What is the difference between Claude Sonnet 5 and Claude Sonnet 4.6?

Claude Sonnet 5 is a drop-in upgrade for Sonnet 4.6, with three specific breaking API changes: adaptive thinking now runs by default on unconfigured requests, manual extended thinking (a fixed budget_tokens configuration) is rejected with an HTTP 400 error, and non-default sampling parameters (temperature, top_p, top_k) also return HTTP 400.

It also ships with a new tokenizer that produces roughly 30% more tokens for the same input text, an added xhigh effort level between high and max, and real-time cyber safeguards designed to detect and block dangerous cyber usage.

Feature-wise, everything else carries over unchanged - the 1M token context window, 128k max output, prompt caching, batch processing, the Files API, PDF support, vision, and the full tool set - except Priority Tier, which Sonnet 5 doesn't support.

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