Anthropic's Claude Sonnet 5.5 Release Date Might Not Be Far Away

Last week, a report hinted that Google’s flagship AI model, Gemini 4, might be closer than expected. But, before that, it seems Anthropic is getting ready to release the Claude Sonnet 5.5 model. The latest leaks suggest the wait could be almost over. A new checkpoint has reportedly reached Anthropic partners, with one leaker claiming it performs better than the earlier build.

Claude Sonnet 5.5 is already showing up in leaked details

According to leaker Lyra on X, partners received another Claude Sonnet 5.5 checkpoint on September 26, with a public release expected sometime this week (via Testing Catalog). The latest leak points to a model with a 1 million-token context window and up to 128K output tokens. It is also reportedly getting adaptive thinking, with multiple reasoning levels available.

There are also signs that Sonnet 5.5 is already being tested in third-party tools. A model entry for claude-sonnet-5-5 has reportedly appeared behind a feature flag in Factory Droid, while some users claim they have encountered the model during testing.

Claude Sonnet 5.5 could be a major upgrade

Sonnet has traditionally sat in the middle of Anthropic’s lineup, offering a balance between capability and cost. Sonnet 5.5 appears to be keeping that positioning while pushing the context window, reasoning and overall capabilities further.

Anthropic has confirmed that Sonnet 5.5 is coming after Opus 5.5, but there is still no official release date or confirmed final specifications. If the latest leak is accurate, we may not have to wait much longer to see what Claude Sonnet 5.5 can actually do.