Mark the boundary between vibe-coded prototype and engineered product, literally.

Simon Willison and a growing 2026 consensus argue you should consciously separate the two modes. Vibe coding optimizes for speed and accepts unread code. Agentic engineering optimizes for correctness with specs, tests, and review. The actionable move is a literal marker, a folder, a comment, or a branch convention, that tells later agents which rigor to apply where. This is the one I've gotten burned by most. Prototype-grade code silently becomes load-bearing because nobody marked it as throwaway, and six weeks later it's in the critical path with no tests. Mark it now or pay for it later.