Anthropic launches Claude Fable 5.1 and Mythos 5.1, now available in GitHub Copilot

Sep 1, 2026 · Issue 5 · 4 sources, 4 articles

Anthropic has introduced the Claude Fable 5.1 and Mythos 5.1 models, which it promotes for coding and knowledge work. Fable 5.1 became generally available in GitHub Copilot on 1 September.

Who is affected and how

senior developer

For long-running, agent-based tasks, it is worth comparing the new model with the current one on your own codebase.

tech lead

It is available in the Copilot model picker, so the team's default model can be re-evaluated on cost and quality.

Sources

• Introducing Claude Fable 5.1 and Claude Mythos 5.1anthropic.com · Sep 1, 2026
• Anthropic launches Claude Fable 5.1 and Mythos 5.1, claims major leap in coding and AI researchwionews.com · Sep 2, 2026
• Claude Fable 5.1 is generally available in GitHub Copilot - GitHub Changeloggithub.blog · Sep 1, 2026
• Gemini 3.8 Flash is now available in GitHub Copilotthe github blog · Sep 3, 2026

Machine-generated summary based on the titles and excerpts of the source articles. The facts come from the linked article.