FrontierThe story, in brief
Claude Fable 5 outpaces GPT-5.5 by 13 points on FrontierMath's toughest problems
13 points. That's how much Claude Fable 5 now leads GPT-5.5 on the hardest math problems AI can solve.
The KeyNews take
Why it matters
Anthropic has dramatically closed the reasoning gap with OpenAI. Claude Fable 5's jump from <10% to 88% accuracy on frontier math signals a fundamental shift in who owns AI's reasoning tier.
The key facts
6 to knowClaude Fable 5: 88% accuracy on FrontierMath hardest tier
Claude Opus 4.5: <10% accuracy on same tier (early 2026)
OpenAI GPT-5.5: 75% accuracy on FrontierMath hardest tier
13-point performance gap favoring Anthropic
Benchmark: FrontierMath toughest problems
Capability focus: Mathematical reasoning at frontier level
Go to the source
The Decoderthe-decoder.com
Publisher excerpt: Anthropic's Claude Fable 5 hits 88 percent accuracy on the hardest FrontierMath tier, a massive jump from Opus 4.5, which sat below 10 percent in early 2026. OpenAI's GPT-5.5 reaches about 75 percent on the same tier. The pace of improvement in AI math keeps accelerating.