Flags SDK is an open-source feature flagging toolkit developed by Vercel for modern web frameworks like Next.js and SvelteKit. It allows developers to define, manage, and evaluate feature flags directly in code, enabling controlled feature rollouts, experimentation, and gradual releases.

Unlike traditional feature flag platforms that rely heavily on external dashboards, Flags SDK follows a “flags as code” approach. Each feature flag is defined as a function in the codebase, making it easier to track usage, enforce type safety, and maintain consistency across the application. This approach improves developer experience by enabling better refactoring, discoverability, and version control.

The SDK acts as an abstraction layer between the application and any feature flag provider. It is provider-agnostic, meaning developers can use third-party services, custom logic, or no provider at all. This reduces vendor lock-in and allows flexibility in how feature flags are evaluated and managed.

Flags SDK integrates natively with modern frameworks and supports server-side evaluation, middleware usage, and edge environments. It also includes support for experimentation workflows such as A/B testing and gradual rollouts, making it suitable for production-grade applications.

The platform emphasizes performance and simplicity, using patterns like precomputation and deterministic evaluation to ensure consistent behavior across requests. It also integrates with tools like Vercel’s Flags Explorer, allowing developers to override and test feature flags locally without affecting production.

Key features include:

• "Flags as code" model with function-based feature flags
• Provider-agnostic architecture (works with any flag service or custom logic)
• Built-in support for Next.js and SvelteKit frameworks
• Type-safe APIs with full TypeScript support
• Support for A/B testing, experimentation, and controlled rollouts

Common use cases include:

• Managing feature rollouts
• Running A/B tests
• Gating new features in production
• Experimenting with UI changes
• Safely deploying incremental updates in web applications

Flags SDK is developed by Vercel as part of its ecosystem for modern web development and experimentation tooling.