How does Opus 5 handle disabling thinking at xhigh or max effort?

Claude Opus 5's API rejects this specific combination outright: setting thinking: {"type": "disabled"} together with effort xhigh or max returns an HTTP 400 error rather than silently ignoring the disable request or falling back to some default behavior.

This is a deliberate restriction - Anthropic requires thinking to stay enabled at those two highest effort levels, so the only ways to actually disable thinking are to also lower the effort level to high or below, or to leave thinking enabled and rely on the effort setting itself to control cost instead.

For teams migrating from Opus 4.8, where no such restriction existed, this specific combination is one of the concrete, testable request patterns worth checking explicitly during migration, since it's a clear pass/fail case rather than a subtler behavioral shift.

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