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Integration Sveltekit

PostHogOfficialCreated by PostHog

  • Skill
  • Tool integration
  • Data and AI
  • MIT
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From the author

PostHog integration for SvelteKit applications
— the description by PostHog

AutoGineer summary

Written by AutoGineer from the skill’s own files, not by the author

This skill provides a comprehensive, step-by-step procedure for adding PostHog analytics to SvelteKit projects. It covers client-side initialization, server-side tracking, user identification, and error tracking. The workflow emphasizes best practices such as using environment variables, implementing reverse proxies to bypass ad blockers, and maintaining consistency with provided example patterns.

What it does

  • Guides users through a multi-step integration process for PostHog in SvelteKit.
  • Provides patterns for both client-side and server-side event tracking.
  • Includes instructions for user identification and error tracking implementation.
  • Details the configuration of reverse proxies to improve data reliability.
  • Requires the use of environment variables for secure credential management.
  • Mandates validation of Svelte components using provided tooling.

When to use it

  • When you need to add analytics to a new or existing SvelteKit application.
  • When you want to track user behavior across both client and server environments.
  • When you need to implement robust error tracking within a SvelteKit project.
  • When you are setting up session replay and require specific SvelteKit configuration adjustments.

Advantages

  • Ensures a consistent and reliable integration pattern.
  • Promotes secure handling of API keys via environment variables.
  • Addresses common SvelteKit-specific pitfalls like SSR compatibility.
  • Facilitates accurate user correlation between frontend and backend.

Usage examples

“How do I start integrating PostHog into my SvelteKit app?”
The agent initiates the workflow, starting with the 'PostHog Setup - Begin' phase to analyze your project structure and create an event tracking plan.
“How should I handle server-side tracking in SvelteKit?”
The agent directs you to use the singleton pattern in 'src/lib/server/posthog.ts' as shown in the reference example.

Install

  1. Download the skill as a ZIP:

    https://mp-staging.autogineer.com/agent-skills/posthog/integration-sveltekit/download

  2. Unzip it into the cross-client skills folder of your project:

    mkdir -p .agents/skills && unzip integration-sveltekit.zip -d .agents/skills

Documentation by PostHog

PostHog integration for SvelteKit

This skill helps you add PostHog analytics to SvelteKit applications.

Workflow

Follow these steps in order to complete the integration:

  1. basic-integration-1.0-begin.md - PostHog Setup - Begin ← Start here

  2. basic-integration-1.1-edit.md - PostHog Setup - Edit

  3. basic-integration-1.2-revise.md - PostHog Setup - Revise

  4. basic-integration-1.3-conclude.md - PostHog Setup - Conclusion

Reference files

  • references/EXAMPLE.md - SvelteKit example project code

  • references/svelte.md - Svelte - docs

  • references/identify-users.md - Identify users - docs

  • references/basic-integration-1.0-begin.md - PostHog setup - begin

  • references/basic-integration-1.1-edit.md - PostHog setup - edit

  • references/basic-integration-1.2-revise.md - PostHog setup - revise

  • references/basic-integration-1.3-conclude.md - PostHog setup - conclusion

The example project shows the target implementation pattern. Consult the documentation for API details.

Key principles

  • Environment variables: Always use environment variables for PostHog keys. Never hardcode them.

  • Minimal changes: Add PostHog code alongside existing integrations. Don't replace or restructure existing code.

  • Match the example: Your implementation should follow the example project's patterns as closely as possible.

Framework guidelines

  • Set paths.relative to false in svelte.config.js — this is required for PostHog session replay to work correctly with SSR and is easy to miss

  • Use the Svelte MCP server tools to check Svelte documentation (list-sections, get-documentation) and validate components (svelte-autofixer) — always run svelte-autofixer on new or modified .svelte files before finishing

Identifying users

Identify users during login and signup events. Refer to the example code and documentation for the correct identify pattern for this framework. If both frontend and backend code exist, pass the client-side session and distinct ID using X-POSTHOG-DISTINCT-ID and X-POSTHOG-SESSION-ID headers to maintain correlation.

