Claude Opus 5 vs Gemini 3.8 Flash
Head-to-head technical comparison of intelligence, capabilities, and API pricing.
Which should you use?
Claude Opus 5 leads on the composite index by 3.1 points (96.6 vs 93.5).
Their Arena Elo differs by only 1 points, inside the confidence intervals the public board reports, so human preference does not separate them.
Cost is the sharper difference: Claude Opus 5 is 6.7x the input price of Gemini 3.8 Flash ($5 against $0.75 per 1M), so at volume the choice is usually decided by budget rather than benchmarks.
Pick Claude Opus 5 when
- Higher composite index — 96.6 against 93.5, a gap of 3.1 points.
- Stronger on MMLU-Pro: 95.7% against 93.2%.
Pick Gemini 3.8 Flash when
- Cheaper input tokens — $0.75 against $5 per 1M, so Claude Opus 5 costs 6.7x as much to feed.
- Better value per point of measured capability: $0.008 per index point against $0.052.
Value per point of measured capability, at list input pricing: Gemini 3.8 Flash $0.008 · Claude Opus 5 $0.052 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.