GPT-5.5 Pro vs Claude Sonnet 5

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

Claude Sonnet 5

Anthropic

AI Mastery Index94

LMSYS Arena1356

Technical (MMLU-Pro)94%

Cost per 1M Tokens$2

Architect's Verdict

When comparing GPT-5.5 Pro and Claude Sonnet 5, GPT-5.5 Pro edges out in overall capabilities according to the AI Mastery Index. However, for cost-sensitive workloads, Claude Sonnet 5 offers a more economical API at $2 per 1M tokens. Your choice should depend heavily on whether you prioritize raw reasoning (MMLU) or scalable deployment economics.