Customers almost always know "What" (they're not making this up). They usually know "Why". Then never know "How". So, the "What" is theirs, the "Why" is ours

Customers almost always know "What" (they're not making this up). They usually know "Why". Then never know "How". So, the "What" is theirs, the "Why" is ours, and the "How" is mine. All documented and agreed upon before a single line of code is written (except for a prototype, wireframe, or proof of concept). Take the time and energy to do this up front or stop caring. Your choice.