Anthropic launches Claude Fable, a public version of Mythos

Anthropic is opening access to its most advanced AI model yet, but with strict safety measures that mark a new chapter in responsible deployment. On Tuesday, the company introduced Claude Fable 5, the first publicly available version of its Mythos model, designed for software engineering, knowledge work, and vision tasks. However, the model comes with hard-coded guardrails: in high-risk areas such as cybersecurity, biology, chemistry, and distillation, it will block responses and defer to the less powerful Claude Opus 4.8.

Mythos first launched as a preview in April, limited to a small group of partners due to cybersecurity concerns. Last week, Anthropic expanded access to hundreds of organizations across 15 countries, focusing on those managing critical infrastructure. Now, a version of that technology is available to anyone through the Claude API and consumption-based Enterprise plans. Subscription access will roll out in stages: through June 22, Fable 5 is included in Pro, Max, Team, and seat-based Enterprise plans at no extra cost. On June 23, Anthropic will remove Fable 5 from those plans, requiring usage credits going forward, with plans to restore it as a standard subscription feature as soon as possible.

Alongside this public launch, Anthropic is also deploying Mythos 5, a new version for organizations already approved to access the advanced model. Fable’s release comes as the company prepares to enter public markets, alongside OpenAI and Elon Musk’s SpaceX. It also follows Anthropic’s call for major global AI labs to establish a coordinated brake pedal on frontier AI development, warning that systems are advancing so rapidly 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 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,” the company said. Still, novel attacks remain possible. As a result, with the launch of Fable 5 and Mythos 5, Anthropic will require a 30-day retention on all traffic, even if enterprises previously had zero-retention agreements. The company 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.” This policy could set an industry precedent, where access to increasingly powerful models comes with mandatory data retention framed as a safety measure.

For those who continue using the model, not every question will get a Fable 5 answer. Anthropic says the cases where Fable defers 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 Fable was the first to achieve a 90% score on its core analytics benchmark of complex, long-running analytical tasks. “On the hardest questions, it shows strong judgement and attention to nuance,” Hex noted. Vibe-coding platform Base44 added 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 cost alone might serve as a deterrent for widespread use. Many enterprises are growing critical of AI costs after seeing their bills or blowing through yearly AI budgets early. Advanced models like Opus 4.8 can exacerbate those issues, with reasoning skills that split a single request into multiple tasks. Anthropic said it expects demand for Fable 5 to be very high and difficult to predict. Still, some companies, like shopping rewards platform Rakuten, see the upside. “At the highest effort, Fable reflects on and validates its own work,” Rakuten said. “For us, that’s what makes highly autonomous operations possible , the extra thinking pays for itself.”