Whichever camp you are in, both end up building in thin pieces that run. Böckeler's objection to heavy specs is that "small, iterative steps" keep you in control, and pstack, a published set of agent instructions, orders its work the same way.
Vertical slice
Also called Tracer bullet
A small piece of a feature that goes through every layer, from what the user sees down to where the data is kept, so it works end to end even though it does very little.
An agent can write a whole layer confidently, and the mismatches only appear where layers meet, by which time a lot is resting on a guess. A slice gives it something real to run and check within the first hour.
“Build the thinnest version first: one button that saves one comment to the real database and shows it after a reload. No editing, no styling. Show me it working end to end before you add anything.”