Why moderate: near-exact agreement (49 of 49 strings) with anthropic/claude-opus-4.8, short of a full exact signature.
Anthropic: Claude Sonnet 5 is Anthropic's model. Its measured tokenizer sits with the Claude group, which is evidence that it builds on the Claude tokenizer. Tokenizer reuse is normal engineering, open vocabularies travel between labs, and this says nothing against Anthropic's authorship of the model itself.
| We checked | What we found | Where it came from |
|---|---|---|
| How it counts tokens | Counted, using the 49 strings that came back reliably | we measured it |
| How its API is set up | Set up identically to 1 other listing, field for field | api_5d3cdea0 |
| How much it can read and write | Reads up to 1,000,000 tokens at once, writes up to 128,000 | its own listing |
| Whether it thinks before answering | Reasons when asked to, at max, xhigh, high, medium, low, high by default | its own listing |
| Who served our requests | Claude Platform on AWS | we measured it, 5 October 2026 |
| Measured twice | The 5 October 2026 reading agreed with the 3 September 2026 reading on every one of the 49 strings clean in both: the tokenizer behind this name held. | 49/49 agree |
Click a row to open the full comparison.
We sent Anthropic: Claude Sonnet 5 fifty short pieces of text and recorded what each one cost it in tokens. The bars below are those costs. Two models built on the same tokenizer produce the same bars; a model built on a different one produces a different set, which is what makes this a fingerprint.
An x marks a row excluded as corrupt (caching or provider interference detected during measurement). Overhead subtracted: 6 prompt tokens.
Sonnet 5 is Anthropic's most capable Sonnet-class model, with frontier performance across coding, agents, and professional work. It supports adaptive thinking with selectable reasoning effort levels (low, medium, high, max,...
Supported parameters
include_reasoningmax_completion_tokensmax_tokensreasoningreasoning_effortresponse_formatstopstructured_outputstool_choicetoolsverbosity
defaults: {}
