green tests prove the code agrees with its mocks

As a non-software dev software person, i have never understood the purpose of mocks.

from Aneesh's Blyg ↗ · snapshot of v1

They suck unless you really need them — generally you really need them if the thing you're mocking is too expensive or slow to run or impossible to run e.g. a 3rd party API that doesn't have a test environment or a payment provider that would charge real money every run, anything that sends real emails or SMS to actual people, hardware you don't have on your dev machine (a printer, a GPU cluster, some IoT device), a huge ML model that takes minutes just to load, or nondeterministic stuff like the current time or randomness that you need to pin down to get a repeatable test.