Anthropic launches Claude Fable 5, first Mythos class model that you can use

Anthropic has announced the launch of Claude Fable 5, the first Mythos-class AI model that can be used by the general public. The company says that Fable 5 can do exceptionally well in tasks like software engineering, or scientific research, but comes with "hard limits" to avoid potential misuse in cybersecurity or distillation.

In Short

• Anthropic releases Claude Fable 5, first Mythos-class model for general public
• Fable 5 comes with hard limits on cybersecurity, biology, chemistry, and distillation
• It is double the cost of Anthropic’s Opus 4.8 model

Anthropic will now let you access Claude Mythos-class AI models. The company announced on Tuesday that Claude Fable 5 is the first publicly available Mythos-class model, but it comes with certain hard limits.

Mythos refers to Anthropic’s most powerful class of AI models. Anthropic has previously hinted that Mythos preview was so powerful that it could potentially hack any software in the world.

According to the AI startup, Fable 5 is its most capable generally available model so far, with strong performance in software engineering, knowledge work, vision, scientific research and long, complex tasks, but that its release comes with added safeguards in high-risk areas.

Mythos has made waves globally with many governments, including India, being worried about the potential disruption it could cause to cybersecurity. Reports also claim that the US may be using Mythos to plan cyberattacks.

What is Claude Fable 5?

Anthropic says that Claude Fable 5 shows "exceptional performance" across software engineering and knowledge work tasks, and that on some benchmarks it scored more than 10 per cent higher than Claude Opus 4.8, which the company announced late last month.

The AI startup stated, "Fable 5's capabilities exceed those of any model we've ever made generally available The longer and more complex the task, the larger Fable 5's lead over our other models."

What about cybersecurity?

When Anthropic announced Claude Mythos in April, governments around the world were concerned over the potential risks associated with cybersecurity due to such AI models. To address this issue, Anthropic says that it has put hard limits on Fable 5 in areas like cybersecurity, biology, chemistry, and distillation. In such cases, the model blocks responses and falls back to Claude Opus 4.8.

Anthropic has previously accused Chinese AI companies of distilling Claude to train their own models. Distillation happens when a smaller model is made to learn from the outputs of a larger model. It is likely that the safeguards against distillation in Fable 5 could help avoid this issue.

Dianne Penn, Anthropic's head of product management for research, told CNBC that the broad release was possible because of the new guardrails. "For us, it's really around what we call 'race to the top,' being able to provide this technology in a valuable fashion, and at the same time providing the right safety guardrails so that it can do asymmetrically more benefits than harm," she said.

Penn said Claude Fable 5 represents a "significant jump" in capability, which is why Anthropic had to put in additional protections. She said if a user asks a high-risk question, such as how to make ricin, the model will block the response and Claude Opus 4.8 will deliver a safe answer instead. "What we wanted to do was to be very intentional about building new types of classifiers and new types of safety guardrails in place for this launch," Penn said.

Anthropic said it tuned the safeguards conservatively, so the fallback may trigger more often than some users expect. At the same time, the company said the cases in which Fable has to defer to Opus 4.8 are rare, with early data showing that at least 95 per cent of Fable sessions run entirely on the model's own responses.

Before releasing the model, Anthropic said it stress-tested its classifiers with jailbreak attempts. "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.

New Mythos 5 model

The launch also includes Claude Mythos 5 for organisations that have already been approved to access the advanced model, including a small group of cyber defenders and critical infrastructure providers through Project Glasswing. Anthropic said Mythos 5 uses the same underlying model as Fable 5, but with safeguards lifted in some areas, where Fable 5 would have fallen back to Claude Opus 4.8.

Anthropic had launched Mythos as a preview in April and initially limited access to a handful of partners because of cybersecurity concerns. The company later expanded access to hundreds of organisations across 15 countries, again focusing on those that manage critical infrastructure.

Claude Fable 5 and Mythos 5 availability and price

The AI startup says that Fable 5 and Mythos 5 will require 30-day retention on all traffic, including for enterprises that previously had zero-retention agreements. Anthropic insists that it would not use that data for training and would use it only to "defend against complex and novel attacks, including new jailbreaks."

Claude Fable 5 is available now through its Claude API and consumption-based Enterprise plans. It also said access on subscriptions will roll out in stages. Through June 22, Fable 5 will be included in Pro, Max, Team and seat-based Enterprise plans at no extra cost. But from June 23, the company said, it will move the model off those plans and require usage credits, with the aim of restoring it as a standard subscription feature when capacity allows.

Claude Mythos 5 remained restricted to Glasswing partners until the trusted access programme was expanded.

Anthropic said it expects demand for Fable 5 to be very high and difficult to predict. Pricing for both Fable 5 and Mythos 5 is set at $10 per million input tokens and $50 per million output tokens, which is double the price of Opus 4.8.

The AI company has released the first Mythos-class model to the public just days after it filed for an IPO. OpenAI and SpaceX are also planning to go public soon.

The rollout of Fable 5 also follows Anthropic's recent call for major AI labs to create a coordinated brake pedal on frontier AI development, with the company warning that systems are advancing so quickly that they may soon reach recursive self-improvement, or RSI, allowing them to improve themselves without human intervention.

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