Claude Opus 5.5 cannot turn thinking off

On Opus 5.5, thinking disabled and manual thinking both return a 400 error, forced tool_choice returns a 400 error, and the older computer use tool is rejected on the Claude API and Google Cloud.

Who is affected:

Code moving from Claude Opus 5 to Opus 5.5.

Action needed:

Omit the thinking field, control depth with effort, and use tool_choice auto.