The ultrathink cheat sheet

From @learningwithcole

One word in your prompt tells Claude Code to think harder for one turn. Here is what it does, how to tell it worked, and when to leave it out.

What it does #

Type ultrathink anywhere in your prompt in Claude Code.

• It asks for deeper reasoning on that turn.
• In the terminal prompt, that turn runs at xhigh effort on Opus 5.5, Sonnet 5.5, Haiku 5.5 and Fable 5.1 through Anthropic's API. You need Claude Code 2.1.311 or later.
• Type it while Claude is already working and it raises the rest of that turn.
• Your saved effort setting does not change. The next turn goes back to normal.

The rainbow rule #

Watch the word itself.

• Rainbow: the word can raise the turn. The raise is live.
• Plain: it does not raise the turn. That includes the word in quotes or inside a file path.

What you typed	What you see	Raises the turn?
ultrathink fix the flaky auth test	rainbow	Yes
what does "ultrathink" do?	plain	No
read docs/ultrathink.md	plain	No

Only ultrathink is a keyword. Claude Code treats "think", "think hard" and "think more" as ordinary prompt text.

When to use it #

One hard problem, one turn:

• A bug you can't explain
• A design choice with real tradeoffs
• A refactor that touches many files

When to skip it #

• Plain lookups
• Small edits
• Anything where you already know the answer and just want it typed

If you want every turn to run higher, set your effort level instead. Save ultrathink for the one-offs.

If the word stays plain on a fresh prompt #

• Check your version. The raise needs 2.1.311 or later.
• Check your model. Run /model and pick Opus 5.5, Sonnet 5.5, Haiku 5.5 or Fable 5.1.
• Some setups can't raise a turn, such as Amazon Bedrock. Set the effort level instead.

VS Code #

Since version 2.1.316 (October 31, 2026), the VS Code chat box does the same thing: the turn runs at xhigh and the word shows in a rainbow.

The off switch #

If you type the word by habit, set CLAUDE_CODE_DISABLE_ULTRATHINK to turn the keyword off. It arrived in 2.1.316. It is in the changelog, and the environment variables docs page didn't list it yet when I checked on October 31.