What's new in Claude Sonnet 5

# What’s new in Claude Sonnet 5 Source: [https://simonwillison.net/2026/Jun/30/claude-sonnet-5/](https://simonwillison.net/2026/Jun/30/claude-sonnet-5/) 30th June 2026 \- Link Blog **[What's new in Claude Sonnet 5](https://platform.claude.com/docs/en/about-claude/models/whats-new-sonnet-5)**\([via](https://news.ycombinator.com/item?id=48736605)\) Claude Sonnet 5 came out[this morning](https://www.anthropic.com/news/claude-sonnet-5)\. I always head straight for the "what's new" developer docs because they tend to have more actionable information than the official announcement post\. Anthropic say of Sonnet 5 that "its performance is close to that of Opus 4\.8, but at lower prices"\. The[system card](https://www-cdn.anthropic.com/9e6a1044980d8c4ed85669faf9c2a8342e2e9f1e/Claude%20Sonnet%205%20System%20Card.pdf)helps explain how they were able to release the model without being blocked by the US government: > Sonnet 5 is significantly less capable at cyber tasks than Mythos 5: its safeguards are thus similar to those we apply to Opus 4\.7 and Opus 4\.8 \(models that are more capable than Sonnet 5 but much less capable than Mythos 5\)\. Of note from the "what's new" API changes: - Sampling parameters`temperature`,`top\_p`,`top\_k`are no longer supported\. - It has a 1 million token context window and 128,000 maximum output tokens\. - It features "the same set of tools and platform features as Claude Sonnet 4\.6" - Adaptive thinking is on by default, unless you specify`"thinking": \{type: "disabled"\}`\. - The pricing is the same as Sonnet 4\.6: $3/million input, $15/million input, with an introductory discount to $2/$10 until 31st August\. But\.\.\. - The model has a new tokenizer, where "The same input text produces approximately 30% more tokens than on Claude Sonnet 4\.6\." \- effectively a 30% price increase\. I used my[Claude Token Counter](https://tools.simonwillison.net/claude-token-counter)tool to try out the new tokenizer\. Here are my results for several larger documents: DocumentSonnet 4\.6Opus 4\.7Sonnet 5[Universal Declaration of Human Rights \(English\)](https://github.com/simonw/udhr-markdown/blob/main/declarations/eng.md)**2,356****3,347** 1\.42x**3,341** 1\.42x[Universal Declaration of Human Rights \(Spanish\)](https://github.com/simonw/udhr-markdown/blob/main/declarations/spa.md)**3,572****4,753** 1\.33x**4,747** 1\.33x[Universal Declaration of Human Rights \(Chinese, Mandarin Simplified\)](https://github.com/simonw/udhr-markdown/blob/main/declarations/cmn_hans.md)**3,334****3,366** 1\.01x**3,360** 1\.01x[sqlite\_utils/db\.py](https://github.com/simonw/sqlite-utils/blob/79117b9d110d72f46dab5fe2cda412ff4789ab55/sqlite_utils/db.py)\(4,279 lines of Python\)**44,014****56,118** 1\.28x**56,113** 1\.27xSo the new token is roughly 1\.4x times more expensive for English, 1\.33x for Spanish, 1\.28x for Python code and effectively the same cost for Simplified Mandarin\. Here's[the pelican](https://gist.github.com/simonw/a89e756b621a31e8ffc210e3428efa77)\. It's nothing to write home about\. Sonnet 5 thinks it looks like a goose\.