Fable 5.1 is now generally available, while Mythos 5.1 is only available through Anthropic’s “trusted access” programmes.

Anthropic has launched the latest iterations of Claude Fable and Claude Mythos, “the world’s most advanced models for coding and knowledge work” according to the company.

The AI giant said in a press release that Fable 5.1 and Mythos 5.1 are “the same model, but with different levels of safeguards”.

Fable 5.1 is now generally available, while Mythos 5.1 is only available through Anthropic’s “trusted access” programmes – Mythos’ safeguards are specifically designed to support work in cybersecurity and the life sciences, according to the release.

The company said that Fable 5.1 is capable of much higher performance than its predecessor and that when set to medium or low effort, it can achieve similar or better results than Fable 5.

The new model can now also be used to discover software vulnerabilities, though it can’t exploit them, according to Anthropic.

The biggest performance jump between Fable 5.1 and Fable 5 appears to be in relation to agentic scientific research, with the new model capable of more than double the capability in comparison to its predecessor – from a score of 24.7pc to 52.6pc to be specific.

In terms of price, Fable 5.1 will cost an estimated 25pc less than Fable 5 for “typical workloads, wherever usage is billed by token”. For “highly agentic work”, customers will be charged up to 45pc less.

Anthropic is also deploying its new enterprise frontier safeguards (EFS) system, which stores data in cloud infrastructure “controlled entirely” by the customer, not Anthropic. EFS gives customers “complete privacy” which Anthropic said equates to a zero data retention policy, and will be made available to enterprise customers in phases, beginning later this fall.

Anthropic said that eligible customers will be able to use Fable 5.1 with zero data retention before EFS is available.

Mythos 5.1, according to Anthropic, is identical to Fable 5.1 but with “more permissive safeguards” for vetted individuals and organisations whose work is affected by cybersecurity and life sciences restrictions.

Currently, Mythos 5.1 is only available to vetted cybersecurity and life science professionals at select US organisations, though Anthropic said it’s coordinating with the US government to expand access to a broader set of domestic and international partners “as quickly as possible”.

Mythos 5.1 is now also being used to power Claude Security – the company’s product for scanning codebases for vulnerabilities and suggesting patches for human review.

OpenAI readying Astra

The debut of Fable 5.1 and Mythos 5.1 comes a few months after their predecessors – particularly Mythos – caused a stir with claims of unprecedented AI capabilities in relation to cybersecurity, including exploitation of vulnerabilities.

Security concerns even led to the US government ordering Anthropic to suspend the models for a few weeks.

Concern around Anthropic’s AI cyber capabilities were reignited recently after the company reported incidents of Claude models (including Mythos) instigating cyberattacks after breaking free during tests.

The buzz caused by Mythos in particular ignited further competition with Anthropic’s chief US AI rival OpenAI.

According to an announcement earlier this week, the ChatGPT creator is preparing to release its new Astra AI model soon, which OpenAI claimed can find previously unknown security flaws and develop ways to exploit them across many well-protected systems without a person guiding each step.

OpenAI said in a recent blogpost that it has delayed parts of Astra’s development and release over the past few weeks to strengthen and test protections against “cyber misuse and unauthorised model actions”.

Astra’s launch is expected soon, though access to “its most advanced cybersecurity capabilities” will be more limited, according to the company.

“We will continue to test these systems, share what we learn, and be clear about what remains uncertain,” said OpenAI. “The models that follow Astra will demand more of us. We will take the time and do the work needed to meet that responsibility.”

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