flags-sdk

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. Vercel Flags is the first-party provider, letting you manage flags from the Vercel dashboard or the vercel flags CLI.

• Docs: https://flags-sdk.dev
• Repo: https://github.com/vercel/flags

Core concepts

Flags as code

Each flag is declared as a function. No string keys at call sites:

import { flag } from 'flags/next';

export const exampleFlag = flag({
  key: 'example-flag',
  decide() { return false; },
});

const value = await exampleFlag();

Server-side evaluation

Flags evaluate server-side to avoid layout shift, keep pages static, and maintain confidentiality. Combine routing middleware with the precompute pattern to serve static variants from CDN.

Adapter pattern

Adapters replace decide and origin on a flag declaration, connecting your flags to a provider. Vercel Flags (@flags-sdk/vercel) is the first-party adapter. Third-party adapters are available for Statsig, LaunchDarkly, PostHog, and others.

import { flag } from 'flags/next';
import { vercelAdapter } from '@flags-sdk/vercel';

export const exampleFlag = flag({
  key: 'example-flag',
  adapter: vercelAdapter,
});

Version note: The SDK is published as flags (renamed from @vercel/flags; that old name still appears in changelog history). flags 4.2.0+ accepts the adapter factory by reference (adapter: vercelAdapter) and resolves it once per declaration. Older versions require calling it (adapter: vercelAdapter()). The called form still works on new versions, so prefer the shorthand unless you're targeting flags < 4.2.0.

Agent workflow: Creating a new flag

When a user asks you to create or add a feature flag, follow these steps in order. Do not leave CLI steps as "next steps" for the user — execute them yourself.