Claude Sonnet 5 changelog

Claude Sonnet 5 becomes the default model in Claude Code

Claude Code 2.1.197 made Claude Sonnet 5 the default model, with a native 1M token context window.

Who is affected:

Claude Code users who had not picked a model.

What to do:

Update to Claude Code 2.1.197 or later. Use /model to pick a different default.

Claude Sonnet 5 removes manual thinking budgets and sampling parameters

On Sonnet 5, manual extended thinking and non-default temperature, top_p or top_k return a 400 error, and a new tokenizer produces about 30% more tokens for the same text.

Who is affected:

Code moving from Claude Sonnet 4.6 to Sonnet 5.

Action needed:

Remove budget_tokens and sampling parameters, and re-measure token costs.