Disk
Also called Storage, SSD
Long-term storage, where files stay when the power goes off. It is much slower than memory. A program that only keeps data in memory loses it when it stops, crashes or the machine restarts.
An agent reaches for whatever makes the demo work, and that can be a list in memory. It passes every test. Then your first restart wipes all your users. So always ask where each piece of data lives.
Say it to your agent
“You stored sessions in a module-level Map, so they vanish on every restart. Store them in the database, then prove it: log in, restart the server, and show me that the session still works.”