flags-sdk

The Flags SDK (flags npm package) is a feature flags toolkit for Next.js and SvelteKit. It turns each feature flag into a callable function, works with any flag provider via adapters, and keeps pages static using the precompute pattern.

Each flag is declared as a function — no string keys at call sites:

Evaluate flags server-side to avoid layout shift, keep pages static, and maintain confidentiality. Use routing middleware + precompute to serve static variants from CDN.

Comprehensive guide for implementing feature flags and A/B tests using the Flags SDK (the `flags` npm package). Use when: (1) Creating or declaring feature flags with `flag()`, (2) Setting up feature flag providers/adapters (Vercel, Statsig, LaunchDarkly, PostHog, GrowthBook, Hypertune, Edge Config, OpenFeature, Flagsmith, Reflag, Split, Optimizely, or custom adapters), (3) Implementing precompute patterns for static pages with feature flags, (4) Setting up evaluation context with `identify` and `dedupe`, (5) Integrating the Flags Explorer / Vercel Toolbar, (6) Working with feature flags in Next.js (App Router, Pages Router, Middleware) or SvelteKit, (7) Writing custom adapters, (8) Encrypting/decrypting flag values for the toolbar, (9) Any task involving the `flags`, `flags/next`, `flags/sveltekit`, `flags/react`, or `@flags-sdk/*` packages. Triggers on: feature flags, A/B testing, experimentation, flags SDK, flag adapters, precompute flags, Flags Explorer, feature gates, flag overrides. Source: vercel/flags.