Feature flags, A/B tests, and gradual rollouts with Vercel Flags, in any app or framework. Use whenever the user is planning, adding, or changing a feature flag, toggle, kill switch, experiment, percentage rollout, or targeting rule, or wants to gate or hide a feature for some users, even before a library or provider is chosen. Covers choosing an integration (Flags SDK, `@vercel/flags-core`, OpenFeature), the Vercel dashboard, the `vercel flags` CLI (create, inspect, set, split, rollout, rules, segments, evaluations, archive), OIDC vs SDK keys, `FLAGS_SECRET`, and Flags Explorer overrides. For `flag()` declarations, adapters, precompute, and the Flags Explorer endpoint in Next.js or SvelteKit code, load the `flags-sdk` skill.