Anthropic Enhances Claude with Fable 5.1 and Mythos 5.1

Wednesday 02 September 2026 - 13:37

Anthropic continues its performance race with two new versions of Claude. Fable 5.1 targets paying users with enhanced capabilities in programming, research, and complex tasks, while Mythos 5.1 is intended for a select group of verified organizations.

Two Models, One Core Technology

Anthropic introduced its new models Claude Fable 5.1 and Claude Mythos 5.1 on September 1st, just three months after the previous versions.

Despite their distinct names, both models are based on the same core architecture. Their main difference lies in the level of security and the access conditions that govern their capabilities.

Fable 5.1 is designed for broader use, while Mythos 5.1 provides access to more advanced features in certain specialized areas and remains limited to selected organizations.

Claude Aims to Tackle Long-Lasting Tasks

With Fable 5.1, Anthropic primarily seeks to improve Claude's ability to handle long and complex missions.

The model is designed to chain multiple steps, use different tools, and continue its work over an extended period. It can be deployed on IT projects, conduct research, review numerous documents, or work with spreadsheets and presentations.

Another highlighted evolution is that when a step fails, the model is better equipped to continue the mission by adapting its approach rather than considering the entire task as completed.

Significant Progress in Programming

Programming is one of the areas where Anthropic claims the most significant gains.

In tests conducted by the company on Terminal-Bench-Science 0.1, which assesses an AI agent's ability to perform scientific research in a computing environment, Fable 5.1 achieved 52.6%, compared to 24.7% for Fable 5.

On Terminal-Bench 4.0, dedicated to autonomously executed programming tasks, the new version scored 55.8%, against 42% for its predecessor.

However, these figures should be interpreted with caution: they are results reported by Anthropic as part of its own evaluations.

Fable 5.1 Available to Subscribers

Unlike Mythos 5.1, Fable 5.1 is offered to general users with a paid Claude subscription.

This availability reflects Anthropic's intention to gradually transition the advanced capabilities of its models into common professional uses, particularly software development, document analysis, and tasks requiring multiple tools.

Nevertheless, the model remains subject to additional restrictions in certain areas considered sensitive.

Mythos 5.1 Takes a Controlled Access Approach

Mythos 5.1 adopts a different approach. Its additional capabilities in certain specialized areas justify, according to Anthropic, limited access to verified organizations.

This separation allows the company to offer more powerful technology in controlled environments while maintaining more safeguards for the public version.

With Fable 5.1 and Mythos 5.1, Anthropic aims to simultaneously meet the needs of professional users and the control requirements associated with the most sensitive applications.