Tests are proof, not paperwork.
The agent writes code fast, and it can't tell whether that code does what you meant. Testing is how you find out before your users do.
Manual
You Click Through
A written checklist anyone can follow.
Day 53.
Automated
Code Checks Code
Runs in seconds, every time.
Day 52.
Real users
Feedback
What you'd never think to try.
Day 78.
When You Find A Bug, And What It Costs
While building
A second to fix
Before commit
Minutes
Before release
Hours
Found by a customer
Days, and trust
Source: NIST Planning Report 02-3 ↗
If you didn't check it, it doesn't work yet.