flags-sdk

name	flags-sdk
description	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.
license	Sustainable Use License 1.0
metadata	{"domain":"development","subdomain":"frontend","tags":"feature-flags, a-b-testing, experimentation, flags-explorer, precompute, vercel","frameworks":"flags-sdk, nextjs, sveltekit","author":"Hayden Bleasel <[e-mail]>","lastUpdated":"12026-02-24","provenance":"ported"}

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.

• 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; },
});

// Usage: just call the function
const value = await exampleFlag();

Server-Side Evaluation

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

Adapter Pattern

Adapters replace decide and origin on a flag declaration, enabling provider-agnostic flags:

import { flag } from 'flags/next';
import { statsigAdapter } from '@flags-sdk/statsig';

export const myGate = flag({
  key: 'my_gate',
  adapter: statsigAdapter.featureGate((gate) => gate.value),
  identify,
});

Declaring Flags

Basic Flag

import { flag } from 'flags/next'; // or 'flags/sveltekit'

export const showBanner = flag<boolean>({
  key: 'show-banner',
  description: 'Show promotional banner',
  defaultValue: false,
  options: [
    { value: false, label: 'Hide' },
    { value: true, label: 'Show' },
  ],
  decide() { return false; },
});

Flag with Evaluation Context

Use identify to establish who the request is for; decide receives the entities:

import { dedupe, flag } from 'flags/next';
import type { ReadonlyRequestCookies } from 'flags';

interface Entities {
  user?: { id: string };
}

const identify = dedupe(
  ({ cookies }: { cookies: ReadonlyRequestCookies }): Entities => {
    const userId = cookies.get('user-id')?.value;
    return { user: userId ? { id: userId } : undefined };
  },
);

export const dashboardFlag = flag<boolean, Entities>({
  key: 'new-dashboard',
  identify,
  decide({ entities }) {
    if (!entities?.user) return false;
    return ['user1', 'user2'].includes(entities.user.id);
  },
});

Flag with Adapter

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

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

Key Parameters

Parameter	Type	Description
key	string	Unique flag identifier
decide	function	Resolves the flag value
defaultValue	any	Fallback if decide returns undefined or throws
description	string	Shown in Flags Explorer
origin	string	URL to manage the flag in provider dashboard
options	{ label?: string, value: any }[]	Possible values, used for precompute + Flags Explorer
adapter	Adapter	Provider adapter implementing decide and origin
identify	function	Returns evaluation context (entities) for decide

Dedupe

Wrap shared functions (especially identify) in dedupe to run them once per request:

import { dedupe } from 'flags/next';

const identify = dedupe(({ cookies }) => {
  return { user: { id: cookies.get('uid')?.value } };
});

Note: dedupe is not available in Pages Router.

Flags Explorer Setup

Next.js (App Router)

// app/.well-known/vercel/flags/route.ts
import { getProviderData, createFlagsDiscoveryEndpoint } from 'flags/next';
import * as flags from '../../../../flags';

export const GET = createFlagsDiscoveryEndpoint(async () => {
  return getProviderData(flags);
});

With External Provider Data

import { getProviderData, createFlagsDiscoveryEndpoint } from 'flags/next';
import { getProviderData as getStatsigProviderData } from '@flags-sdk/statsig';
import { mergeProviderData } from 'flags';
import * as flags from '../../../../flags';

export const GET = createFlagsDiscoveryEndpoint(async () => {
  return mergeProviderData([
    getProviderData(flags),
    getStatsigProviderData({
      consoleApiKey: process.env.STATSIG_CONSOLE_API_KEY,
      projectId: process.env.STATSIG_PROJECT_ID,
    }),
  ]);
});

SvelteKit

// src/hooks.server.ts
import { createHandle } from 'flags/sveltekit';
import { FLAGS_SECRET } from '$env/static/private';
import * as flags from '$lib/flags';

export const handle = createHandle({ secret: FLAGS_SECRET, flags });

FLAGS_SECRET

Required for precompute and Flags Explorer. Must be 32 random bytes, base64-encoded:

node -e "console.log(crypto.randomBytes(32).toString('base64url'))"

Store as FLAGS_SECRET env var. On Vercel: vc env add FLAGS_SECRET then vc env pull.

Precompute Pattern (Overview)

Use precompute to keep pages static while using feature flags. Middleware evaluates flags and encodes results into the URL via rewrite. The page reads precomputed values instead of re-evaluating.

High-level flow:

1. Declare flags and group them in an array
2. Call precompute(flagGroup) in middleware, get a code string
3. Rewrite request to /${code}/original-path
4. Page reads flag values from code: await myFlag(code, flagGroup)

For full implementation details, see framework-specific references:

• Next.js: See references/nextjs.md — covers proxy middleware, precompute setup, ISR, generatePermutations, multiple groups
• SvelteKit: See references/sveltekit.md — covers reroute hook, middleware, precompute setup, ISR, prerendering

Custom Adapters

Create an adapter factory returning an object with origin and decide:

import type { Adapter } from 'flags';

export function createMyAdapter(/* options */) {
  return function myAdapter<ValueType, EntitiesType>(): Adapter<ValueType, EntitiesType> {
    return {
      origin(key) {
        return `https://my-provider.com/flags/${key}`;
      },
      async decide({ key }): Promise<ValueType> {
        // evaluate against your provider
        return false as ValueType;
      },
    };
  };
}

Encryption Functions

For keeping flag data confidential in the browser (used by Flags Explorer):

Function	Purpose
encryptFlagValues	Encrypt resolved flag values
decryptFlagValues	Decrypt flag values
encryptFlagDefinitions	Encrypt flag definitions/metadata
decryptFlagDefinitions	Decrypt flag definitions
encryptOverrides	Encrypt toolbar overrides
decryptOverrides	Decrypt toolbar overrides

All use FLAGS_SECRET by default. Example:

import { encryptFlagValues } from 'flags';
import { FlagValues } from 'flags/react';

async function ConfidentialFlags({ values }) {
  const encrypted = await encryptFlagValues(values);
  return <FlagValues values={encrypted} />;
}

React Components

import { FlagValues, FlagDefinitions } from 'flags/react';

// Renders script tag with flag values for Flags Explorer
<FlagValues values={{ myFlag: true }} />

// Renders script tag with flag definitions for Flags Explorer
<FlagDefinitions definitions={{ myFlag: { options: [...], description: '...' } }} />

References

Detailed framework and provider guides are in separate files to keep context lean:

• references/nextjs.md: Next.js quickstart, App Router, Pages Router, middleware/proxy, precompute, dedupe, dashboard pages, marketing pages, suspense fallbacks
• references/sveltekit.md: SvelteKit quickstart, hooks setup, toolbar, precompute with reroute + middleware, dashboard pages, marketing pages
• references/providers.md: All provider adapters — Vercel, Edge Config, Statsig, LaunchDarkly, PostHog, GrowthBook, Hypertune, Flagsmith, Reflag, Split, Optimizely, OpenFeature, and custom adapters
• references/api.md: Full API reference for flags, flags/react, flags/next, and flags/sveltekit