> "I actually want to have a good mental picture of what I'm doing before I start prototyping. And while I had a high-level notion of what I wanted, I didn't have enough of a idea of the details to really start coding." This I like. The race away from the waterfall straw man has also stripped us of the advantages of BDUF. While rigid phase-driven project management helps nobody, I think there's still room for speccin…
You should be speccing what you want to achieve: the goals, the why, the impact, the external limitations, measures of success and so forth. This also allows you to describe and plan testing up front. The "how" is best handled in an iterative manner. A lot of people use AGILE to avoid planning at all, which is a particular destructive anti-pattern, and the exact opposite of what you need.
I think that the first "how" should be planned as much as anything else. I understand how you refactor from v0.0.1 to v5.34.2 iteratively, but I think that getting from vNothing to v0.0.1 is qualitatively different.
If I don't have a complete idea of how my minimally functional thing will work that is small enough that I can completely hold it in my head, and instead just architect by agglutination and test writing, 1) my results are going to be hacky garbage, 2) my first 50 iterations are going to be devoted to replacing it all haphazardly to fix bugs, and 3) the code and interface will become increasingly more complex, harder to work with, and strewn with special cases.
When v0.0.1 is well planned, v2.5.2 may not look anything like the plan anymore, but in my experience it becomes shorter, cleaner, and more correct rather than a giant ball of band-aids propped up with tests.