Error tracking

Add PostHog error tracking to relevant files, particularly around critical user flows and API boundaries.

Files

references/EXAMPLE.md

PostHog SvelteKit Example Project

Repository: https://github.com/PostHog/context-mill Path: basics/sveltekit

README.md

SvelteKit PostHog example

This example demonstrates how to integrate PostHog with a SvelteKit application, including:

  • Client-side PostHog initialization using SvelteKit hooks

  • Server-side PostHog tracking with the Node.js SDK

  • Reverse proxy to avoid ad blockers

  • User identification and event tracking

  • Error tracking with captureException

  • Session replay configuration

Getting started

1. Install dependencies
npm install
2. Configure environment variables

Copy the example environment file and add your PostHog credentials:

cp .env.example .env

Edit .env with your PostHog project token:

PUBLIC_POSTHOG_PROJECT_TOKEN=your_posthog_project_token_here
PUBLIC_POSTHOG_HOST=https://us.i.posthog.com

You can find your project token in your PostHog project settings.

3. Run the development server
npm run dev

Open http://localhost:5173 to view the app.

Project structure

src/
├── lib/
│   ├── auth.svelte.ts              # Auth context with Svelte 5 runes
│   ├── components/
│   │   └── Header.svelte           # Navigation component
│   └── server/
│       └── posthog.ts              # Server-side PostHog singleton
├── routes/
│   ├── +layout.svelte              # Root layout with auth provider
│   ├── +page.svelte                # Home/login page
│   ├── burrito/
│   │   └── +page.svelte            # Event tracking demo
│   ├── profile/
│   │   └── +page.svelte            # Error tracking demo
│   └── api/
│       └── auth/
│           └── login/
│               └── +server.ts      # Login API with server-side tracking
├── hooks.client.ts                 # Client-side PostHog init + error handling
├── hooks.server.ts                 # Server hooks with reverse proxy
├── app.css                         # Global styles
└── app.html                        # HTML template

Key integration points

Client-side initialization (src/hooks.client.ts)

PostHog is initialized in the SvelteKit client hooks init function, which runs once when the app starts:

import posthog from 'posthog-js';

export async function init() {
  posthog.init(PUBLIC_POSTHOG_PROJECT_TOKEN, {
    api_host: '/ingest',
    ui_host: 'https://us.posthog.com',
    defaults: '2026-01-30',
    capture_exceptions: true
  });
}
Server-side tracking (src/lib/server/posthog.ts)

A singleton pattern ensures one PostHog client instance for server-side tracking:

import { PostHog } from 'posthog-node';

let posthogClient: PostHog | null = null;

export function getPostHogClient() {
  if (!posthogClient) {
    posthogClient = new PostHog(PUBLIC_POSTHOG_PROJECT_TOKEN, {
      host: PUBLIC_POSTHOG_HOST,
      flushAt: 1,
      flushInterval: 0
    });
  }
  return posthogClient;
}
Reverse proxy (src/hooks.server.ts)

The server hooks handle proxies requests through /ingest to avoid ad blockers:

export const handle: Handle = async ({ event, resolve }) => {
  if (event.url.pathname.startsWith('/ingest')) {
    const pathname = event.url.pathname.replace('/ingest', '');
    const host = pathname.startsWith('/static')
      ? 'https://us-assets.i.posthog.com'
      : 'https://us.i.posthog.com';
    // Proxy to PostHog...
  }
  return resolve(event);
};
User identification

When a user logs in, they are identified in PostHog:

import posthog from 'posthog-js';

// On login
posthog.identify(userId, { username });
posthog.capture('user_logged_in', { username });

// On logout
posthog.capture('user_logged_out');
posthog.reset();
Error tracking

Errors are automatically captured via the handleError hook:

export const handleError: HandleClientError = async ({ error }) => {
  posthog.captureException(error);
  return { message: 'An error occurred' };
};

