Four Opus 5.5 API changes make Opus 5 requests fail

Ask Opus 5.5 to skip thinking and it returns a 400 error. Anthropic’s migration guide lists four breaks like that, so the price cut to $4 in and $20 out per million tokens comes with homework.

• Thinking is always on, effort is the only dial, and its default drops from high to medium. • Forced tool calls through tool_choice fail, even on the token counting endpoint. • Editing earlier turns invalidates thinking blocks, a 400 error on accounts created on or after August 31, 2026. • Computer use needs the new toolset everywhere except Amazon Bedrock.

One trap throws no error: narration between tool calls arrives as thinking blocks, empty by default, so agent UIs go silent. And agents that don’t handle the new refusal categories will stop mid-task.