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[ARTICLE · art-45602] src=simonwillison.net ↗ pub= topic=large-language-models verified=true sentiment=· neutral

What's new in Claude Sonnet 5

Anthropic released Claude Sonnet 5, a new AI model with performance close to Opus 4.8 at lower prices, and updated its API by removing temperature, top_p, and top_k parameters. The model's tokenizer is roughly 1.4x more expensive for English text compared to previous versions.

read1 min views1 publishedJun 30, 2026

Anthropic say of Sonnet 5 that "its performance is close to that of Opus 4.8, but at lower prices". The system card 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:

temperature

, top_p

, top_k

are no longer supported."thinking": {type: "disabled"} .I used my Claude Token Counter tool to try out the new tokenizer. Here are my results for several larger documents:

Document Sonnet 4.6 Opus 4.7 Sonnet 5

So 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. It's nothing to write home about. Sonnet 5 thinks it looks like a goose.

Via [Hacker News](https://news.ycombinator.com/item?id=48736605)

Tags: [ai](https://simonwillison.net/tags/ai), [generative-ai](https://simonwillison.net/tags/generative-ai), [llms](https://simonwillison.net/tags/llms), [anthropic](https://simonwillison.net/tags/anthropic), [claude](https://simonwillison.net/tags/claude), [llm-pricing](https://simonwillison.net/tags/llm-pricing), [pelican-riding-a-bicycle](https://simonwillison.net/tags/pelican-riding-a-bicycle), [llm-release](https://simonwillison.net/tags/llm-release)
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