2026-06-30 21:23 UTCOriginal source2 min readUpdated: 2026-06-30 22:23 UTC

What's new in Claude Sonnet 5

Anthropic releases Claude Sonnet 5, with performance close to Opus 4.8 at lower prices. The model introduces a new tokenizer that increases token count by approximately 30% for the same input, effectively raising costs. API changes include removal of sampling parameters and adaptive thinking enabled by default.

SourceSimon Willison's Weblog

Simon Willison’s Weblog

30th June 2026 - Link Blog

What's new in Claude Sonnet 5 (via) Claude Sonnet 5 came out this morning. 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 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 tool to try out the new tokenizer. Here are my results for several larger documents:

Document Sonnet 4.6 Opus 4.7 Sonnet 5

Universal Declaration of Human Rights (English) 2,356 3,347 1.42x 3,341 1.42x

Universal Declaration of Human Rights (Spanish) 3,572 4,753 1.33x 4,747 1.33x

Universal Declaration of Human Rights (Chinese, Mandarin Simplified) 3,334 3,366 1.01x 3,360 1.01x

sqlite_utils/db.py (4,279 lines of Python) 44,014 56,118 1.28x 56,113 1.27x

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.