flags-sdk

name

flags-sdk

description

Guide for feature flags and A/B tests with the Flags SDK (`flags` npm package) and Vercel Flags. Use when: declaring flags with `flag()`, using `vercelAdapter` or `vercel flags` CLI (add, list, enable, disable, inspect, archive, rm, sdk-keys), setting up providers/adapters (Vercel, Statsig, LaunchDarkly, PostHog, GrowthBook, Hypertune, Global Config, OpenFeature, Split, Flagsmith, Reflag, Optimizely, or custom adapters), implementing precompute patterns for static pages, setting up `identify`/`dedupe`, integrating Flags Explorer/Toolbar, working with flags in Next.js (App Router, Pages Router, Middleware) or SvelteKit, writing custom adapters, or encrypting/decrypting flag values. Triggers: feature flags, A/B testing, experimentation, flags SDK, flag adapters, precompute, Flags Explorer, feature gates, flag overrides, Vercel Flags, vercel flags CLI, vercel flags add, vercel flags list, vercel flags enable, vercel flags disable, `flags/next`, `flags/sveltekit`, `flags/react`, `@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.

Before you start

Check the project state to adapt commands and decide which steps you can skip:

• Which lockfile is present (pnpm-lock.yaml, package-lock.json, yarn.lock, bun.lockb)? → Adapt all package manager commands accordingly (pnpm add, npm install, yarn add, bun add).
• Is flags in package.json? → Skip install (step 1)
• Does .vercel/ directory exist? → Project is linked, skip vercel link in step 2
• Does .env.local contain FLAGS=? → Env vars already pulled, skip step 3
• Does flags.ts (or lib/flags.ts, src/flags.ts) exist? → Add to it rather than creating from scratch (step 4)
• Is @vercel/toolbar in package.json? → Skip toolbar setup (step 6)
• Does app/.well-known/vercel/flags/route.ts exist? → Flags Explorer already set up, skip step 7

Steps

1. Install packages (if not already in package.json):

   pnpm i flags @flags-sdk/vercel
   

2. Register the flag with Vercel: Run vercel flags add <flag-key> --kind boolean --description "<description>".

   Prerequisite: The vercel flags commands require the Vercel CLI to be installed and authenticated. If the Vercel CLI is not installed, run pnpm i -g vercel. For authentication or linking issues, follow the vercel-cli skill if it's already available. If it isn't, suggest the user install it with npx skills add vercel/vercel@vercel-cli (don't auto-install it) and rely on vercel <command> --help in the meantime.

   Before running vercel flags add, verify the project is linked to Vercel. Check for a .vercel directory in the project root. If it doesn't exist, run vercel link first.

3. Pull environment variables: Run vercel env pull to write FLAGS and FLAGS_SECRET to .env.local. Without these environment variables, vercelAdapter will not be able to evaluate flags. This step is mandatory after creating a flag.

4. Declare the flag in code: Add it to flags.ts (or create the file if it doesn't exist) using vercelAdapter:

   import { flag } from 'flags/next';
   import { vercelAdapter } from '@flags-sdk/vercel';
   
   export const myFlag = flag({
     key: 'my-flag',
     adapter: vercelAdapter,
   });
   

5. Use the flag: Call it in your page or component and conditionally render based on the result:

   import { myFlag }  ;
   
       () {
      enabled =  ();
      ;
   }
   

Vercel Flags

Vercel Flags is Vercel's feature flags platform. You create and manage flags from the Vercel dashboard or the vercel flags CLI, then connect them to your code with the @flags-sdk/vercel adapter. When you create a flag in Vercel, the FLAGS and FLAGS_SECRET environment variables are configured automatically.

To create a flag end-to-end, follow the Agent workflow above.

For the full Vercel provider reference — user targeting, vercel flags CLI subcommands, custom adapter configuration, and Flags Explorer setup — see references/providers.md.

Declaring flags

When using Vercel Flags, declare flags with vercelAdapter as shown in the Agent workflow. For other providers, see references/providers.md. Below are the general flag() patterns.

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. The returned entities are passed to decide:

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 another adapter

Adapters connect flags to third-party providers. Each adapter replaces decide and origin:

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,
});

See references/providers.md for all supported adapters.

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.

Bulk evaluation

To evaluate multiple flags at once, call evaluate() (from flags/next) instead of awaiting flags one at a time or using Promise.all(). To evaluate a single flag, just call it: await myFlag().

import { evaluate } from 'flags/next';
import { flagA, flagB } from '../flags';

// avoid: each await blocks the next, so the flags resolve sequentially
const a = await flagA();
const b = await flagB();

// avoid: parallel, but each flag is evaluated in isolation
const [a, b] = await Promise.all([flagA(), flagB()]);

// prefer: shares work across the batch
const [a, b] = await evaluate([flagA, flagB]);

evaluate() is faster than both approaches. Awaiting flags one at a time makes total latency the sum of every flag's evaluation instead of the slowest single flag, while Promise.all() runs them in parallel but evaluates each in isolation. evaluate() pre-reads headers, cookies, and overrides once for the whole batch and lets adapters resolve a group in a single call, which reduces the number of parallel promises the runtime manages and leaves less room for the async work to be interrupted by other microtasks.

It accepts either an array (positional results) or an object (keyed results):

const [a, b] = await evaluate([flagA, flagB]);
const { a, b } = await evaluate({ a: flagA, b: flagB });

Outside App Router (Pages Router getServerSideProps/API routes, or routing middleware), pass the request as the second argument: await evaluate([flagA, flagB], request).

evaluate() always evaluates flags at request time. It is not for reading precomputed (static) values — for those, use getPrecomputed (or call the flag with the code, await myFlag(code, flagGroup)).

Adapters can opt into batching by implementing the optional bulkDecide hook. The Vercel adapter (@flags-sdk/vercel) implements it — roughly a 10x reduction in evaluation time when resolving hundreds of flags. See references/providers.md — Custom Adapters for implementing bulkDecide, and references/api.md — evaluate for the full signature.

Flags Explorer setup

Next.js (App Router)

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

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

With external provider data

When using a third-party provider alongside Vercel Flags, combine their data with mergeProviderData. Each provider adapter exports its own getProviderData — see the provider-specific examples in references/providers.md.

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'))"

Use a separate FLAGS_SECRET value for each environment (Development, Preview, Production), and mark the Preview and Production values as Sensitive. Run the generator once per environment to produce distinct values, then store each on Vercel:

vercel env add FLAGS_SECRET production --sensitive --value <production-secret>
vercel env add FLAGS_SECRET preview --sensitive --value <preview-secret>
vercel env add FLAGS_SECRET development --value <development-secret>

Then run vc env pull to sync to local.

Precompute pattern

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 that returns an object with origin and decide. For the full pattern (including default adapter and singleton client examples), see references/providers.md.

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, toolbar, App Router, Pages Router, middleware/proxy, precompute, dedupe, dashboard pages, marketing pages, suspense fallbacks
• references/sveltekit.md: SvelteKit quickstart, toolbar, hooks setup, precompute with reroute + middleware, dashboard pages, marketing pages
• references/providers.md: All provider adapters — Vercel, Global 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