Svelte

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.

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

Client-side setup

Install posthog-js using your package manager:

npm install --save posthog-js

yarn add posthog-js

pnpm add posthog-js

bun add posthog-js

If your site sets a Content-Security-Policy, it needs to allow PostHog. This applies to the snippet and to package installs alike: the SDK lazy-loads extra bundles (session replay, surveys) from PostHog's CDN, and sends events to the ingestion host. PostHog serves from subdomains of posthog.com that change over time, so allow the wildcard:

PostHog AI

script-src 'self' https://*.posthog.com;
connect-src 'self' https://*.posthog.com;
worker-src 'self' blob: data:;

script-src covers the snippet and the lazy-loaded bundles, connect-src covers event ingestion and feature flags, and worker-src covers session replay. The toolbar needs a few more, or use a reverse proxy so everything is first-party. Failing to do so causes silent failures where capture and identify calls never send, so the integration looks complete while zero events arrive. Remember connect-src falls back to default-src, so default-src 'self' blocks event delivery even when the script itself is bundled.

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.

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: '2026-05-30',
    })
  }

  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.

Use a stable ID from your auth system when possible, not an email or display name. Send those as person properties instead. If your app has no other stable key, email works as a fallback if they are unique. Never a shared literal like "anonymous" or "user", which pools many people onto one person and corrupts their data. When no ID is available at all, skip the identify and retain the anonymous distinct ID that's automatically assigned.

Call posthog.reset() on logout, so the next person to use the browser doesn't inherit the last one's identity.

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

If your app calls your own backend, tracing_headers adds X-POSTHOG-DISTINCT-ID and X-POSTHOG-SESSION-ID to matching fetch and XMLHttpRequest requests. This lets server-side SDKs link backend events, errors, and LLM traces back to frontend sessions and replays. Use hostnames only, without protocols or paths.

posthog.init('<ph_project_token>', {
  api_host: 'https://us.i.posthog.com',
  // Optional: send PostHog session/user context to your backend
  tracing_headers: ['api.example.com'],
})

This works in local development too, but match on the hostname alone: use 'localhost', not 'localhost:3000'. Ports are never part of a hostname, so a value with one in it never matches anything. localhost and 127.0.0.1 are also different hostnames — use whichever your app actually calls.

Tracing headers help you attribute events across front and backend consistently. When this isn't available, use your server-side stable IDs to deduce the matching distinctId, and pass it in when capturing the event.

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

Server-side setup

Install posthog-node using your package manager:

npm install posthog-node --save

yarn add posthog-node

pnpm add posthog-node

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:

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).

<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.

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:

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