You can also manually capture errors:

try {
  // Some operation
} catch (err) {
  posthog.captureException(err);
}
Session replay configuration

For session replay to work correctly, add this to svelte.config.js:

export default {
  kit: {
    paths: {
      relative: false
    }
  }
};

Features demonstrated

  1. Login page (/) - User authentication with PostHog identification

  2. Burrito page (/burrito) - Custom event tracking with properties

  3. Profile page (/profile) - Error tracking demonstration

Learn more

.env.example

# PostHog configuration
# Get your PostHog project token from: https://app.posthog.com/project/settings
PUBLIC_POSTHOG_PROJECT_TOKEN=your_posthog_project_token_here
PUBLIC_POSTHOG_HOST=https://us.i.posthog.com

.npmrc

engine-strict=true

src/app.d.ts

// See https://svelte.dev/docs/kit/types#app.d.ts
// for information about these interfaces
declare global {
	namespace App {
		// interface Error {}
		// interface Locals {}
		// interface PageData {}
		// interface PageState {}
		// interface Platform {}
	}
}

export {};

src/app.html

<!doctype html>
<html lang="en">
	<head>
		<meta charset="utf-8" />
		<meta name="viewport" content="width=device-width, initial-scale=1" />
		%sveltekit.head%
	</head>
	<body data-sveltekit-preload-data="hover">
		<div style="display: contents">%sveltekit.body%</div>
	</body>
</html>

src/hooks.client.ts

import posthog from 'posthog-js';
import { PUBLIC_POSTHOG_PROJECT_TOKEN } from '$env/static/public';
import type { HandleClientError } from '@sveltejs/kit';

// Initialize PostHog when the app starts in the browser
export async function init() {
	posthog.init(PUBLIC_POSTHOG_PROJECT_TOKEN, {
		api_host: '/ingest',
		ui_host: 'https://us.posthog.com',
  defaults: '2026-01-30',
		capture_exceptions: true
	});
}

// Capture client-side errors with PostHog
export const handleError: HandleClientError = async ({ error, status, message }) => {
	posthog.captureException(error);

	return {
		message,
		status
	};
};

src/hooks.server.ts

import type { Handle, HandleServerError } from '@sveltejs/kit';
import { getPostHogClient } from '$lib/server/posthog';

// Handle requests - includes reverse proxy for PostHog
export const handle: Handle = async ({ event, resolve }) => {
	const { pathname } = event.url;

	// Reverse proxy for PostHog - route /ingest requests to PostHog servers
	if (pathname.startsWith('/ingest')) {
		const hostname = pathname.startsWith('/ingest/static/')
			? 'us-assets.i.posthog.com'
			: 'us.i.posthog.com';

		const url = new URL(event.request.url);
		url.protocol = 'https:';
		url.hostname = hostname;
		url.port = '443';
		url.pathname = pathname.replace(/^\/ingest/, '');

		const headers = new Headers(event.request.headers);
		headers.set('host', hostname);
		headers.set('accept-encoding', '');

		const clientIp = event.request.headers.get('x-forwarded-for') || event.getClientAddress();
		if (clientIp) {
			headers.set('x-forwarded-for', clientIp);
		}

		const response = await fetch(url.toString(), {
			method: event.request.method,
			headers,
			body: event.request.body,
			// @ts-expect-error - duplex is required for streaming request bodies
			duplex: 'half'
		});

		return response;
	}

	return resolve(event);
};

// Capture server-side errors with PostHog
export const handleError: HandleServerError = async ({ error, status, message }) => {
	const posthog = getPostHogClient();

	posthog.capture({
		distinctId: 'server',
		event: 'server_error',
		properties: {
			error: error instanceof Error ? error.message : String(error),
			status,
			message
		}
	});

	return {
		message,
		status
	};
};

src/lib/auth.svelte.ts

import { getContext, setContext } from 'svelte';
import posthog from 'posthog-js';
import { browser } from '$app/environment';

export interface User {
	username: string;
	burritoConsiderations: number;
}

const AUTH_KEY = Symbol('auth');

