Instrument Integration

AI agents: this is one page from PostHog's docs. Full index of Markdown docs for LLMs: https://posthog.com/llms.txt

Astro

Canonical URL: https://posthog.com/docs/libraries/astro

Cloud region: This page uses US Cloud hosts. For an EU Cloud project, use https://eu.i.posthog.com instead of https://us.i.posthog.com (ingestion host), and use https://eu.posthog.com instead of https://us.posthog.com (app host for links).

On this page

• Install with the wizard

• Installation

• Node.js server context and traces

• Identifying users

• Next steps

PostHog makes it easy to get data about traffic and usage of your Astro 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 Astro app using the JavaScript Web SDK.

Install with the wizard

Run this command in your project directory to add PostHog to your Astro project. The wizard detects your framework, installs the SDK, and sets up your first events.

npx @posthog/wizard

Or set it up manually by following the rest of this guide.

Installation

In your src/components folder, create a posthog.astro file:

cd ./src/components
# or 'cd ./src && mkdir components && cd ./components' if your components folder doesn't exist
touch posthog.astro

In this file, add your Web snippet which you can find in your project settings. Be sure to include the is:inline directive to prevent Astro from processing it, or you will get Typescript and build errors that property 'posthog' does not exist on type 'Window & typeof globalThis'.

posthog.astro

---
// src/components/posthog.astro
---
<script is:inline>
  !function(t,e){var o,n,p,r;e.__SV||(window.posthog=e,e._i=[],e.init=function(i,s,a){function g(t,e){var o=e.split(".");2==o.length&&(t=t[o[0]],e=o[1]),t[e]=function(){t.push([e].concat(Array.prototype.slice.call(arguments,0)))}}(p=t.createElement("script")).type="text/javascript",p.crossOrigin="anonymous",p.async=!0,p.src=s.api_host+"/static/array.js",(r=t.getElementsByTagName("script")[0]).parentNode.insertBefore(p,r);var u=e;for(void 0!==a?u=e[a]=[]:a="posthog",u.people=u.people||[],Object.defineProperty(u,"toString",{configurable:!0,enumerable:!0,writable:!0,value:function(t){var e="posthog";return"posthog"!==a&&(e+="."+a),t||(e+=" (stub)"),e}}),Object.defineProperty(u.people,"toString",{configurable:!0,enumerable:!0,writable:!0,value:function(){return u.toString(1)+".people (stub)"}}),o="capture identify alias people.set people.set_once set_config register register_once unregister opt_out_capturing has_opted_out_capturing opt_in_capturing reset isFeatureEnabled onFeatureFlags getFeatureFlag getFeatureFlagResult reloadFeatureFlags group updateEarlyAccessFeatureEnrollment getEarlyAccessFeatures getActiveMatchingSurveys getSurveys getNextSurveyStep onSessionId".split(" "),n=0;n<o.length;n++)g(u,o[n]);e._i.push([i,s,a])},e.__SV=1)}(document,window.posthog||[]);
  posthog.init('<ph_project_token>', {
    api_host:'https://us.i.posthog.com',
    defaults: '2026-08-30'
  })
</script>

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:

Copy

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.

Using with Astro's view transitions (ClientRouter)

If you've opted in to Astro's <ClientRouter> component for client-side navigation, you'll need to add an initialization guard to prevent PostHog from running multiple times during page transitions.

Update your posthog.astro file to wrap the snippet with a check:

posthog.astro

---
// src/components/posthog.astro
---
<script is:inline>
  if (!window.__posthog_initialized) {
    window.__posthog_initialized = true;

    !function(t,e){var o,n,p,r;e.__SV||(window.posthog=e,e._i=[],e.init=function(i,s,a){function g(t,e){var o=e.split(".");2==o.length&&(t=t[o[0]],e=o[1]),t[e]=function(){t.push([e].concat(Array.prototype.slice.call(arguments,0)))}}(p=t.createElement("script")).type="text/javascript",p.crossOrigin="anonymous",p.async=!0,p.src=s.api_host+"/static/array.js",(r=t.getElementsByTagName("script")[0]).parentNode.insertBefore(p,r);var u=e;for(void 0!==a?u=e[a]=[]:a="posthog",u.people=u.people||[],Object.defineProperty(u,"toString",{configurable:!0,enumerable:!0,writable:!0,value:function(t){var e="posthog";return"posthog"!==a&&(e+="."+a),t||(e+=" (stub)"),e}}),Object.defineProperty(u.people,"toString",{configurable:!0,enumerable:!0,writable:!0,value:function(){return u.toString(1)+".people (stub)"}}),o="capture identify alias people.set people.set_once set_config register register_once unregister opt_out_capturing has_opted_out_capturing opt_in_capturing reset isFeatureEnabled onFeatureFlags getFeatureFlag getFeatureFlagResult reloadFeatureFlags group updateEarlyAccessFeatureEnrollment getEarlyAccessFeatures getActiveMatchingSurveys getSurveys getNextSurveyStep onSessionId".split(" "),n=0;n<o.length;n++)g(u,o[n]);e._i.push([i,s,a])},e.__SV=1)}(document,window.posthog||[]);

