Claude Sonnet 5.5 changes thinking and tool use in ways that break Sonnet 5 code

To turn off up-front thinking on Sonnet 5.5 you send between_tools instead of disabled. Forced tool use returns a 400 error. Thinking blocks only work in the account that produced them.

Who is affected:

Code moving from Claude Sonnet 5 to Sonnet 5.5.

Action needed:

Follow the Sonnet 5.5 migration guide before switching model IDs.