Dominik Ferber
a2f7367671
shutdown() unwires the stream and polling event handlers, but nothing wired them again. A client initialized after a shutdown opened a new connection and then ignored every datafile that connection delivered, so evaluations returned the default value with an error reason. The failure was silent: the socket was healthy and the fetch happened, only the handlers were missing. wireSourceEvents() now runs at the start of tryInitializeStream() and tryInitializePolling(), the only two places that start a source. Repeat calls cannot duplicate a subscription, because the handlers are stable instance properties and TypedEmitter keeps them in a Set. Two tests cover that invariant: one across a shutdown, and one for a source that starts twice within a single lifecycle, which happens when a failed initialization is followed by an evaluation. This also affects the VercelProvider OpenFeature provider, whose onClose() hook shuts the client down, since section 2.5.2 of the provider spec allows a provider to be initialized again after it is closed. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Flags SDK
The feature flags toolkit for Next.js and SvelteKit.
From the creators of Next.js, the Flags SDK is a free open-source library that gives you the tools you need to use feature flags in Next.js and SvelteKit applications.
- Works with any flag provider, custom setups or no flag provider at all
- Compatible with App Router, Pages Router, and Routing Middleware
- Built for feature flags and experimentation
See flags-sdk.dev for full docs and examples.
Upgrading from version 3? See the Upgrade to v4 guide.
Installation
Install the package using your package manager:
npm install flags
Setup
Create an environment variable called FLAGS_SECRET.
The FLAGS_SECRET value must have a specific length (32 random bytes encoded in base64) to work as an encryption key. Create one using node:
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>
This secret is required to use the SDK. It is used to read overrides and to encrypt flag values in case they are sent to the client and should stay secret.
Usage
Create a file called flags.ts in your project and declare your first feature flag there:
// app/flags.tsx
import { flag } from "flags/next";
export const exampleFlag = flag<boolean>({
key: "example-flag",
decide() {
return true;
},
});
Call your feature flag in a React Server Component:
// app/page.tsx
import { exampleFlag } from "./flags";
export default async function Page() {
const example = await exampleFlag();
return <div>{example ? "Flag is on" : "Flag is off"}</div>;
}
Feature Flags can also be called in Routing Middleware and API Routes.
Adapters
The Flags SDK has adapters for popular feature flag providers including LaunchDarkly, Optimizely, and Statsig.
Documentation
There is a lot more to the Flags SDK than shown in the example above.
See the full documentation and examples on flags-sdk.dev.