// Class-based auth state using Svelte 5 $state in class fields
// This is the recommended pattern for encapsulating reactive state + behavior
export class AuthState {
	user = $state<User | null>(null);

	constructor() {
		// Restore user from localStorage on creation (browser only)
		if (browser) {
			const storedUsername = localStorage.getItem('currentUser');
			if (storedUsername) {
				this.user = { username: storedUsername, burritoConsiderations: 0 };
			}
		}
	}

	login = async (username: string, password: string): Promise<boolean> => {
		try {
			const response = await fetch('/api/auth/login', {
				method: 'POST',
				headers: { 'Content-Type': 'application/json' },
				body: JSON.stringify({ username, password })
			});

			if (response.ok) {
				const { user: userData } = await response.json();
				this.user = userData as User;

				if (browser) {
					localStorage.setItem('currentUser', username);
					posthog.identify(username, { username });
					posthog.capture('user_logged_in', { username });
				}

				return true;
			}
			return false;
		} catch (error) {
			console.error('Login error:', error);
			return false;
		}
	};

	logout = (): void => {
		if (browser) {
			posthog.capture('user_logged_out');
			posthog.reset();
			localStorage.removeItem('currentUser');
		}
		this.user = null;
	};

	incrementBurritoConsiderations = (): void => {
		if (this.user) {
			this.user = {
				...this.user,
				burritoConsiderations: this.user.burritoConsiderations + 1
			};
		}
	};
}

export function setAuthContext(auth: AuthState) {
	setContext(AUTH_KEY, auth);
}

export function getAuthContext(): AuthState {
	return getContext<AuthState>(AUTH_KEY);
}

src/lib/components/Header.svelte

<script lang="ts">
	import { getAuthContext } from '$lib/auth.svelte';

	const auth = getAuthContext();
</script>

<header class="header">
	<div class="header-container">
		<nav>
			<a href="/">Home</a>
			{#if auth.user}
				<a href="/burrito">Burrito</a>
				<a href="/profile">Profile</a>
			{/if}
		</nav>
		<div class="user-section">
			{#if auth.user}
				<span>Welcome, {auth.user.username}</span>
				<button class="btn-logout" onclick={() => auth.logout()}>Logout</button>
			{/if}
		</div>
	</div>
</header>

src/lib/index.ts

// place files you want to import through the `$lib` alias in this folder.

src/lib/server/posthog.ts

import { PostHog } from 'posthog-node';
import { PUBLIC_POSTHOG_PROJECT_TOKEN, PUBLIC_POSTHOG_HOST } from '$env/static/public';

let posthogClient: PostHog | null = null;

export function getPostHogClient() {
	if (!posthogClient) {
		posthogClient = new PostHog(PUBLIC_POSTHOG_PROJECT_TOKEN, {
			host: PUBLIC_POSTHOG_HOST,
			flushAt: 1,
			flushInterval: 0
		});
	}
	return posthogClient;
}

export async function shutdownPostHog() {
	if (posthogClient) {
		await posthogClient.shutdown();
	}
}

src/routes/+layout.svelte

<script lang="ts">
	import { AuthState, setAuthContext } from '$lib/auth.svelte';
	import Header from '$lib/components/Header.svelte';
	import '../app.css';

	let { children } = $props();

	// Create and provide auth context
	const auth = new AuthState();
	setAuthContext(auth);
</script>

<svelte:head>
	<title>Burrito consideration app</title>
	<meta name="description" content="Consider the potential of burritos with PostHog analytics" />
</svelte:head>

