Vercel's design.md item is recorded as an agent-contract signal. The useful change is that brand and page guidance can move from human-only prose into a file that an agent can read while building.
For New Runtime, this is part of the work-artifact layer around agents. Durable instructions, evaluation harnesses, and production feedback need to sit outside a single chat transcript. The next thing to watch is whether design.md becomes a portable convention across tools or remains a Vercel-specific workflow pattern.