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Last verified: 2026-08-06

Disabling thinking on Opus 5 returns HTTP 400

A Claude Cowork error and how to fix it. The exact message, why it happens, and the steps that resolve it.

Claude Cowork 
POST /v1/messages
{ "model": "claude-opus-5", "thinking": { "type": "disabled" }, "effort": "xhigh" }

→ HTTP 400 (invalid_request_error)
Fix:

On claude-opus-5, disabling thinking is only accepted at effort high or below. Drop to high/medium/low, or keep xhigh/max and remove the thinking block (Opus 5 thinks by default).

Changed from Opus 4.8, where this was legal — swapping the model string alone can break a working request · changelog

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