Anthropic Releases Fable 5, Its First Public Mythos-Class AI Model

Credit: Anthropic

Key Takeaways

1. Anthropic released its first “Mythos-class” model to the general public on Tuesday: Fable 5.
2. Fable 5 comes with many of the powerful capabilities of Mythos, with strict guardrails for safety.
3. Fable 5 cannot answer questions about cybersecurity, biology, chemistry, or AI distillation. Instead, the AI drops these requests to Opus 4.8.
4. Mythos 5 is available with these capabilities, but not for the general public.

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Back in April, Anthropic introduced its “Mythos” model to the world. Mythos Preview, reportedly, is such a powerful model that it can find security flaws across all kinds of software. In the wrong hands, bad actors could abuse the model to find vulnerabilities in programs, services, and sites most of us rely on for modern digital life. In effect, Mythos could open up the biggest hacking opportunity in history.

As such, Anthropic pulled the brakes on Mythos. While it maintained that it would eventually release the model to the public, it first needed to trial it with a limited pool of trusted testers, in what it calls “Project Glasswing.” To start, that meant opening up the model to the U.S. and other governments. While Mythos is still not available to the likes of you or me, Anthropic is releasing a new model that promises many of the capabilities of Mythos, without the accompanying cybersecurity risks.

What are Anthropic’s Fable 5 and Mythos 5?

On Tuesday, Anthropic announced its latest model, Claude Fable 5, which it calls a “Mythos-class model” that is “safe for general use.” The company says Fable 5 is supposedly better and more capable than any of its other public models. Anthropic claims Fable 5 scores at the top of most benchmarks, including software engineering, knowledge work, vision tasks, and research. The company goes so far as to say “the longer and more complex the task, the larger Fable 5’s lead over our other models.” There’s also Mythos 5, which seems to be Fable 5 without certain limitations, but isn’t available to the general public.

According to Anthropic’s benchmarking, Fable 5 and Mythos 5 alike outperform Mythos Preview, Opus 4.8, OpenAI’s GPT-5.5, and Google’s Gemini 3.1 Pro, in the following categories: agentic coding, knowledge work, spatial reasoning, tool use, legal, multidisciplinary reasoning (without tools), biology, cybersecurity, and health. Mythos Preview ekes out a win in computer use and multidisciplinary reasoning (with tools), but it’s a clean sweep over all other models.

Credit: Anthropic

Anthropic says Fable 5 was able to complete a coding project that would have taken a team over two months to finish in just a day. It can rebuild a web app’s source code from only screenshots. It can beat Pokémon FireRed with a “minimal, vision-only harness,” while other Claude models struggled to play at all. It was able to play Slay the Spire and reached the final act three times more often than Opus 4.8. Mythos 5 builds on its research abilities, with improved stats in drug design, as well as novel hypotheses regarding questions of molecular biology, and the ability to produce novel research in genomics.

How is Anthropic keeping Fable 5 safe?

That’s the big question: If Fable 5 is Mythos-class, how can you ensure that it’s safe to release to the general public? Couldn’t a bad actor take advantage of Fable 5’s capabilities and force it to discover and disclose security vulnerabilities?

Anthropic says it has that figured out. While Fable 5 may be Mythos-level in many ways, the company says that its Project Glasswing testing has produced a model with the proper safeguards for a public release. Fable 5 looks out for “classifiers,” or highly sensitive topics, that it knows it should not answer. When Fable 5 receives a request that it thinks has to do with cybersecurity, biology, chemistry, or distillation, it doesn’t answer the question itself. Instead, it passes the query off to Opus 4.8, Anthropic’s “next-most-capable” model. The model should still be powerful enough to provide accurate answers, but not capable of providing malicious users with the tools necessary to exploit others.

Anthropic says its new guardrails are cautious and conservative, and may be overkill. Benign requests may accidentally trip Fable 5’s security alarms, but that supposedly happens around 5% of the time. As such, Anthropic says Fable 5 is able to handle requests itself roughly 95% of the time.