Claude Code's ultrathink now runs the turn at xhigh effort

Version 2.1.311 of Claude Code changes what the ultrathink keyword does. In the interactive terminal, a prompt containing it now runs that turn at xhigh effort. This applies on Opus 5.5, Sonnet 5.5, Haiku 5.5, and Fable 5.1 through Anthropic's API.

The release notes add one detail about timing. If you type ultrathink while Claude is working, it raises the rest of that turn.

The highlight now means something

The rainbow highlight on ultrathink now appears only where the keyword can raise effort. It no longer shows when the word is quoted or is part of a file path. If the word is not highlighted, it is not doing anything.

ultrathink: why does the retry loop in src/queue.ts drop the last job?

What the notes do not say

The changelog entry names the interactive terminal, the four models, and Anthropic's API. It says nothing about Bedrock, Vertex, Foundry, or non-interactive runs, so check the highlight before relying on the keyword there.

See the Claude Code sheet for the rest of the surface.

Source: Claude Code changelog, 2.1.311.