How to run a single Claude Code turn at xhigh effort with ultrathink

Starting with Claude Code 2.1.311 (October 26), putting ultrathink in a prompt in the interactive terminal actually runs that turn at xhigh effort. Your session setting stays as it is, so use it when you want Claude to think harder about one difficult question. Before this, the keyword only added an in-context instruction and the effort level sent to the API was unchanged.

1. Check your version and update if needed.

claude --version
claude update

1. Put ultrathink anywhere in your prompt.

ultrathink figure out why this race condition only reproduces sometimes

The keyword turns rainbow only where it can raise effort. An ultrathink inside quotes or as part of a file path gets no rainbow and doesn't raise effort.

1. You can also type ultrathink and send it while Claude is working. The rest of that turn then runs at xhigh.

The changelog lists the conditions as Opus 5.5, Sonnet 5.5, Haiku 5.5 and Fable 5.1 through Anthropic's API. It doesn't mention other providers such as Bedrock or Vertex, or claude -p runs. The model configuration docs still carry the older description (effort unchanged, an instruction added). Other phrases such as "think hard" still aren't keywords and pass through as ordinary prompt text. To raise effort for the whole session, use /effort xhigh.