Models & Stacks
Claude Fable 5 Build Guide
What Claude Fable 5 is, the benchmarks behind it, and how to put the first Mythos-class model to work.
Published June 2026
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Claude Fable 5 is Anthropic's first publicly available Mythos-class model, released June 9, 2026, sitting above Opus in the capability hierarchy. It scores 80.3 percent on SWE-bench Pro and runs unattended. Mythos Preview launched in April through Project Glasswing, too capable at cybersecurity to release publicly. Fable 5 is the same frontier capability wrapped in classifiers that redirect high-risk topics to Opus 4.8.
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