Feature Flags Nextjs

Next.js - Docs

PostHog makes it easy to get data about traffic and usage of your Next.js app. Integrating PostHog into your site enables analytics about user behavior, custom events capture, session recordings, feature flags, and more.

This guide walks you through integrating PostHog into your Next.js app using the React and the Node.js SDKs.

You can see a working example of this integration in our Next.js demo app.

Next.js has both client and server-side rendering, as well as pages and app routers. We'll cover all of these options in this guide.

Try @posthog/next (pre-release): A simplified Next.js integration with synchronized client/server identity, server-side flag bootstrapping, and a built-in API proxy. Read the setup guide →

Prerequisites

To follow this guide along, you need:

1. A PostHog instance (either Cloud or self-hosted)

2. A Next.js application

Beta: integration via LLM

Install PostHog for Next.js in seconds with our wizard by running this prompt with LLM coding agents like Cursor and Bolt, or by running it in your terminal.

npx @posthog/wizard@latest

Learn more

Or, to integrate manually, continue with the rest of this guide.

Client-side setup

Install posthog-js using your package manager:

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npm

npm install --save posthog-js

Yarn

yarn add posthog-js

pnpm

pnpm add posthog-js

Bun

bun add posthog-js

Add your environment variables to your .env.local file and to your hosting provider (e.g. Vercel, Netlify, AWS). You can find your project token in your project settings.

.env.local

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NEXT_PUBLIC_POSTHOG_TOKEN=<ph_project_token>
NEXT_PUBLIC_POSTHOG_HOST=https://us.i.posthog.com

These values need to start with NEXT_PUBLIC_ to be accessible on the client-side.

Integration

Next.js provides the instrumentation-client.ts|js file for client-side setup. Add it to the root of your Next.js app (for both app and pages router) and initialize PostHog in it like this:

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instrumentation-client.js

import posthog from 'posthog-js'
posthog.init(process.env.NEXT_PUBLIC_POSTHOG_TOKEN, {
  api_host: process.env.NEXT_PUBLIC_POSTHOG_HOST,
  defaults: '2026-01-30'
});

instrumentation-client.ts

import posthog from 'posthog-js'
posthog.init(process.env.NEXT_PUBLIC_POSTHOG_TOKEN!, {
  api_host: process.env.NEXT_PUBLIC_POSTHOG_HOST,
  defaults: '2026-01-30'
});

Bootstrapping with instrumentation-client

When using instrumentation-client, the values you pass to posthog.init remain fixed for the entire session. This means bootstrapping only works if you evaluate flags before your app renders (for example, on the server).

If you need flag values after the app has rendered, you’ll want to:

• Evaluate the flag on the server and pass the value into your app, or

• Evaluate the flag in an earlier page/state, then store and re-use it when needed.

Both approaches avoid flicker and give you the same outcome as bootstrapping, as long as you use the same distinct_id across client and server.

See the bootstrapping guide for more information.

Identifying users

Identifying users is required. Call posthog.identify('your-user-id') after login to link events to a known user. This is what connects frontend event captures, session replays, LLM traces, and error tracking to the same person — and lets backend events link back too.

See our guide on identifying users for how to set this up.

Set up a reverse proxy (recommended)

We recommend setting up a reverse proxy, so that events are less likely to be intercepted by tracking blockers.

We have our own managed reverse proxy service, which is free for all PostHog Cloud users, routes through our infrastructure, and makes setting up your proxy easy.

If you don't want to use our managed service then there are several other options for creating a reverse proxy, including using Cloudflare, AWS Cloudfront, and Vercel.

Grouping products in one project (recommended)

If you have multiple customer-facing products (e.g. a marketing website + mobile app + web app), it's best to install PostHog on them all and group them in one project.

This makes it possible to track users across their entire journey (e.g. from visiting your marketing website to signing up for your product), or how they use your product across multiple platforms.

Add IPs to Firewall/WAF allowlists (recommended)

For certain features like heatmaps, your Web Application Firewall (WAF) may be blocking PostHog’s requests to your site. Add these IP addresses to your WAF allowlist or rules to let PostHog access your site.

EU: 3.75.65.221, 18.197.246.42, 3.120.223.253

US: 44.205.89.55, 52.4.194.122, 44.208.188.173

These are public, stable IPs used by PostHog services (e.g., Celery tasks for snapshots).

Accessing PostHog

Once initialized in instrumentation-client.js|ts, import posthog from posthog-js anywhere and call the methods you need on the posthog object.

JavaScript

PostHog AI

'use client'
import posthog from 'posthog-js'
export default function Home() {
  return (
    <div>
      <button onClick={() => posthog.capture('test_event')}>
        Click me for an event
      </button>
    </div>
  );
}

Using React hooks

The React feature flag hooks work automatically when PostHog is initialized via instrumentation-client.ts. The hooks use the initialized posthog-js singleton:

JavaScript

PostHog AI

'use client'
import { useFeatureFlagEnabled } from 'posthog-js/react'
export default function FeatureComponent() {
  const showNewFeature = useFeatureFlagEnabled('new-feature')
  return showNewFeature ? <NewFeature /> : <OldFeature />
}

Usage

See the React SDK docs for examples of how to use:

• posthog-js functions like custom event capture, user identification, and more.

