Anthropic today is launching Claude Fable 5, a safe-for-use Mythos class model, and is launching Claude Mythos 5 to a small group of security and infrastructure providers that has some safefguards lifted.
Fable 5 has safeguards that can prevent its cybersecurity capabilities from being misused. The safeguards ensure that queries on certain topics will be answered by Claude Opus 4.8. Meanwhile, Mythos 5 will be deployed through the company’s Project Glasswing as an uprade to Mythos Preview. The company claims this new model has the strongest cybersecurity capabilities of any model. A trusted access program will soon be in place to expand Mythos 5.
In its blog announcement, Anthropic wrote: “The capabilities of models like Fable 5 and Mythos 5 have the potential to do profound good for the world. We’ve seen the beginnings of this in Project Glasswing, where the models have helped cyber defenders secure critically important software. We’ve also seen it in life sciences research, where the models are positing novel hypotheses and speeding up the development of new therapeutics.”
The new models have been shown to work more autonomously than any previous Claude models, the company wrote. For software engineers, Anthropic reported that Fable 5 “compressed months of engineering into days.” In their test, they said a Ruby codebase with 50 million lines was migrated in a day, which would have taken an entire team more than two months to complete by hand. The model’s improved capabilities also extend to knowledge work, vision, memory, and life sciences research.
“Fable 5 is a real step forward for the developers GitHub serves,” said a spokesperson for GitHub. “In our early testing, it took on complex, long-horizon coding tasks with a level of autonomy and reliability that exceeded previous benchmarks. But what excites us most is the direction it
points: a future where developers can hand increasingly ambitious work to agents and trust the results across the software lifecycle.”
Fable 5 and Mythos 5 are being offered at $10 per million input tokens and $50 per million output tokens—less than half the price of Claude Mythos Preview. Today’s joint launch is another step towards our goal of bringing advanced AI capabilities to as many users as possible, as quickly and as safely as we can.
The company is also adding data privacy protections, such as logging human access to the data, and ensuring the data will be deleted after 30 days, in most cases.
Availability
While Claude Fable 5 is available everywhere today, Mythos 5 is limited to Glasswing partners, with some of the cyber safeguards lifted. Developers can use claude-fable-5 via the Claude API.
A conservative staged rollout of subscription plans is as follows, per the company:
- From today through June 22, Fable 5 is included on Pro, Max, Team, and seat-based Enterprise plans at no extra cost.
- On June 23, we’ll remove Fable 5 from those plans. Using it after that will require usage credits. If capacity allows, we’ll extend the included window.
- After this point—when sufficient capacity allows us to do so—we aim to restore Fable 5 as a standard part of subscription plans. We intend to do this as quickly as we can.
