Anthropic Offers Mythos Upgrade for Cyber Partners and a ‘Safe’ Version for the Rest of You

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Anthropic released two new AI models called Claude Fable 5 and Claude Mythos 5 on Tuesday, which the company says have greater capabilities than the Mythos Preview model it released in April to a limited set of tech industry partners. Anthropic has said the initial, limited release stemmed from concerns that the model’s capabilities could be exploited by bad actors to develop hacking tools that could catch defenders off guard.

Anthropic is currently only releasing Claude Mythos 5 to a limited set of industry partners, many of which received access to Mythos Preview, and the company says it is collaborating with the US government on the rollout.

Claude Fable 5, which is being publicly released, uses the same underlying model as Mythos 5, but will have “guardrails” in place at launch, the company said Tuesday, that will block the model from answering many user questions related to cybersecurity, biology, and chemistry. These requests will instead be re-routed to an older AI model, Claude Opus 4.8. If Anthropic suspects a user is trying to conduct distillation—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, the company says.

In an interview with WIRED, Anthropic’s head of Product Management, Diane Penn, says that the company has been grappling with the question of how to handle Mythos’s software vulnerability-discovery abilities and other advanced capabilities since before its April release, but that testing and user input since then helped to hone the strategy.

“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,” Penn says. “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.”

For now, Penn says that the protective mechanism is built to err on the side of caution, meaning some user queries may be routed to the less capable AI model even if they’re benign. Over time, Anthropic hopes to make its classifiers more precise, but Penn says this was the only safe way the company could release the model broadly at this time.

The company said on Tuesday that in addition to offering Claude Mythos 5 to Project Glasswing partners, it is also giving access to “select biology researchers.” Additionally, Anthropic noted in its blog post about Tuesday’s launch that it is providing unrestricted versions to these small groups of customers “until our trusted access program is available,” hinting at future plans to expand access even more. Since the Mythos launch in April, Anthropic has repeatedly emphasized that eventually its competitors in both the private and even open weight spaces will inevitably also offer models with Mythos-level capabilities.

The ability for Claude Mythos and other new AI models to design hacking tools that can find and exploit vulnerabilities in both new and legacy software has forced tech companies and governments around the world to secure their software defenses before AI models of this level are made broadly available to attackers. Anthropic first released Mythos to industry partners under a consortium called Project Glasswing, with the idea that this could give members a head start in preparing their own systems and weighing global solutions to the threat before a broader release.

Anthropic wrote in an update about Project Glasswing last week: “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 says Claude Fable 5—named after the literary form, much like the company’s existing Haiku, Sonnet, and Opus models—offers increased performance on software engineering and tasks that require visual understanding. But that added performance comes at a price. Claude Fable 5 and Claude Mythos 5 will cost developers $10 per million input tokens and $50 per million output tokens—twice as much as Anthropic’s publicly available AI models, but cheaper than Mythos Preview.

The neutered release of Claude Fable 5 hints at 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. In April, OpenAI also privately launched a model that it said has advanced cybersecurity capabilities and convened a working group similar to Project Glasswing. Both OpenAI and Anthropic have confidentially filed for IPOs, and are racing to impress prospective investors before they become public companies as soon as this year.

Even as an interim solution, though, it remains to be seen how resistant Claude Fable 5’s safeguards are in the wild. Anthropic says in more than 1,000 hours of red-teaming, its testers found no universal jailbreaks for the model. Still, fears about the ability to develop adequate protections underpinned the company’s original justification for why it did not release Mythos-class models to the public in April, and these fears have seemingly persisted.