What is the difference between disabling thinking on Sonnet 5 and on Opus 5?

On Claude Opus 5, thinking: {"type": "disabled"} is only accepted at effort high or below - attempting to combine it with xhigh or max effort returns an HTTP 400 error, since Opus 5 requires thinking to stay enabled at its two highest effort levels.

On Claude Sonnet 5, by contrast, thinking: {"type": "disabled"} is accepted regardless of the configured effort level, without that same restriction tying disabling thinking to a specific effort ceiling.

This is a concrete API-level difference worth checking specifically when porting request-building logic between the two models, since code that correctly disables thinking at any effort on Sonnet 5 could hit an unexpected 400 error if the same pattern is reused unchanged against Opus 5 at xhigh or max.

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