<Header />
<main>
	{@render children()}
</main>

src/routes/+page.svelte

<script lang="ts">
	import { getAuthContext } from '$lib/auth.svelte';

	const auth = getAuthContext();

	let username = $state('');
	let password = $state('');
	let error = $state('');

	async function handleSubmit(e: Event) {
		e.preventDefault();
		error = '';

		try {
			const success = await auth.login(username, password);
			if (success) {
				username = '';
				password = '';
			} else {
				error = 'Please provide both username and password';
			}
		} catch (err) {
			console.error('Login failed:', err);
			error = 'An error occurred during login';
		}
	}
</script>

<div class="container">
	{#if auth.user}
		<h1>Welcome back, {auth.user.username}!</h1>
		<p>You are now logged in. Check out the navigation to explore features.</p>
		<ul>
			<li><a href="/burrito">Consider a burrito</a></li>
			<li><a href="/profile">View your profile</a></li>
		</ul>
	{:else}
		<h1>Welcome to Burrito consideration app</h1>
		<p>Sign in to start considering burritos.</p>

		<form class="form" onsubmit={handleSubmit}>
			<div class="form-group">
				<label for="username">Username:</label>
				<input type="text" id="username" bind:value={username} required />
			</div>

			<div class="form-group">
				<label for="password">Password:</label>
				<input type="password" id="password" bind:value={password} required />
			</div>

			{#if error}
				<p class="error">{error}</p>
			{/if}

			<button type="submit" class="btn-primary">Sign In</button>
		</form>

		<p class="note">
			Enter any username and password to sign in. This is a demo app.
		</p>
	{/if}
</div>

src/routes/api/auth/login/+server.ts

import { json } from '@sveltejs/kit';
import type { RequestHandler } from './$types';
import { getPostHogClient } from '$lib/server/posthog';

const users = new Map<string, { username: string; burritoConsiderations: number }>();

export const POST: RequestHandler = async ({ request }) => {
	const { username, password } = await request.json();

	if (!username || !password) {
		return json({ error: 'Username and password required' }, { status: 400 });
	}

	let user = users.get(username);
	const isNewUser = !user;

	if (!user) {
		user = { username, burritoConsiderations: 0 };
		users.set(username, user);
	}

	// Capture server-side login event with user context
	const posthog = getPostHogClient();
	posthog.withContext(
		{
			distinctId: username,
			personProperties: {
				username,
				createdAt: isNewUser ? new Date().toISOString() : undefined
			}
		},
		() => {
			posthog.capture({
				event: 'server_login',
				properties: {
					isNewUser,
					source: 'api'
				}
			});
		}
	);

	// Flush events to ensure they're sent
	await posthog.flush();

	return json({ success: true, user });
};

src/routes/burrito/+page.svelte

<script lang="ts">
	import { goto } from '$app/navigation';
	import { browser } from '$app/environment';
	import posthog from 'posthog-js';
	import { getAuthContext } from '$lib/auth.svelte';

	const auth = getAuthContext();

	let hasConsidered = $state(false);

	// Redirect to home if not logged in
	$effect(() => {
		if (browser && !auth.user) {
			goto('/');
		}
	});

	function handleConsideration() {
		if (!auth.user) return;

		auth.incrementBurritoConsiderations();
		hasConsidered = true;
		setTimeout(() => (hasConsidered = false), 2000);

		// Capture burrito consideration event with PostHog
		posthog.capture('burrito_considered', {
			total_considerations: auth.user.burritoConsiderations,
			username: auth.user.username
		});
	}
</script>

<div class="container">
	{#if auth.user}
		<h1>Burrito consideration zone</h1>
		<p>This is where you consider the infinite potential of burritos.</p>
		<p>Current considerations: <strong>{auth.user.burritoConsiderations}</strong></p>

		<button class="btn-burrito" onclick={handleConsideration}>
			I have considered the burrito potential
		</button>

		{#if hasConsidered}
			<p class="success">
				Thank you for your consideration! Count: {auth.user.burritoConsiderations}
			</p>
		{/if}

