What
"Ultrathink" is a word you can type in a prompt that Claude Code treats specially. A new environment variable (a named value set outside Claude Code), CLAUDE_CODE_DISABLE_ULTRATHINK, turns that handling off.
- When the variable is set, the check
!i.CLAUDE_CODE_DISABLE_ULTRATHINK && v("tengu_turtle_carbon", !0)returns false, whatever the remote switchtengu_turtle_carbonsays. - Claude Code looks for the word ultrathink in your prompt only when that check is true.
- When the variable is unset, nothing changes: the check follows
tengu_turtle_carbon, which defaults to on. - The variable is added to several lists of recognised environment variables, including one next to
DISABLE_TELEMETRYandCLAUDE_CODE_DISABLE_WEB_FETCH.
In readings taken before this release, tengu_turtle_carbon came back on for this site's account and on for an anonymous check. No reading has been taken under this release.
Why
You, or an administrator setting up Claude Code for others, can now stop the word ultrathink from triggering its special handling without depending on a remote switch.
tengu_turtle_carbon On for this account, and not off by defaultThe flag server returned on for the one account this site reads, and nothing in this release compiles it off by default. The compiled default is shown below, and says which it is when we cannot read one: a fifth of gates compile in a string or a number rather than on or off, and most published releases have no gate table behind them at all. No client can see what the server returns for your account.
This account: on · anonymous baseline: on · compiled default in v2.1.315: on
These values were read against a different version of Claude Code, so treat them as the nearest reading available instead of one taken on this release.
Read once, for one account on one subscription tier, against v2.1.315. It isn't a statement about your account. What a flag value here can and cannot tell you
New in this build: CLAUDE_CODE_DISABLE_ULTRATHINK