The bottom line: Sonnet 5 offers Opus 4.8-like performance at theoretically the same price as Sonnet 4.6, but a new tokenizer increases practical operating costs by approximately 30 percent.
Anthropic has released Claude Sonnet 5 and positions the model as a cost-effective alternative to Opus 4.8. However, a revised tokenization leads to an effective price increase of approximately 30 percent compared to Sonnet 4.6.
Claude Sonnet 5 is now available. Anthropic is marketing the model with performance comparable to Opus 4.8, at lower costs. The company’s System Card indicates that Sonnet 5 is significantly less capable on cyber tasks than Myth 5 and therefore operates with the safety measures of Opus 4.7 and Opus 4.8 — an approach that circumvents approval barriers from the U.S. government.
The API changes are substantial: temperature, top_p, and top_k sampling parameters are no longer supported. The model features a context window of 1 million tokens and can output a maximum of 128,000 tokens. Adaptive Thinking is enabled by default and can only be disabled by specifying “thinking”: {type: “disabled”}. In terms of features, it matches Sonnet 4.6.
The official pricing structure remains identical to Sonnet 4.6: $3 per million input tokens and $15 per million output tokens, with an introductory discount to $2 and $10 respectively until August 31. However, practical cost calculation is altered by the new tokenization. The new tokenizer generates approximately 30 percent more tokens for the same input text as Sonnet 4.6.
Concrete measurements confirm this shift: for the Universal Declaration of Human Rights in English, Sonnet 5 causes approximately 42 percent more token consumption than Sonnet 4.6 (3,341 versus 2,356 tokens). For Spanish, the increase is 33 percent (4,747 versus 3,572 tokens), for Python code 27 percent (56,113 versus 44,014 tokens). Simplified Chinese shows no significant change. The effective price increase varies depending on the language domain and thus falls well short of the advertised price parity.
Source: simonwillison.net · Published June 30, 2026
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