Integration Sveltekit

Svelte - Docs

PostHog makes it easy to get data about traffic and usage of your Svelte 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 SvelteKit app using the JavaScript Web and Node.js SDKs.

Beta: integration via LLM

Install PostHog for Svelte 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:

PostHog AI

npm

npm install --save posthog-js

Yarn

yarn add posthog-js

pnpm

pnpm add posthog-js

Bun

bun add posthog-js

Then, if you haven't created a root layout already, create a new file called +layout.js in your src/routes folder In this file, check the environment is the browser, and initialize PostHog if so. You can get both your API key and instance address in your project settings.

routes/+layout.js

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import posthog from 'posthog-js'
import { browser } from '$app/environment';
export const load = async () => {
  if (browser) {
    posthog.init('<ph_project_token>', {
      api_host: 'https://us.i.posthog.com',
      defaults: '2026-01-30',
    })
  }
  return
};

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.

❗️ If you intend on using session replays with a server-side rendered Svelte app ensure that your asset URLs are configured to be relative.

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).

Server-side setup

Install posthog-node using your package manager:

PostHog AI

npm

npm install posthog-node --save

Yarn

yarn add posthog-node

pnpm

pnpm add posthog-node

Bun

bun add posthog-node

Then, initialize the PostHog Node client where you'd like to use it on the server side. For example, in a load function:

routes/+page.server.js

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import { PostHog } from 'posthog-node';
export async function load() {
  const posthog = new PostHog('<ph_project_token>', { host: 'https://us.i.posthog.com' });
  posthog.capture({
    distinctId: 'distinct_id_of_the_user',
    event: 'event_name',
  })
  await posthog.shutdown()
}

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

Feature flags

To use client-side feature flags, import PostHog into your Svelte component and check if the feature is enabled (while ensuring the code only runs in the browser).

routes/+page.svelte

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<script>
  import posthog from 'posthog-js'
  import { browser } from '$app/environment'
  import { onMount } from 'svelte'
  let coolFeature = $state(false)
  onMount(() => {
    if (browser) {
      coolFeature = posthog.isFeatureEnabled('cool-feature')
    }
  })
</script>
{#if coolFeature}
  <p>Welcome to the cool feature!</p>
{/if}

To use server-side feature flags, import PostHog into your SvelteKit load function and check if the feature is enabled.

routes/+page.server.js

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import { PostHog } from 'posthog-node';
const client = new PostHog(
  '<ph_project_token>',
  { host: 'https://us.i.posthog.com' }
);
export async function load() {
  const distinctId = 'distinct_id_of_the_user';
  const megaFeature = await client.isFeatureEnabled(
    'mega-feature',
    distinctId
  );
  return {
    megaFeature
  };
}

See our JavaScript Web and Node docs for more details.

Configuring session replay for server-side rendered apps

By default, Svelte uses relative asset paths during server-side rending. This causes issues with PostHog's ability to record sessions.

To fix this, set the config to not use relative paths in svelte.config.js:

JavaScript

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kit: {
     paths: {
         relative: false,
     },
 },

Next steps

For any technical questions for how to integrate specific PostHog features into Svelte (such as analytics, feature flags, A/B testing, surveys, etc.), have a look at our JavaScript Web and Node SDK docs.

Alternatively, the following tutorials can help you get started:

• How to set up Svelte analytics, feature flags, and more

• How to set up A/B tests in Svelte

• How to set up surveys in Svelte

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