and user input has helped hone its strategy.

Guardrails for Claude Fable 5

Anthropic’s strategy of making Claude Fable 5 safe for widespread use involves its older AI technology. When Claude users input queries related to topics that could be perceived as too risky, Anthropic said that these requests will instead be rerouted to an older AI model, Claude Opus 4.8, which was released last month and is designed to avoid the security risks of Mythos.

These guardrails have been designed to block responses related to cybersecurity, biology and other vulnerable areas. While they are designed to avoid hackers using Fable to attack computer networks, these measures may also make businesses and cybersecurity experts struggle to defend networks using the new model.

Notably, Anthropic said that if a user is suspected of trying to conduct distillation, the technique of training a smaller AI model off a larger AI model’s responses, on Claude Fable 5, those requests will also be rerouted to Claude Opus 4.8 without informing the user about it.

The protective mechanism is built to err on the side of caution, which means that some user queries may be routed to the less capable AI model even if they are benign, according to Anthropic’s Diane Penn. In the past, Anthropic has expressed concerns that competitors, including open-weight model developers, could also eventually build AI models with Mythos-level capabilities.

The company said that it carried out more than 1,000 hours of red-teaming, and its testers found no universal jailbreaks for the model. However, it remains to be seen how resistant Claude Fable 5’s safeguards are in the wild. Several research studies in the past have demonstrated that guardrails fitted on AI systems are not always reliable.