Fixed one thing. Broke another.
A regression is something that used to work and now doesn't. Agents cause them often because changes spread further than you asked.
Why agents cause them
One fix can touch shared code used elsewhere.
It doesn't remember why something was done a certain way.
It tidies things you didn't ask it to.
Four Defences
Keep the tests
Every test stays and runs every time.
Re-run the script
The full manual script, not just the fix.
Small commits
So you can see exactly when it broke.
Fence it in
“Only change files related to the export.”
Every bug fixed gets a test that stays.