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DisclaimerUnofficial, and not affiliated with Anthropic. Nearly all of this is read straight out of what ships: npm bundles, captured prompts, published docs. Anthropic's own notes go in verbatim, marked as theirs. The rest is my reading, and every entry carries the strings behind it. If one looks wrong, vote it down and say why.

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Claude Code v2.1.8 15 Jan 2026

Ultrathink Keyword Deprecated

What

The ultrathink keyword no longer has any effect.

Details

Previous behavior: typing "ultrathink" in a message would enable extended thinking
New behavior: Thinking budget is now max by default for all messages
Users may see a tip: "Ultrathink no longer does anything. Thinking budget is now max by default."
The regex pattern still exists but the parsing/detection logic has been removed

Evidence

Tip at line 528403 ("ultrathink-deprecated", "Ultrathink no longer does anything. Thinking budget is now max by default.")

Strings lifted out of the shipped bundle, so the claim above can be checked against them.

See this entry in the whole of v2.1.8 →