What makes an agent run: you, a schedule, or something a contact does.
Open the builder: Agents → "Create an agent".
Pick a template to start from, e.g. "Find & email new prospects".
Drag "Start on an event" from the rail. It runs when a contact replies or is added, instead of waiting for you.
"Not scheduled" at the bottom runs the agent daily or weekly.
Without either, it runs only when you press "Run".