Sonnet 5.5 Free vs GPT-5.6 Luna

Anthropic has released the Sonnet 5.5 model, now available in the free version of the Claude service. This move directly challenges OpenAI, which still uses GPT-5.6 Luna for its free tier.

Capabilities Comparison

GPT-5.6 Luna is positioned as a lightweight and cost-effective solution for high-load tasks. In contrast, Sonnet 5.5 demonstrates significant superiority in complex coding tasks and interactive application development.

The Artificial Analysis Intelligence Index highlights the performance gap:

• Claude Sonnet 5.5 Max — 56 points
• GPT-5.6 Luna Max — 37 points

The score difference indicates that these models belong to different performance classes. Meanwhile, ChatGPT Plus subscribers continue using the Sol 5.6 version without access to newer updates.