Anthropic just crossed a threshold it had been carefully guarding since April. Claude Fable 5 -- the first Mythos-class model available to the general public -- is now live, and the Zed editor is one of the first places you can reach for it, available in both Zed v1.5.5 (stable) and v1.6.2 (preview).

This is not a routine model refresh. Mythos was initially limited to a handful of partners due to cybersecurity concerns when it launched in April. What changed is that Anthropic spent the intervening months building classifiers strong enough to make a public release defensible. The result is Fable 5: same underlying model, with hard guardrails on the most dangerous capabilities.

A tier above Opus

Fable 5 is a Mythos-class model made safe for general use, and its capabilities exceed those of any model Anthropic has ever made generally available. It is state-of-the-art on nearly all tested benchmarks, showing exceptional performance in software engineering, knowledge work, vision, and scientific research.

The longer and more complex the task, the larger Fable 5's lead over other models. That framing is backed by numbers:

• Fable 5 reaches 95.0% on SWE-bench Verified and 80.0% on SWE-bench Pro, compared to Opus 4.8 at 88.6% and 69.2% respectively.
• On SWE-Bench Pro, Fable 5 scored 80.3%, ahead of GPT-5.5 at 58.6% and Gemini 3.1 Pro at 54.2%. On FrontierCode Diamond, it scored 29.3% compared to 13.4% for Opus 4.8 and just 5.7% for GPT-5.5.
• During early testing, Stripe reported that Fable 5 compressed months of engineering into days -- performing a codebase-wide migration in a 50-million-line Ruby codebase in a single day, a task that would otherwise have taken a whole team over two months.