Feature flags

Nothing to install from here. The Flags SDK is framework-agnostic at its core, and its Next integration — flags/next, the one with flag(), dedupe() and the adapters — needs one thing from Next: next/headers, for headers() and cookies(). Those exist here under the same names and with the same shapes, one object per request, which is what the SDK’s per-request dedupe keys on. The build answers next/headers with them, so the SDK runs as written:

bun add flags

import { flag, dedupe } from 'flags/next'

const visitor = dedupe(async ({ cookies, headers }) => ({
  id: cookies.get('visitor')?.value ?? 'anonymous',
  country: headers.get('x-vercel-ip-country') ?? headers.get('cf-ipcountry') ?? '',
}))

export const showBanner = flag<boolean, { id: string; country: string }>({
  key: 'show-banner',
  identify: visitor,
  decide: ({ entities }) => entities?.country === 'GB',
})

import { showBanner } from '../flags'

export default async function Home() {
  const banner = await showBanner()

  return banner ? <Banner /> : null
}

A flag reads the request, so the page that awaits one renders per visitor — the build says headers() in run, cookies() in run stream per request and stores the rest as a shell, given a <Suspense> or a loading.tsx above the read. That is the same line any headers() call earns; see Static generation. An adapter — Statsig, LaunchDarkly, Vercel’s own — is the SDK’s, configured the way its docs say.

The discovery endpoint is a route handler in the shape a route.ts already has:

import { createFlagsDiscoveryEndpoint, getProviderData } from 'flags/next'
import * as flags from '../../../../flags'

export const GET = createFlagsDiscoveryEndpoint(async () => getProviderData(flags))
export const openapi = false

A test is not built, so it needs the alias too: one line in the test preload, which the scaffold ships. Without it flags/next finds Next’s own next/headers — installed by some other app in a monorepo — and throws that headers was called outside a request scope.

mock.module('next/headers', () => import('@rsc-kit/core/request'))

A flag then evaluates inside withRequest(), the way anything that reads the request does in a test; see Testing.

What does not carry over is precompute(): it is built on a Next middleware rewriting the url to a permutation, which is Next’s routing. Read the flag in the page instead; a flag read under a boundary costs one streamed hole, not the page.