		<div class="note">
			<p>Each consideration is tracked as a PostHog event with custom properties.</p>
		</div>
	{:else}
		<p>Please log in to consider burritos.</p>
	{/if}
</div>

src/routes/profile/+page.svelte

<script lang="ts">
	import { goto } from '$app/navigation';
	import { browser } from '$app/environment';
	import posthog from 'posthog-js';
	import { getAuthContext } from '$lib/auth.svelte';

	const auth = getAuthContext();

	// Redirect to home if not logged in
	$effect(() => {
		if (browser && !auth.user) {
			goto('/');
		}
	});

	function triggerTestError() {
		try {
			throw new Error('Test error for PostHog error tracking');
		} catch (err) {
			posthog.captureException(err);
			console.error('Captured error:', err);
			alert('Error captured and sent to PostHog!');
		}
	}
</script>

<div class="container">
	{#if auth.user}
		<h1>User profile</h1>

		<div class="stats">
			<h2>Your information</h2>
			<p><strong>Username:</strong> {auth.user.username}</p>
			<p><strong>Burrito considerations:</strong> {auth.user.burritoConsiderations}</p>
		</div>

		<h2 style="margin-top: 2rem;">Error tracking demo</h2>
		<p>Click the button below to trigger a test error that will be captured by PostHog.</p>

		<button class="btn-primary" onclick={triggerTestError} style="margin-top: 1rem;">
			Trigger test error (for PostHog)
		</button>

		<div class="note">
			<p>This demonstrates PostHog's error tracking capabilities.</p>
			<p>The error will appear in your PostHog error tracking dashboard.</p>
		</div>
	{:else}
		<p>Please log in to view your profile.</p>
	{/if}
</div>

static/robots.txt

# allow crawling everything by default
User-agent: *
Disallow:

svelte.config.js

import adapter from '@sveltejs/adapter-auto';
import { vitePreprocess } from '@sveltejs/vite-plugin-svelte';

/** @type {import('@sveltejs/kit').Config} */
const config = {
	// Consult https://svelte.dev/docs/kit/integrations
	// for more information about preprocessors
	preprocess: vitePreprocess(),

	kit: {
		// adapter-auto only supports some environments, see https://svelte.dev/docs/kit/adapter-auto for a list.
		// If your environment is not supported, or you settled on a specific environment, switch out the adapter.
		// See https://svelte.dev/docs/kit/adapters for more information about adapters.
		adapter: adapter(),
		// Required for PostHog session replay to work correctly with SSR
		paths: {
			relative: false
		}
	}
};

export default config;

vite.config.ts

import { sveltekit } from '@sveltejs/kit/vite';
import { defineConfig } from 'vite';

export default defineConfig({
	plugins: [sveltekit()]
});

Requirements and compatibility

Works with: Claude Code, Claude apps, OpenAI Codex, GitHub Copilot, Cursor, Gemini CLI, Kiro, OpenCode, Goose, Amp, Roo Code, JetBrains Junie, Factory Droid, Devin (ex-Windsurf), Mistral Vibe, OpenHands, Letta

“Works with” means the tool reads the open Agent Skills format; a skill written for one tool may use features others lack.

Needs: Needs an MCP server

Security

No issues found by our static scan· No issues found

A static scan is not a guarantee. Skills are third-party content: review a skill before you let an agent use it.

Attribution and licence

Created by
PostHog
Publisher
PostHog (Official)
Source
PostHog/skills · skills/posthog/integration/skills/sveltekit · commit a5d30e7
Licence
MIT License (redistribution allowed)
  • LICENSE (added by AutoGineer from LICENSE)
  • ATTRIBUTION.md (written by AutoGineer)
Synced
Also distributed in
PostHog/skills

AutoGineer is not affiliated with or endorsed by PostHog.

Download ZIP

The ZIP holds the skill’s files, its licence files and an ATTRIBUTION.md. Download integration-sveltekit.zip