GPT-5.5 Pro vs Gemini 3.1 Ultra

Head-to-head technical comparison of intelligence, capabilities, and API pricing.

GPT-5.5 Pro

OpenAI

AI Mastery Index94.2

LMSYS Arena1350

Technical (MMLU-Pro)94.2%

Cost per 1M Tokens$30

Gemini 3.1 Ultra

Google

AI Mastery Index92.5

LMSYS Arena1320

Technical (MMLU-Pro)92.5%

Cost per 1M Tokens$20

Architect's Verdict

When comparing GPT-5.5 Pro and Gemini 3.1 Ultra, GPT-5.5 Pro edges out in overall capabilities according to the AI Mastery Index. However, for cost-sensitive workloads, Gemini 3.1 Ultra offers a more economical API at $20 per 1M tokens. Your choice should depend heavily on whether you prioritize raw reasoning (MMLU) or scalable deployment economics.