set-up-feature-flags

name	set-up-feature-flags
description	Use when adding feature flags to a frontend — wires the vendor-agnostic OpenFeature SDK (a useFlag hook in React, a composable in Vue) with safe defaults, an evaluation context for user targeting, a dev override, and a pattern for gating routes behind flags. Swap providers (PostHog/GrowthBook/LaunchDarkly/…) in one line.

Set Up Feature Flags

1. Audit current state

grep -E '"(@openfeature/web-sdk|@openfeature/react-sdk)"' package.json 2>/dev/null
grep -rn "import.meta.env.VITE_FEATURE_\|featureFlag\|useFlag" src/ 2>/dev/null | head

Look for ad-hoc env-var flags (VITE_FEATURE_X) — those are build-time toggles, not runtime flags; this skill replaces them where you need runtime control. Pairs with set-up-auth (targeting context = the logged-in user) and set-up-routing (flag-gated routes).

2. Decide what to do

• No flags → full setup.
• Env-var toggles only → migrate the ones that need runtime/gradual rollout to OpenFeature; leave pure build-time switches as env.
• Vendor SDK used directly in components → put it behind the OpenFeature seam (step 5).

3. Detect framework

React → @openfeature/react-sdk (provider + useFlag). Vue → @openfeature/web-sdk + a small composable.

4. Install

pnpm add @openfeature/web-sdk @openfeature/react-sdk   # Vue: just @openfeature/web-sdk

5. Set the provider (vendor-agnostic seam)

// src/libs/featureFlags.ts
import { OpenFeature, InMemoryProvider } from '@openfeature/web-sdk';

// Dev/test: in-memory flags. Prod: your vendor's OpenFeature provider
// (PostHog / GrowthBook / LaunchDarkly / ConfigCat / Flagsmith) — a one-line swap.
const provider = new InMemoryProvider({
  'new-dashboard': { variants: { on: true, off: false }, defaultVariant: 'off', disabled: false },
});

export function initFeatureFlags() {
  void OpenFeature.setProviderAndWait(provider);
}

6. React — OpenFeatureProvider + useFlag

// main.tsx
import { OpenFeatureProvider } from '@openfeature/react-sdk';
import { initFeatureFlags } from '@/libs/featureFlags';

initFeatureFlags();

<OpenFeatureProvider>
  <App />
</OpenFeatureProvider>

// usage — query-style hook with a mandatory safe default
import { useFlag } from '@openfeature/react-sdk';

function Dashboard() {
  const { value: showNew } = useFlag('new-dashboard', false); // default = control
  return showNew ? <NewDashboard /> : <ClassicDashboard />;
}

The React SDK re-renders on flag/context change and supports Suspense while the provider initializes.

7. Vue — a composable

// src/composables/useFlag.ts
import { OpenFeature } from '@openfeature/web-sdk';
import { ref, onUnmounted } from 'vue';

export function useFlag(key: string, defaultValue: boolean) {
  const client = OpenFeature.getClient();
  const value = ref(client.getBooleanValue(key, defaultValue));
  const handler = () => { value.value = client.getBooleanValue(key, defaultValue); };
  client.addHandler('PROVIDER_CONFIGURATION_CHANGED', handler);
  onUnmounted(() => client.removeHandler('PROVIDER_CONFIGURATION_CHANGED', handler));
  return value;
}

8. Targeting context (who gets the flag)

Set the evaluation context from the current user — the provider targets on it (plan, % rollout, allow-list). Update it on login (set-up-auth):

import { OpenFeature } from '@openfeature/web-sdk';
await OpenFeature.setContext({ targetingKey: user.id, plan: user.plan, country: user.country });

Never hand-code if (user.id === 'me') — that's what targeting is for.

9. Gate a route behind a flag

// React (TanStack Router): src/routes/new-dashboard.tsx
import { OpenFeature } from '@openfeature/web-sdk';

export const Route = createFileRoute('/new-dashboard')({
  beforeLoad: () => {
    if (!OpenFeature.getClient().getBooleanValue('new-dashboard', false)) throw notFound();
  },
});

Dev override: seed the InMemoryProvider from a local panel / localStorage so you can flip flags without the backend; OpenFeature also supports a multi-provider for layering overrides on top of the real provider.

10. Verify

pnpm tsc --noEmit
pnpm dev

Flip defaultVariant (or the dev override) → the gated UI/route appears; the safe default (control) renders when the provider is unreachable.

References

• ./feature-flags-patterns.md — vendor-agnostic seam, fail-closed defaults, targeting-not-hardcoding, flags-are-debt cleanup, build-time-env vs runtime-flag.
• ../set-up-auth/SKILL.md — the user that feeds the targeting context.
• ../set-up-routing/SKILL.md — the route guard hook.