• Feature flags including variants and payloads.

You can also read the full posthog-js documentation for all the usable functions.

Server-side analytics

Next.js enables you to both server-side render pages and add server-side functionality. To integrate PostHog into your Next.js app on the server-side, you can use the Node SDK.

First, install the posthog-node library:

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npm

npm install posthog-node --save

Yarn

yarn add posthog-node

pnpm

pnpm add posthog-node

Bun

bun add posthog-node

Router-specific instructions

App router

For the app router, we can initialize the posthog-node SDK once with a PostHogClient function, and import it into files.

This enables us to send events and fetch data from PostHog on the server – without making client-side requests.

JavaScript

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// app/posthog.js
import { PostHog } from 'posthog-node'
export default function PostHogClient() {
  const posthogClient = new PostHog(process.env.NEXT_PUBLIC_POSTHOG_TOKEN, {
    host: process.env.NEXT_PUBLIC_POSTHOG_HOST,
    flushAt: 1,
    flushInterval: 0
  })
  return posthogClient
}

Note: Because server-side functions in Next.js can be short-lived, we set flushAt to 1 and flushInterval to 0.

• flushAt sets how many capture calls we should flush the queue (in one batch).

• flushInterval sets how many milliseconds we should wait before flushing the queue. Setting them to the lowest number ensures events are sent immediately and not batched. We also need to call await posthog.shutdown() once done.

To use this client, we import it into our pages and call it with the PostHogClient function:

JavaScript

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import Link from 'next/link'
import PostHogClient from '../posthog'
export default async function About() {
  const posthog = PostHogClient()
  const flags = await posthog.getAllFlags(
    'user_distinct_id' // replace with a user's distinct ID
  );
  await posthog.shutdown()
  return (
    <main>
      <h1>About</h1>
      <Link href="/">Go home</Link>
      { flags['main-cta'] &&
        <Link href="http://posthog.com/">Go to PostHog</Link>
      }
    </main>
  )
}

Pages router

For the pages router, we can use the getServerSideProps function to access PostHog on the server-side, send events, evaluate feature flags, and more.

This looks like this:

JavaScript

PostHog AI

// pages/posts/[id].js
import { useContext, useEffect, useState } from 'react'
import { getServerSession } from "next-auth/next"
import { PostHog } from 'posthog-node'
export default function Post({ post, flags }) {
  const [ctaState, setCtaState] = useState()
  useEffect(() => {
    if (flags) {
      setCtaState(flags['blog-cta'])
    }
  })
  return (
    <div>
      <h1>{post.title}</h1>
      <p>By: {post.author}</p>
      <p>{post.content}</p>
      {ctaState &&
        <p><a href="/">Go to PostHog</a></p>
      }
      <button onClick={likePost}>Like</button>
    </div>
  )
}
export async function getServerSideProps(ctx) {
  const session = await getServerSession(ctx.req, ctx.res)
  let flags = null
  if (session) {
    const client = new PostHog(
      process.env.NEXT_PUBLIC_POSTHOG_TOKEN,
      {
        host: process.env.NEXT_PUBLIC_POSTHOG_HOST,
      }
    )
    flags = await client.getAllFlags(session.user.email);
    client.capture({
      distinctId: session.user.email,
      event: 'loaded blog article',
      properties: {
        $current_url: ctx.req.url,
      },
    });
    await client.shutdown()
  }
  const { posts } = await import('../../blog.json')
  const post = posts.find((post) => post.id.toString() === ctx.params.id)
  return {
    props: {
      post,
      flags
    },
  }
}

Note: Make sure to always call await client.shutdown() after sending events from the server-side. PostHog queues events into larger batches, and this call forces all batched events to be flushed immediately.

Server-side configuration

Next.js overrides the default fetch behavior on the server to introduce their own cache. PostHog ignores that cache by default, as this is Next.js's default behavior for any fetch call.

You can override that configuration when initializing PostHog, but make sure you understand the pros/cons of using Next.js's cache and that you might get cached results rather than the actual result our server would return. This is important for feature flags, for example.

TSX

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posthog.init(process.env.NEXT_PUBLIC_POSTHOG_TOKEN, {
  // ... your configuration
  fetch_options: {
    cache: 'force-cache', // Use Next.js cache
    next_options: {       // Passed to the `next` option for `fetch`
      revalidate: 60,     // Cache for 60 seconds
      tags: ['posthog'],  // Can be used with Next.js `revalidateTag` function
    },
  }
})

Configuring a reverse proxy to PostHog

To improve the reliability of client-side tracking and make requests less likely to be intercepted by tracking blockers, you can setup a reverse proxy in Next.js. Read more about deploying a reverse proxy using Next.js rewrites, Next.js middleware, and Vercel rewrites.

Further reading

• How to set up Next.js analytics, feature flags, and more

• How to set up Next.js pages router analytics, feature flags, and more

• How to set up Next.js A/B tests

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