How do you troubleshoot an HTTP 400 error disabling thinking on Opus 5?

Check the effort level configured on the failing request first - this specific error occurs when thinking: {"type": "disabled"} is combined with effort xhigh or max, since Claude Opus 5 requires thinking to remain enabled at those two highest effort levels.

If the workload genuinely needs xhigh or max effort for its capability requirements, the fix is to remove the disabled-thinking configuration and let thinking run, using the effort level itself, rather than disabling thinking outright, to manage cost and depth.

If disabling thinking is the actual priority for this specific request, whether for latency, cost, or output-format requirements, the fix is instead to lower the effort level to high or below, which is the range where explicitly disabling thinking remains a supported configuration.

Because this is a hard, deterministic restriction rather than a probabilistic behavior, it's straightforward to catch in automated testing by explicitly exercising this exact combination as part of migration or regression tests, rather than only discovering it when it surfaces unexpectedly in production traffic.

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