How to turn off the ultrathink keyword in Claude Code with an environment variable

Claude Code 2.1.316 (October 31) adds the CLAUDE_CODE_DISABLE_ULTRATHINK environment variable, which turns the ultrathink keyword off. Since 2.1.311, a prompt containing ultrathink runs that turn at xhigh effort, so the word showing up by accident in code or logs can send an unintended turn into deep thinking and spend extra time and tokens. Set the variable to prevent that.

export CLAUDE_CODE_DISABLE_ULTRATHINK=1
claude

The changelog gives only the variable name and "turns the keyword off", with no value format. The 1 above is an example that follows the convention of other CLAUDE_CODE_DISABLE_* variables. The environment variables reference doesn't list it yet. If you only want to raise effort for hard questions, leave it unset and keep using ultrathink.