Anthropic’s Claude Fable is a version of Mythos the public can access today

Anthropic is bringing its most powerful AI model to the general public for the first time, but it’s doing it with guardrails. 

On Tuesday, the AI firm launched Claude Fable 5, the first publicly available version of its Mythos model. Anthropic says Fable 5 excels at software engineering, knowledge work, and vision, but it comes with hard safety limits. In high-risk areas like cybersecurity, biology, chemistry, and distillation, the model blocks responses and falls back to Claude Opus 4.8.

Launched as a preview in April, Mythos was initially limited to a handful of partners due to cybersecurity concerns. Last week, Anthropic expanded access to hundreds of organizations across 15 countries, again focusing on organizations that manage critical infrastructure.

Now, a version of that technology is available to anyone through Anthropic’s Claude API and consumption-based Enterprise plans. Access on subscriptions will roll out in stages: through June 22, Fable 5 is be included in Pro, Max, Team, and seat-based Enterprise plans at no extra cost. On June 23, Anthropic will pull Fable 5 from those plans, requiring usage credits going forward, with plans to restore it as a standard subscription feature as soon as possible.

Anthropic is also deploying a new version of Mythos, called Mythos 5, to organizations that have already been approved to access the advanced model.

Fable’s launch comes as Anthropic prepares to enter the public markets, alongside OpenAI and Elon Musk’s SpaceX. It also follows the AI firm’s plea urging major global AI labs to establish a coordinated brake pedal on frontier AI development. Anthropic warned that systems are advancing so rapidly that they may soon achieve recursive self-improvement (RSI), autonomously improving themselves without human intervention. 

Wary of what a Mythos-class model could do in the wrong hands, Anthropic says it stress-tested its classifiers with jailbreak attempts before releasing Fable 5. 

“Internally, we ran an external bug bounty that produced no universal jailbreaks in over 1,000 hours of testing. We then worked with external red-teaming orgs which also failed to find universal jailbreaks.”

That said, there could still be novel attacks remain possible. As a result, with the launch of Fable 5 and Mythos 5, Anthropic said it will require a 30-day retention on all traffic, even if enterprises previously had zero-retention agreements. Anthropic said it won’t use the data for training, only to “defend against complex and novel attacks, including new jailbreaks,” and “identify and reduce false positives.” The policy could set an industry precedent in which access to increasingly powerful models comes with mandatory data retention policies framed as a safety measure.

For those that continue to use the model, not every question will get a Fable 5 answer. Anthropic says the cases in which Fable has to defer to Opus 4.8 are rare, with early data showing at least 95% of Fable sessions running entirely on the model’s own responses. 

In third-party testing, analytics company Hex said in a statement that Fable was the first to get a 90% on its core analytics benchmark of complex, long-running analytical tasks. 

“On the hardest questions, it shows strong judgement and attention to nuance,” Hex said.

Vibe-coding platform Base44 noted in a statement that Fable is better at “one-shotting full apps” and has excellent tool-calling. AI-powered workspace and agent platform Genspark said Fable beat every other model in its evaluations, and performed significantly better on tasks like UI design and game coding. 

Pricing for both Fable 5 and Mythos 5 is $10 per million input tokens and $50 per million output tokens, double the price of Opus 4.8. That price alone might serve as a deterrent for widespread use.

Many enterprises are growing critical of AI costs after seeing the bills come in or blowing through their yearly AI budgets early. Advanced models like Opus 4.8 can exacerbate those issues, with advanced reasoning skills that can split a single request into multiple tasks.

Anthropic said it expects demand for Fable 5 to be very high and difficult to predict. And indeed some, like shopping rewards platform Rakuten, might think the upside is worth the price point. 

“At the highest effort, Fable reflects on and validates its own work,” Rakuten said in a statement. “For us, that’s what makes highly autonomous operations possible — the extra thinking pays for itself.”