Vercel frontend contract

client.ts is a stateless browser/SSR Store API adapter with explicit shop, channel, locale, cart token and revisions. It accepts no merchant credential and redirects through the existing single-use native checkout handoff. Do not share cart handles across SSR visitors. vercel.json proxies public Store API routes only; configure the actual Commerce origin and keep responses uncached.

Use the frontend's own origin for origin and the native Commerce origin for checkoutOrigin. A proxy is needed for browser calls; do not assume Core CORS. The example does not provision a Vercel project or provide a complete account, iframe, return-navigation or custom storefront implementation.

The actual cart/handoff owner remains Core; component/transport regressions are in frontend/tests/unit/service-integrations.test.tsx, and native checkout transfer acceptance remains scripts/checkout_handoff.py. See the integration guide.