Sonnet 5.5 ships, and Simon Willison says it now powers Claude's free tier

Anthropic released Claude Sonnet 5.5 on Sept 28, 2026; Simon Willison says it is now the model behind Claude's free tier.

Anthropic released Claude Sonnet 5.5 on September 28, 2026. According to Simon Willison, the bigger story for most people is that it now powers the free tier on claude.ai, "so all of this stuff can be done by free users." If that holds, builders who prototype on free accounts get a much stronger model without paying. Anthropic has not confirmed the free-tier change in the sources we found, and free-tier rate limits are unknown.

On the API, the official model page lists a 1M-token context window, 128K max output and $2/$10 per million tokens. It also lists breaking changes for Sonnet 5 code: forced tool use returns an error, and non-default temperature, top_p or top_k values return a 400 error. CNBC reports it is available on AWS, Google Cloud and Microsoft Azure.

Anthropic says Sonnet 5.5 is more than 30% faster and up to 30% cheaper per task than Sonnet 5. That claim has not been independently reproduced. Kingy AI cites Artificial Analysis data showing $7.60 per task at max effort, above Opus 5.5's $5.98 and GPT-6 Sol's $1.06. So check your effort setting before you assume the model will save you money.

Willison says ChatGPT's free tier is still on GPT-5.6 Luna, which he calls "a lot less capable." 9to5Google reports that GPT-6 Luna reached Free users only in the desktop app. It is unclear which model free ChatGPT chat currently uses.

Sources