Use ultrathink keyword to trigger extended reasoning

You can use the ultrathink keyword in your prompts to encourage Claude to engage in deeper, more extended reasoning. This is particularly useful for complex problems that require multiple steps of thought or analysis.

NB, previously thinking was controlled also by using think, think hard, and think harder keywords. But it looks like now ultrathink is the only one that works. It seems like these keywords were superseded by Tab shortcut to switch the reasoning mode.