That is indeed a drawback of this approach, and is problematic if and only if `prep` and `finish` are part of the public interface. If they're not, it's a worthwhile exercise to ask yourself what you want from your test suite, and whether or not individual tests for prep and finish support that goal. For me personally, my ultimate goal for a test suite is to get as close to the equivalence "the tests do not pass <-> pushing this to production would break the product" as humanly possible, as I really don't like spending time "fixing" tests. This leads me to test at the system boundaries, i.e., to leave the lower level tests out when I can. What value does testing stuff that will not affect production in any way bring me?

> but that essentially just re-introduces the coupling to implementation details at a higher level

The definition of coupling that I find useful is the following. A thing f is coupled to a thing g w.r.t. a change d in g if changing g by d requires you to change f. I find that it captures exactly the phenomenon which makes maintenance and extension of brownfield projects so costly.

So, for example, both test suites I gave as an example are coupled to sort with respect to the change "sort sorts stuff in descending order instead of ascending". Making this change requires you change pretty much every assert but the trivial ones, in both test suites.

With respect to the change "move some code from prep to finish" or "get rid of prep and finish entirely and move everything to the body of sort", only the second suite is coupled to sort.

This may not be the definition of coupling that you like to use. If it is, I don't see how the test suite of the first kind including edge cases for prep and finish at the level of sort is still coupled to implementation details.

I strongly do agree with your approach, for the record

I suppose it really comes down to what kind of coverage is important

In your case you seem to take the approach of having coverage on the high level surface area

Mostly I see coverage referring to the actual lines of code and branches

Having high coverage of lines and branches does require having tests that even dig into your private interfaces