SDK Overview

Featureflip provides server-side and client-side SDKs. Pick the one that matches where your code runs.

Which SDK should I use?

Are you evaluating flags in a backend service (Node.js, Python, C#, Java, Go, PHP, Ruby)? → Use a server-side SDK.

Are you evaluating flags in a browser or frontend app? → Using React? Use the React SDK. → Any other framework (or no framework)? Use the JavaScript / TypeScript SDK.

Are you evaluating flags in a mobile app? → iOS / macOS / tvOS / watchOS? Use the Swift SDK. → Flutter? Use the Flutter SDK. → Android? Use the Android SDK.

Client-side vs server-side

	Server-side	Client-side
Use case	Backend services, APIs, server-rendered pages	Browser apps, SPAs, frontend frameworks
SDK key	Server SDK key — keep secret, never expose to users	Client SDK key — safe to embed in frontend code
Flag visibility	All flags in the project	Only flags marked Client-side visible
Evaluation context	Multi-user — pass a different context per evaluation call	Single-user — set context once, call `identify()` on login
Real-time updates	SSE streaming or polling	SSE streaming
Security	Key stays on your server	Key is public by design — only exposes client-visible flag values

SDK keys

Server-side and client-side SDKs use different keys. You can find both in your project settings.

Server SDK keys have access to all flags and should never be included in client-side bundles.
Client SDK keys only return flags explicitly marked as “Client-side visible” in the dashboard. They are safe to ship in browser code.

Client-side SDK quick start

JavaScript / TypeScript SDK

npm install @featureflip/browser-sdk

import { FeatureflipClient } from '@featureflip/browser-sdk';

const client = FeatureflipClient.get({
  clientKey: 'your-client-sdk-key',
});

await client.initialize();

const showBanner = client.boolVariation('show-banner', false);

See the full JavaScript / TypeScript SDK reference.

React SDK

npm install @featureflip/react-sdk @featureflip/browser-sdk

import { FeatureflipProvider, useFeatureFlag } from '@featureflip/react-sdk';

function App() {
  return (
    <FeatureflipProvider clientKey="your-client-sdk-key">
      <Banner />
    </FeatureflipProvider>
  );
}

function Banner() {
  const showBanner = useFeatureFlag('show-banner', false);
  if (!showBanner) return null;
  return <div>New feature available!</div>;
}

See the full React SDK reference.

Swift SDK

In Xcode: File → Add Package Dependencies, then enter the repository URL.

import Featureflip

let config = FeatureflipConfig(clientKey: "your-client-sdk-key")
let client = FeatureflipClient(config: config)
await client.initialize()

let showBanner = client.boolVariation("show-banner", default: false)

See the full Swift SDK reference.

Android SDK

dependencies {
    implementation("io.featureflip:featureflip-android:1.0.0")
}

import io.featureflip.android.FeatureflipClient
import io.featureflip.android.FeatureflipConfig

val config = FeatureflipConfig(clientKey = "your-client-sdk-key")
val client = FeatureflipClient.create(config)
client.initialize()

val showBanner = client.boolVariation("show-banner", false)

See the full Android SDK reference.

Server-side SDKs

These SDKs use server SDK keys and are intended for backends or other controlled environments.

Client-side SDKs

JavaScript / TypeScript — framework-agnostic, works with any frontend
React SDK — React bindings with provider and hooks
Swift SDK — iOS, macOS, tvOS, and watchOS
Flutter SDK — cross-platform Flutter apps
Android SDK — Android and Kotlin/JVM