Anthropic just dropped the biggest flex in the AI arms race. Claude Fable 5, the company’s first publicly available “Mythos-class” model, scored 161 on the Epoch Capabilities Index, edging past OpenAI’s GPT-5.5 Pro, which landed at 159.
The numbers that matter
On FrontierMath tier 4, widely considered one of the hardest mathematical reasoning evaluations in AI, Fable 5 scored 88 percent. GPT-5.5 Pro managed roughly 75 percent. That’s a 13-point gap on a test designed to push frontier models to their limits.
Fable 5 posted an 80.3 percent on SWE-Bench Pro, a benchmark that tests AI models on real-world software engineering tasks pulled from actual GitHub repositories. Anthropic’s own previous flagship, Opus 4.8, scored 69.2 percent on the same benchmark. OpenAI’s GPT-5.5 trailed further at 58.6 percent.
In English: Fable 5 can solve roughly four out of five real coding problems thrown at it, compared to three out of five for its predecessor and barely more than half for GPT-5.5. For companies evaluating which AI to plug into their development workflows, that difference translates directly into productivity.
