A program on disk does nothing. Starting it is what makes it matter.
Process
Also called PID
One running copy of a program, with its own private memory and an ID number called a PID. Start a program twice and you have two processes that know nothing about each other. Each runs until it finishes or something stops it.
Agents start servers and leave them running, then start them again. The leftovers keep hold of resources and go on serving old code, so you can lose an hour debugging a change that isn't even the thing running.
Say it to your agent
“Before you start the dev server, run lsof -i :3366 to see if one is already running. If it is, tell me its PID and don't start another. Stop anything you started when you finish.”