On Claude Opus 5, disabling thinking is allowed only at effort high or below: thinking: {"type": "disabled"}…

Friday, 24 July 2026

AI summary

Written by AI from the official notes. Check them for exact details.

Claude Opus 5 restricts disabling thinking to high effort or below, introducing a breaking change from previous versions.

Why it matters: Users of Claude Opus 5 should be aware of the new restrictions on disabling thinking and plan accordingly.

On Claude Opus 5, disabling thinking is allowed only at effort high or below: thinking: {"type": "disabled"} with effort xhigh or max returns a 400 error, a breaking change from Claude Opus 4.8. See What's new in Claude Opus 5.
Effort is the primary control for steering Claude Opus 5: the model supports the full ladder (low, medium, high, xhigh, max), with max for capability-critical work.
Mid-conversation tool changes are now in beta on Claude Fable 5, Claude Mythos 5, Claude Opus 4.8, and Claude Opus 5: add or remove tools between turns of a conversation while preserving the prompt cache. Include the mid-conversation-tool-changes-2026-07-01 beta header in your requests.
The fallbacks parameter now supports a "default" mode, which applies Anthropic's recommended fallback models by refusal category. Server-side fallback is in beta, and the "default" mode requires the server-side-fallback-2026-07-01 beta header. See Refusals and fallback.
We've removed fast mode for Claude Opus 4.7. Requests to claude-opus-4-7 with speed: "fast" now return an error; unlike Claude Opus 4.6, they do not fall back to standard speed. Claude Opus 4.7 itself remains available at standard speed. To continue using fast mode, migrate to Claude Opus 5 or Claude Opus 4.8. Read more in Fast mode.

Full release notes5 changes

On Claude Opus 5, disabling thinking is allowed only at effort high or below: thinking: {"type": "disabled"} with effort xhigh or max returns a 400 error, a breaking change from Claude Opus 4.8. See What's new in Claude Opus 5.
Effort is the primary control for steering Claude Opus 5: the model supports the full ladder (low, medium, high, xhigh, max), with max for capability-critical work.
Mid-conversation tool changes are now in beta on Claude Fable 5, Claude Mythos 5, Claude Opus 4.8, and Claude Opus 5: add or remove tools between turns of a conversation while preserving the prompt cache. Include the mid-conversation-tool-changes-2026-07-01 beta header in your requests.
The fallbacks parameter now supports a "default" mode, which applies Anthropic's recommended fallback models by refusal category. Server-side fallback is in beta, and the "default" mode requires the server-side-fallback-2026-07-01 beta header. See Refusals and fallback.
We've removed fast mode for Claude Opus 4.7. Requests to claude-opus-4-7 with speed: "fast" now return an error; unlike Claude Opus 4.6, they do not fall back to standard speed. Claude Opus 4.7 itself remains available at standard speed. To continue using fast mode, migrate to Claude Opus 5 or Claude Opus 4.8. Read more in Fast mode.