Ultrathink
Claude Code keyword that activates the highest thinking token budget (extended thinking). By prefixing a request with "ultrathink", Claude allocates significantly more tokens to internal reasoning before responding. Hierarchy: think < think hard < think harder < ultrathink. Reserve for complex problems, higher token cost.
Strengths
- Significantly higher reasoning quality on complex and ambiguous problems
- Zero install, a single keyword in your prompt is enough to activate it
- Explicit hierarchy (think → think hard → think harder → ultrathink), precise budget control
Limitations
- Token cost 3-5× higher, monitor consumption on intensive sessions
- Higher latency, the thinking phase takes several additional seconds
- Overkill for simple tasks, does not improve responses on trivial requests
Best for
- Complex architecture decisions requiring evaluation of multiple trade-offs
- Debugging intermittent bugs or race conditions difficult to reproduce
- Strategic refactors requiring global system understanding before acting