Anthropic is making its most advanced AI model available to the general public for the first time, though it comes with built-in safeguards. On Tuesday, the company introduced Claude Fable 5, the first publicly accessible version of its Mythos model. According to Anthropic, Fable 5 performs exceptionally well in software engineering, knowledge work, and visual tasks, but it enforces strict safety boundaries. In sensitive areas like cybersecurity, biology, chemistry, and distillation, the model will refuse to respond and instead defer to Claude Opus 4.8.

Originally launched as a preview in April, Mythos was restricted to a select group of partners due to cybersecurity worries. Last week, Anthropic broadened access to hundreds of organizations across 15 countries, again prioritizing those managing critical infrastructure. Now, a version of this technology is open to everyone through Anthropic’s Claude API and consumption-based Enterprise plans. Subscription access will be phased in: until June 22, Fable 5 is included in Pro, Max, Team, and seat-based Enterprise plans at no extra charge. On June 23, Anthropic will remove Fable 5 from those plans, requiring usage credits going forward, with plans to reinstate it as a standard subscription feature as soon as possible.

Anthropic is also releasing an updated version of Mythos, called Mythos 5, for organizations that have already been approved to use the advanced model. The launch of Fable comes as Anthropic prepares to go public, alongside OpenAI and Elon Musk’s SpaceX. It also follows the company’s call for major global AI labs to establish a coordinated emergency brake on frontier AI development. Anthropic has warned that systems are progressing so quickly that they might soon achieve recursive self-improvement (RSI), meaning they could autonomously enhance themselves without human intervention.

Concerned about what a Mythos-class model could do if misused, Anthropic says it thoroughly 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 stated. Still, novel attacks may 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. Anthropic says 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 rules framed as safety measures.

For those using the model, not every question will receive a Fable 5 answer. Anthropic says instances where Fable must defer to Opus 4.8 are rare, with early data indicating that at least 95% of Fable sessions run entirely on the model’s own responses. In third-party testing, analytics company Hex noted in a statement that Fable was the first to achieve a 90% score on its core analytics benchmark for complex, long-running analytical tasks. “On the hardest questions, it shows strong judgement and attention to nuance,” Hex said. Vibe-coding platform Base44 observed that Fable excels at “one-shotting full apps” and has excellent tool-calling capabilities. AI-powered workspace and agent platform Genspark reported that Fable outperformed every other model in its evaluations, particularly excelling in tasks like UI design and game coding.

Pricing for both Fable 5 and Mythos 5 is set at $10 per million input tokens and $50 per million output tokens, double the cost of Opus 4.8. This price alone might discourage widespread adoption. Many enterprises are growing critical of AI costs after seeing their bills or blowing through annual AI budgets early. Advanced models like Opus 4.8 can worsen these issues, as their sophisticated reasoning skills can split a single request into multiple tasks. Anthropic expects demand for Fable 5 to be very high and difficult to predict. However, some, like shopping rewards platform Rakuten, believe the benefits justify the cost. “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.”