What's New in Claude Sonnet 5: The Breaking API Changes Builders Must Handle
Simon Willison / Anthropic Platform Docs·high signal
Claude Sonnet 5 shipped June 30 as a drop-in for Sonnet 4.6, but migration is not free: a new tokenizer emits ~30% more tokens for the same text (recount prompts and max_tokens budgets), adaptive thinking is on by default, and both manual extended thinking and non-default sampling params (temperature/top_p/top_k) now return HTTP 400. It ships a 1M-token context window, 128k max output, $2/$10 intro pricing through Aug 31, no Priority Tier, and is the first Sonnet-tier model with real-time cybersecurity safeguards (refusals arrive as HTTP 200 with stop_reason of 'refusal'). Simon Willison's writeup plus Anthropic's platform docs are the primary sources.