whats-new-opus-5 changedmodels/opus-5/whats-new-opus-5
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316316 ```
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319Then review the two [behavior changes](https://platform.claude.com/docs/en/models/opus-5/whats-new-opus-5#behavior-changes): thinking is on by default, and disabling thinking with effort `xhigh` or `max` returns a 400 error. See the [migration guide](https://platform.claude.com/docs/en/about-claude/models/migration-guide#migrating-from-claude-opus-4-8-to-claude-opus-5) for step-by-step instructions.
319Then review the two [behavior changes](https://platform.claude.com/docs/en/models/opus-5/whats-new-opus-5#behavior-changes): thinking is on by default, and disabling thinking with effort `xhigh` or `max` returns a 400 error. See the [migration guide](https://platform.claude.com/docs/en/models/opus-5/migration-guide#migrating-from-claude-opus-4-8-to-claude-opus-5) for step-by-step instructions.
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321321## Next steps
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