I don't agree. I believe that when the solutions' space is big design docs work best. If they are done correctly they are a (shallow) breadth-first search in th

I don't agree. I believe that when the solutions' space is big design docs work best. If they are done correctly they are a (shallow) breadth-first search in the space. Specifically if in your design doc include "alternatives considered". Additionally, having multiple people looking at your doc you can prune some possible paths. A breadth-first search is useful when you don't know much about the intricacies of the problem/topic at hand, and so you need some ground work to take informed decisions. If, instead, the solution space is narrow then by all means do a prototype/throwaway PR/... to validate the approach.