Opus 5 rejects disabled thinking at xhigh and max effort

Opus 5 now rejects any request that pairs thinking: {"type": "disabled"} with effort xhigh or max, returning a 400 error, per Anthropic's Opus 5 docs. That's a break from Opus 4.8, where thinking mode and effort level were independent settings, and it will trip up any integration that hardcoded both.

The bigger issue sits below that ceiling. With thinking off, Opus 5 sometimes writes a tool call into user-facing text instead of a real tool_use block, per the docs.

The call looks like it ran. It didn't. In an agentic loop, that phantom call becomes part of the context every later turn reads.

Anthropic's fix is blunt: stop disabling thinking. Thinking enabled at low effort beats thinking disabled at similar cost, per the docs.

If you still need it off, Anthropic ships one combined mitigation instruction to paste in. It says that beats the DIY fix teams reach for, naming <thinking> tags in the prompt.

The pattern matters more than either fix. Anthropic's Opus 5 docs also tell builders to drop their own fallback model lists for a new default mode instead. Two workarounds swapped out in one doc update, not tuned, is a pattern worth watching before you port an Opus 4.8 config forward.