Claude Fable 5 Beats GPT-5.5 on Hardest Math Tier
Key insights
- Claude Fable 5 scored 88% on FrontierMath tier 4, outpacing GPT-5.5's roughly 75% by 13 points under controlled Epoch AI evaluation conditions.
- Opus 4.5 scored below 10% on the same tier earlier in 2026, making Fable 5's 88% a near-complete capability reversal within a single year.
- Real-world corroboration is emerging alongside benchmarks, with AI models including Claude Mythos tackling previously unsolved Erdős mathematical problems.
Why this matters
A 13-point lead on FrontierMath tier 4 matters because this benchmark is designed to be saturation-resistant, so large gaps between frontier models signal genuine capability differences rather than evaluation overfitting. The jump from Opus 4.5's sub-10% to Fable 5's 88% on the hardest tier within a single year compresses what historically required a generation of research progress. For practitioners building autonomous research tools, quantitative modeling pipelines, or scientific agents, the data suggests Anthropic's current flagship is operating at a qualitatively different ceiling than its nearest competitor on math-intensive tasks.
Summary
Anthropic's Claude Fable 5 has reached 88% accuracy on FrontierMath's hardest tier 4, outpacing OpenAI's GPT-5.5 by 13 percentage points on a benchmark widely considered one of the toughest AI math evaluations available.
The gap becomes starker against Anthropic's own recent history: Opus 4.5 scored below 10% on tier 4 earlier in 2026. All models were tested on Epoch AI's standard scaffold with maximum reasoning effort, making the comparison controlled and direct.
Essentially: (Anthropic, OpenAI) are now separated by double digits on the benchmark that matters most for frontier math.
- Fable 5 hit 87% on tiers 1-3 and 88% on the hardest tier 4 (v2).
- GPT-5.5 reached approximately 75% on tier 4, placing it well behind Fable 5.
- Real-world evidence is accumulating alongside benchmarks: AI models including Claude Mythos have tackled longstanding Erdős problems.
A 13-point lead on the benchmark most resistant to saturation is now large enough to influence which models researchers and engineers reach for first.
Potential risks and opportunities
Risks
- OpenAI faces developer preference erosion in math-heavy segments if GPT-5.5's 13-point deficit on FrontierMath tier 4 becomes a standard reference point in model selection conversations over the next quarter.
- Anthropic risks credibility damage if FrontierMath tier 4 (v2) is later found to be materially easier than the original tier 4, undermining the dramatic framing of the jump from Opus 4.5's sub-10%.
- Epoch AI's role as sole evaluator creates a single point of methodological trust: if their scaffold or scoring is challenged, results across all models in this comparison lose their comparative validity simultaneously.
Opportunities
- Anthropic can accelerate enterprise positioning for Fable 5 in quantitative finance, pharmaceutical research, and scientific computing where mathematical reasoning is a hard procurement requirement.
- Epoch AI gains leverage as the de facto third-party evaluator for frontier math capability, positioning the organization for paid evaluation partnerships with other leading labs seeking credible external validation.
- AI-native math tooling companies building automated theorem proving, computational chemistry, or quantitative research platforms can now unlock applications that were inaccessible on predecessor models like Opus 4.5.
What we don't know yet
- FrontierMath tier 4 (v2) methodology: whether the v2 revision made the tier materially easier than the original, which would affect how the comparison to Opus 4.5's sub-10% score is interpreted.
- Epoch AI's standard scaffold is not publicly documented in the article, leaving open whether third parties can independently replicate these evaluation results outside Epoch AI's environment.
- Scores for other frontier models referenced in the article are not detailed, leaving the full competitive landscape across all current models incomplete.
Originally reported by the-decoder.com
Read the original article →Original headline: Epoch AI Data: Claude Fable 5 Scores 88% on FrontierMath Tier 4, Outpacing GPT-5.5 by 13 Points on AI's Hardest Math Benchmark