Anthropic's Claude Fable 5 reaches 88% accuracy on FrontierMath's hardest problems, outpacing OpenAI's GPT-5.5 by a substantial margin.

What happened: Anthropic released Claude Fable 5, which scores 87 percent accuracy on FrontierMath tiers 1–3 and 88 percent on the hardest tier 4 (v2). OpenAI's GPT-5.5 reaches about 75 percent on tier 4. Both models were tested on Epoch AI's standard evaluation with maximum reasoning effort. The jump from the earlier Claude Opus 4.5—which scored below 10 percent on tier 4 in early 2026—shows rapid improvement in Anthropic's math reasoning capability.

Why it matters: FrontierMath is widely considered one of the toughest benchmarks for AI math reasoning. The gap between Fable 5 and GPT-5.5 signals a meaningful shift in the competitive landscape for advanced reasoning tasks. Real-world validation is starting to appear alongside benchmark gains: both an OpenAI model and Claude Mythos recently solved longstanding Erdős problems, suggesting these improvements translate beyond test scores.

What to watch: OpenAI already has GPT-5.6 in development, indicating the competition in frontier AI reasoning is not static. These models are being pushed toward solving previously unsolved mathematical problems, which may determine their practical value in research and engineering domains.