I think the problem I have with this is the idea that the design is static. In practice, if you’ve got five layers, their exact behaviour can be subject to subt

I think the problem I have with this is the idea that the design is static. In practice, if you’ve got five layers, their exact behaviour can be subject to subtle change. Integration tests (not necessarily whole system tests) are invaluable for finding where changed assumptions actually break required behaviour. This, in turn, can inform a better design. Never thought I’d be disagreeing with Kent Beck about anything in his bailiwick, but there you go.