Claude Haiku 5.5 rejects budget thinking, sampling parameters and prefill

Manual extended thinking, temperature, top_p, top_k and assistant prefill can each return a 400 error on Haiku 5.5, and adaptive thinking is on by default.

Who is affected:

Apps calling claude-haiku-4-5 that plan to switch to Haiku 5.5.

Action needed:

Follow the Haiku 5.5 migration guide before switching model IDs.