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adding-feature-flags

Skill2 starsChanged 6 months agoMITView on GitHub

Add feature flags to an application for gradual rollouts, A/B testing, and kill switches using PostHog, LaunchDarkly, or a simple local implementation. Use when the user says "/adding-feature-flags", "adding feature flags", or asks about adding feature flags.

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We split this skill into the rules it gives an agent

3 rules. About 0 are common, 3 are distinctive, and 0 are very specific to this project.

  • ≈0

    Common · estimate

    Many files like this have a rule of this kind.

  • ≈3

    Distinctive · estimate

    This author's own choices. Worth a look.

  • 0

    Very specific

    Tied to their project's names, files or tools. Probably skip.

  • Common
  • Distinctive
  • Very specific
  1. Distinctive:

    Use this skill when the user asks to add feature flags, feature toggles, gradual rollouts, or A/B testing.

    line 8
  2. Distinctive:

    Install PostHog: if not already present:

    line 12
  3. Distinctive:

    Use feature flags for all user-facing changes during rollout, not just experiments.

    line 73

≈ “Common” is an estimate: at least 1 in 5 of the 308,232 skill files we read have a rule of the same kind, not the same words. “Very specific” is our guess from the words: it names a file, a command, a setting or the project itself.

Keep what fits and drop the rest. Your version, ready for your agents.
Files
SKILL.md

Add Feature Flags

Use this skill when the user asks to add feature flags, feature toggles, gradual rollouts, or A/B testing.

Option A: PostHog Feature Flags (Recommended)

  1. Install PostHog if not already present:

    npm install posthog-js

  2. Use feature flags in code:

    import { useFeatureFlagEnabled } from "posthog-js/react";

function MyComponent() {
  const showNewFeature = useFeatureFlagEnabled("new-checkout-flow");
  if (showNewFeature) return <NewCheckout />;
  return <OldCheckout />;
}

  3. Server-side evaluation — for server components or API routes:

    import { PostHog } from "posthog-node";

const posthog = new PostHog(process.env.POSTHOG_API_KEY!);
const isEnabled = await posthog.isFeatureEnabled("new-checkout-flow", userId);
  4. Create flags in the PostHog dashboard — set up targeting rules based on user properties, percentage rollouts, or cohorts.

Option B: Simple Local Feature Flags

For projects that don't need a third-party service:

  1. Create a flags config — lib/feature-flags.ts:

    export const FLAGS = {
  NEW_CHECKOUT: process.env.NEXT_PUBLIC_FF_NEW_CHECKOUT === "true",
  DARK_MODE: process.env.NEXT_PUBLIC_FF_DARK_MODE === "true",
} as const;

  2. Use in components:

    import { FLAGS } from "@/lib/feature-flags";

function App() {
  return FLAGS.NEW_CHECKOUT ? <NewCheckout /> : <OldCheckout />;
}

  3. Add env vars — add flags to .env and .env.example:

    NEXT_PUBLIC_FF_NEW_CHECKOUT=false
NEXT_PUBLIC_FF_DARK_MODE=true

Notes

  • Use feature flags for all user-facing changes during rollout, not just experiments.
  • Clean up stale flags — remove the flag and the old code path once a feature is fully rolled out.
  • For server-side flags, cache the evaluation result to avoid per-request API calls.
  • Name flags descriptively: new-checkout-flow not flag-1.

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