[bee_rider]

It sounds like it is a “specific phrase mapped directly” based on another comment here? I guess that means hardcoded? Not completely sure, though.

[simonw]

It's hard-coded - this isn't a weird model thing, Claude Code detects the exact string "ultrathink" and sets the thinking token budget to 31999.

I included that de-obfuscated code in my post: https://simonwillison.net/2025/Apr/19/claude-code-best-pract...