Claude Opus 5.5 / Sonnet 5.5: 5 settings that return 400 when you only change the model name

Five settings on Claude Opus 5.5 and Sonnet 5.5 will return an HTTP 400 error when only the model name is changed:

1. Omitting the "thinking" parameter and setting the output effort results in 400. Sonnet 5.5 uses the "between_tools" setting with a higher effort level to turn thinking off. Opus 5.5 rejects both "disabled" and "enabled" settings for thinking.

2. Attempting to force a tool choice using "tool_choice": "any" or "tool" is rejected on both models, including on the token-counting endpoint. Use "tool_choice": "auto" with a strict tool and specify when to call the tool.

3. Using the "computer_20251124" toolset is rejected by both models. Replace it with "computer_toolset_20260801". Sonnet 5.5 also rejects "computer_20250124".

4. Editing conversation history, such as changing system messages, tools, or earlier turns, and sending thinking blocks back results in a 400 error. Keep history append-only and use system messages for changes. Update tools using tool_addition or tool_removal.

5. Sonnet 5.5 rejects Opus 4.x and Sonnet 5 as advisors, returning 400. The advisor must be one of the specified models. Opus 5.5 and Sonnet 5.5 have additional refusal categories, and server-side fallback only retries specific categories.