What is Claude Fable 5? 7 things to know about Anthropic's first Mythos model

After months of warning about the potential dangers of releasing Mythos, Anthropic is bringing its powerful model to the public, but with a few caveats. The new model is called Fable 5 and comes with restrictions in areas where Anthropic thinks it could be misused.

However, despite the restrictions, Fable 5 is claimed to be the most powerful AI model available not just from Anthropic, but from any AI provider at the moment.

Here are the top 7 features of Fable 5 you should know:

1) Fable 5 is Anthropic's most powerful public model yet

Anthropic says Fable 5 outperforms every previous Claude model and has achieved state-of-the-art performance on nearly all tested AI benchmarks, including software engineering, vision, scientific reasoning, and knowledge work.

2) Fable 5 takes a lead over GPT-5.5 and Gemini 3.1 Pro

Anthropic also says that Fable 5 outperforms rival AI models from OpenAI and Google across a variety of benchmarks.

On SWE-Bench Pro, a benchmark that measures how well AI models can solve real-world software engineering problems, Anthropic says Fable 5 scored 80.3%, while GPT-5.5 (58.6%) and Gemini 3.1 Pro (54.2%) ranked behind even Claude Opus 4.8 (69.2%).

Similarly, on FrontierCode (Diamond), a benchmark that measures the real-world production quality of AI-generated code, Fable 5 scored 29.3%, while GPT-5.5 lagged behind at 5.7%.

Fable 5 also took the lead in the coveted Humanity's Last Exam benchmark, which measures the limits of AI reasoning and the depth of scientific knowledge. Fable 5 came out on top with a score of 59% without tool use and 64.5% with tools, while GPT-5.5 and Gemini 3.1 Pro trailed with scores of 41.4% and 44.4%, respectively.

3) Fable 5 can compress months of coding into days

Anthropic says that during early testing with Stripe, Fable 5 was able to compress months of software engineering work into days. The AI model is said to have completed a 50-million-line Ruby codebase migration in a single day that would otherwise have required a team of engineers working for more than two months.

4) Rebuild apps from screenshots

Anthropic says Fable 5 is also excellent at tasks that require vision capabilities. This means the model can extract information from scientific charts, understand complex diagrams, and even recreate a web application's source code using screenshots alone.

5) Fable 5 vs Mythos 5

While Fable 5 and Mythos 5 are being released together, Anthropic is placing significant restrictions on many topics that Fable 5 cannot discuss. Some of these restrictions are being lifted for Mythos 5, particularly around cybersecurity, biological research, and advanced scientific reasoning, which the company will make available only to cybersecurity professionals and researchers.

6) Guardrails designed to prevent misuse

To prevent misuse, Fable 5 automatically routes conversations involving cybersecurity, biology, and AI model distillation to Claude Opus 4.8 instead.

Anthropic, however, claims that 95% of Fable sessions do not need to fall back on Opus 4.8 at all.

7) Anthropic's AI is helping with drug discovery

Anthropic claims in its blog that Mythos 5 is helping accelerate parts of the drug design process by around ten times. The company says its model is capable of completing stages of drug design that are usually performed by humans, such as choosing binding sites, selecting and running protein design tools, and recovering from failures.

How to use Fable 5?

Fable 5 is available through the Claude app, Claude website, and API.

To use the new AI model, you will need a Pro, Max, Team, or seat-based Enterprise plan.

However, Anthropic says Fable 5 will remain included in these subscription plans only until 22 June. From 23 June, users will have to begin paying for usage credits to access the model.