Sonnet 5.5 returns a 400 for `thinking: {"type": "disabled"}` and for `tool_choice` of type `any` or `tool`, both of which Sonnet 5 accepts.
The replacement for "thinking off" is `thinking: {"type": "between_tools"}`, which accepts only `low`, `medium` and `high` effort and takes no other thinking field.
Sonnet 5 stays available, with retirement not sooner than June 30, 2027, so you can migrate on your own schedule.