Claude Opus 5.5 breaking changes: thinking can't be disabled, forced tool_choice removed

UpdateSep 22, 2026VerifiedAdded Sep 22, 2026

On Claude Opus 5.5, disabling or manually enabling thinking returns a 400; control depth with the effort parameter instead. tool_choice any and tool also return 400, so use auto with strict tool use, and computer use requires the computer_toolset_20260801 toolset.

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Read Anthropic's release notes

https://platform.claude.com/docs/en/release-notes/overview#september-22-2026

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