Very well said. When I'm working on a hard problem I'll often spend a few weeks sweating details like algorithms, API shapes, wire formats, database schemas, etc. These things are all really easy to change while they're just in a design document. Once you start implementing, big sweeping edits get a lot more difficult. So better to frontload as much of that as possible in the design phase. AI coding agents don't change this dynamic. However all that frontloaded work pays off big when it does come time to implement, because the search space has been narrowed considerably.