When Claude Code requires input from a human, by default that is communicated exclusively via Claude Code outputting a message to the terminal. No sound plays, no visual notification displays outside of what is displayed via the terminal.

That means that if a human is not actively monitoring their terminal, there is a delta between when an agent requires feedback and when a human becomes aware of that. Repeated dozens of times across a workday, the cumulative cost of these idle intervals adds up. A few minutes here and there compounds into hours of idle agent time, which can prove extremely expensive as measured by throughput.