GPT-5.6 Luna vs GPT-5.5 Pro
Head-to-head technical comparison of intelligence, capabilities, and API pricing.
Which should you use?
GPT-5.6 Luna supersedes GPT-5.5 Pro in OpenAI's line-up, so this is an upgrade question rather than a bake-off.
GPT-5.6 Luna is also cheaper — $0.2 against $30 per 1M input tokens — so staying on GPT-5.5 Pro costs more for less.
Their Arena Elo differs by only 13 points, inside the confidence intervals the public board reports, so human preference does not separate them.
Cost is the sharper difference: GPT-5.5 Pro is 150x the input price of GPT-5.6 Luna ($30 against $0.2 per 1M), so at volume the choice is usually decided by budget rather than benchmarks.
Pick GPT-5.6 Luna when
- Cheaper input tokens — $0.2 against $30 per 1M, so GPT-5.5 Pro costs 150x as much to feed.
- Better value per point of measured capability: $0.002 per index point against $0.318.
Pick GPT-5.5 Pro when
- Higher composite index — 94.2 against 92, a gap of 2.2 points.
- Stronger on MMLU-Pro: 94.2% against 92%.
Value per point of measured capability, at list input pricing: GPT-5.6 Luna $0.002 · GPT-5.5 Pro $0.318 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 2026-09-14. 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.
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