CLAUDE_CODE_DISABLE_ULTRATHINK

Updated October 31, 2026 by Alex Sorokoletov

CLAUDE_CODE_DISABLE_ULTRATHINK is a Claude Code environment variable: Turns the ultrathink keyword off, so the word has no effect in a prompt. Added in v2.1.315 and still present in v2.1.315. It is not in the official reference.

How to set it

For one session, export it in the shell that starts Claude Code:

export CLAUDE_CODE_DISABLE_ULTRATHINK=<value>
claude

To apply it to every session, add it to the env object of a settings file:

// ~/.claude/settings.json
{
  "env": {
    "CLAUDE_CODE_DISABLE_ULTRATHINK": "<value>"
  }
}

Release notes that mention it

• Claude Code Version Tracker: Undocumented Env Vars, Feature Gates & Models
• claude-code-env-vars-v2-1-315

Related variables

• CLAUDE_CODE_DISABLE_ADMIN_ENV_UNION: When set to true, uses only the leading admin tier's environment settings instead of merging (unioning) the environment across all admin tiers.
• CLAUDE_CODE_DISABLE_ARTIFACT
• CLAUDE_CODE_DISABLE_ATTACHMENTS: File/image attachments.
• CLAUDE_CODE_DISABLE_AWAITING_USER_IDLE: When set, stops the session from being treated as idle while it waits for user input, so time parked on the user is not counted as idle.
• CLAUDE_CODE_DISABLE_CFC_PROMPT
• CLAUDE_CODE_DISABLE_COMMAND_INJECTION_CHECK: Previously allowed disabling command injection detection.
• CLAUDE_CODE_DISABLE_CRON: Disable cron/scheduled task functionality.
• CLAUDE_CODE_DISABLE_DIR_SYNC: Turns off the background directory-sync worker on a remote session.

Frequently asked questions

All variables, searchable: Claude Code environment variables.