Claude Fable 5.1 vs Claude Opus 4.8
Head-to-head technical comparison of intelligence, capabilities, and API pricing.
Which should you use?
Claude Fable 5.1 supersedes Claude Opus 4.8 in Anthropic's line-up, so this is an upgrade question rather than a bake-off.
Claude Fable 5.1 costs more ($10 against $5 per 1M input), so the upgrade is worth it only where the extra capability changes your output.
Cost is the sharper difference: Claude Fable 5.1 is 2x the input price of Claude Opus 4.8 ($10 against $5 per 1M), so at volume the choice is usually decided by budget rather than benchmarks.
Pick Claude Fable 5.1 when
- Higher composite index — 96.9 against 94.6, a gap of 2.3 points.
- Ahead on Arena Elo by 41 points (1504 vs 1463), which is outside the board's published confidence intervals.
- Stronger on MMLU-Pro: 95.9% against 94.5%.
Pick Claude Opus 4.8 when
- Cheaper input tokens — $5 against $10 per 1M, so Claude Fable 5.1 costs 2x as much to feed.
- Better value per point of measured capability: $0.053 per index point against $0.103.
Value per point of measured capability, at list input pricing: Claude Opus 4.8 $0.053 · Claude Fable 5.1 $0.103 per index point. Run your own token mix through the token cost calculator — a comparison at list price ignores caching and batch discounts, which move real bills more than this gap does.
Where these numbers come from
Figures as of 2026-09-07. Arena Elo and pricing are read from the public board and each vendor's own pricing page; see the full leaderboard for all 34 models and which columns are measured rather than estimated, or the benchmark matrix for scores benchmark by benchmark.