Purpose The vertical slice answers a different question from the concept prototype: "Can we build this full game loop at production quality, on schedule?" Default use — run late in Pre-Production, after GDDs, architecture, and UX specs are complete. It is a near-production-quality build demonstrating one complete [start → challenge → resolution] cycle. Post-pivot? If a PIVOT verdict from an earlier vertical slice sent you back to revise GDDs and architecture, run this again after revisions to re-validate. It can be run as many times as needed until a PROCEED or KILL verdict is reached. It val…