The feature flags toolkit

Flags SDK is a free, open-source library for using feature flags in Next.js and SvelteKit.

$npx skills add vercel/flags@flags-sdk

Using flags as code

The SDK sits between your application and the source of your flags, helping you follow best practices and keep your website fast.

Works with any provider

Use any flag provider, or none at all. Flexible integrations for your projects.

Effortless integration

Integrate with App Router, Pages Router, and Routing Middleware.

Release strategically

Ideal for A/B testing and controlled rollouts. Experiment with confidence.

Effortless setup

With a simple declarative API to define and use your feature flags.

Read the Docs

flags.ts

import { flag } from 'flags/next';

export const exampleFlag = flag({
  key: 'example-flag',
  decide() {
    return Math.random() > 0.5;
  },
});

Declaring a flag

app/page.tsx

import { exampleFlag } from "../flags";

export default async function Page() {
  const example = await exampleFlag();

  return <div>Flag {example ? "on" : "off"}</div>;
}

Using a flag

What builders say about the Flags SDK

Julius

@jullerino

man @vercel flags are so nice

Sully

@SullyOmarr

Ngl the feature flag from @vercel is goated

i can ship my spaghetti to prod and have the entire team test without worrying about breaking things

no more "it worked on local/staging" but breaks in prod

Max Stoiber

@mxstbr

Vercel's new Flags SDK is a brilliant show of taste in developer experience 💯

It introduces significant "artificial" limitations (server-only, no arguments at call site)—but those actually end up guiding users down the pit of success: