Dominik Ferber
e7360242b0
Lifts the per-request evaluation pipeline into src/shared/ and points both frameworks at it, so behavior is defined once. The Next-specific isInternalNextError check becomes an injected `isFrameworkError` hook on applyResult (Next passes it; SvelteKit defaults to none). - shared/evaluation.ts: evaluationCache (+ getUsedFlags accessor), getEntities + identify-args dedupe, and applyResult (cache -> override -> produce -> defaultValue/error -> report). - shared/flag-meta.ts: resolveAdapter, getDecide (with adapter validation), getIdentify, getOrigin. SvelteKit's flag() now runs through this pipeline. This is a behavior change that brings it to parity with Next — SvelteKit now: - falls back to defaultValue when decide throws or returns undefined - honors config.reportValue / adapter.config.reportValue - resolves adapter.origin (value or (key) => origin) - dedupes evaluation via the shared headers-keyed cache (replacing the per-store usedFlags/identifiers maps); createHandle's transformPageChunk reads used flags via getUsedFlags() - getProviderData emits defaultValue + declaredInCode Adds `defaultValue?` to the SvelteKit Flag type (additive) and 9 parity tests. Next's public .d.ts is unchanged. Co-Authored-By: Claude Opus 4.8 (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.