feature
Claude Code 2.1.311: ultrathink now runs at xhigh effort by default on Opus 5.5, Sonnet 5.5, Haiku 5.5, and Fable 5.1
code.claude.comagent
The ultrathink keyword now automatically raises the effort level to xhigh on recent Claude models when running through Anthropic's API. This gives users a more capable reasoning mode without manual configuration. The ultrathink rainbow indicator now appears only where the keyword can raise effort, making the UI clearer.
Stacks this affects
Claude Code ships in 4 tested stacks here.
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