Anthropic today launched two new AI models, Claude Fable 5 and Claude Mythos 5, marking the company's first broad release of the powerful "Mythos-class" AI capabilities it previously made available only to participating organizations in its restricted cybersecurity program, Project Glasswing.
Fable 5's capabilities exceed those of any model Anthropic has ever made generally available. It is state-of-the-art on nearly all tested benchmarks of AI capability, showing exceptional performance in software engineering, knowledge work, vision, scientific research, and many other areas.
The timing is notable. Claude Fable 5 arrives just days after Anthropic confidentially filed its IPO prospectus with the SEC, as the company's revenue run rate has ballooned to $47 billion, up from roughly $10 billion in annual revenue last year. This is a company building momentum fast, and Fable 5 is the most visible proof point.
What Is the Mythos Class?
Mythos-class models are a tier of Claude models that sit above the Opus class in capability. The first, Claude Mythos Preview, was released in April through Project Glasswing. That is followed today by Claude Fable 5 and Claude Mythos 5.
Anthropic captivated Wall Street and government officials in April with the unveiling of Mythos, which excels at identifying security flaws within software. The company did not plan to make the model generally available, limiting the rollout to a select group of companies as part of Project Glasswing.
Anthropic created Glasswing after internal evaluations showed Mythos-class models could find and exploit software vulnerabilities at a level that raised meaningful misuse concerns. The question was never whether to release it more broadly, but how.
Fable 5 vs. Mythos 5: The Key Difference
These two models share the same underlying architecture. They are the same underlying model. The distinction is access control. Claude Mythos 5 has no safety classifiers on cybersecurity and biology queries. Claude Fable 5 has safety classifiers layered on top.
Fable is from the Latin fabula, "that which is told," akin to the Greek mythos. The safeguards are what distinguish the two models and are why Anthropic gave them different names.
In practice, this means:
- The general-purpose Fable 5 wraps the same underlying Mythos-class capability in new safeguards. Requests involving certain high-risk areas, including cybersecurity, biology and chemistry, and model distillation, are automatically routed to Claude Opus 4.8 instead, with users notified when that happens.
- Mythos 5 is the same underlying model as Fable 5 but with the safeguards lifted in some areas. It has the strongest cybersecurity capabilities of any model in the world.
