Claude Sonnet 5.5

About Claude Sonnet 5.5

Claude Sonnet 5.5 is Anthropic's Sonnet-class model for well-scoped everyday work. It succeeds Claude Sonnet 5 as a direct upgrade and is especially strong at building features and fixing bugs. It became available in September 2026.

The model accepts text, image, and file input and produces text. Its context window is 1,000,000 tokens and a response can be up to 128,000 tokens. It supports reasoning with an effort setting, tool calling, and structured outputs. With tool calling, the free-pi agent can read and edit files, run commands, and use free-pi's built-in web search and documentation lookup tools.

It scores 56 on the Artificial Analysis Intelligence Index, the highest score among the free-pi models that have one. On Design Arena it ranks first in game development, second in data visualization, third in 3D, UI components, and code categories overall, and fifth in website design. It also ranks fifth in the mobile apps agent category.

In free-pi it is free to use with no API key and no card, but it is a premium model. It counts as about 0.02x usage, which means it uses your daily allowance quickly.

When to pick it

For a task where quality matters most, such as a tricky bug, a feature that touches many files, or front-end work where the visual result matters, pick Claude Sonnet 5.5. Expect to use a large share of your daily allowance in a short session.

For routine work that should last, pick the default DeepSeek V4 Flash. For a lighter Anthropic model, pick Claude Haiku 5.5. Other premium options are GPT-6.1 Sol and Gemini 4 Argon.