    posthog.init('<ph_project_token>', {
      api_host: 'https://us.i.posthog.com',
      defaults: '2026-08-30',
      capture_pageview: 'history_change'
    })
  }
</script>

Without this guard, ClientRouter's soft navigation can re-execute the inline script during page transitions, causing a stack overflow error. The capture_pageview: 'history_change' option ensures pageviews are tracked automatically as users navigate.

The next step is to a create a Layout where we will use posthog.astro. Create a new file PostHogLayout.astro in your src/layouts folder:

cd .. && cd .. # move back to your base directory if you're still in src/components/posthog.astro
cd ./src/layouts
# or 'cd ./src && mkdir layouts && cd ./layouts' if your layouts folder doesn't exist yet
touch PostHogLayout.astro

Add the following code to PostHogLayout.astro:

PostHogLayout.astro

---
import PostHog from '../components/posthog.astro'
---
<head>
    <PostHog />
</head>

Lastly, update index.astro to wrap your existing app components with the new Layout:

index.astro

---
import PostHogLayout from '../layouts/PostHogLayout.astro';
---
<PostHogLayout>
  <!-- your existing app components -->
</PostHogLayout>

Node.js server context and traces

For on-demand rendering with @astrojs/node in middleware mode, use the shared Node SDK on your Express server. The browser setup above does not add server context.

Reuse one server posthog-node client. Add PostHogContextSpanProcessor from posthog-node/otel to your application's existing OpenTelemetry provider. Keep your exporter, sampler, and other processors. Load your tracing setup, including HTTP and Express instrumentation, before importing the server modules below.

Configure the Node adapter for middleware mode:

import { defineConfig } from 'astro/config'
import node from '@astrojs/node'

export default defineConfig({
    output: 'server',
    adapter: node({ mode: 'middleware' }),
})

Place the request middleware before the generated Astro handler. Pass the same client through the adapter's public locals argument so pages, endpoints, actions, and Astro middleware can reuse it:

import express from 'express'
import { handler as ssrHandler } from './dist/server/entry.mjs'
import { setupExpressRequestContext } from 'posthog-node'
import posthog from './posthog.server.mjs'

const app = express()
app.use(express.static('dist/client'))
setupExpressRequestContext(posthog, app)
app.use((req, res, next) => ssrHandler(req, res, next, { posthog }))
app.listen(4321)

The middleware reads PostHog tracing headers. An existing application identity takes precedence over the browser fallback. The headers are untrusted analytics context and must not be used for authentication or authorization. Only requests that carry the headers receive browser IDs; standard browser document navigations do not gain fetch/XHR-added tracing headers.

The optional processor adds sessionId and posthogDistinctId to recording application spans. Explicit canonical attributes keep their values. At request entry, it also fills missing IDs on the nearest recording HTTP server ancestor through parent objects it has observed. A sampler-dropped intermediate span breaks that verified chain, so the earlier server span stays unchanged. New recording spans inside the request still receive context.

Express owns the request context. It stays active for work that the request starts, including while a response body streams and after the response ends. A background job that needs its own IDs must use a new explicit context.

Astro can handle errors before they reach Express's error middleware. To capture errors thrown by next(), use the same client from your existing Astro middleware chain:

import { defineMiddleware } from 'astro:middleware'

export const onRequest = defineMiddleware(async ({ locals }, next) => {
    try {
        return await next()
    } catch (error) {
        locals.posthog.captureException(error)
        throw error
    }
})

Keep your other middleware when adding error capture. Capture errors from later streaming work where they occur, because next() may already have returned.

This setup covers the Node adapter's middleware mode. Prerendered pages and static assets use the browser integration; standalone and edge deployments need their own verified context and response lifetime setup. See Astro's Node adapter guide and the Node.js tracing guide.

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.

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.

PostHog captures heatmap screenshots using Browserless, which has its own IP addresses. Browserless publishes the current list here.

An allowlist does not help when your app has a private address. For apps on an internal network, see internal and intranet applications.

Next steps

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

Alternatively, the following tutorials can help you get started:

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

• How to set up A/B tests in Astro

• How to set up surveys in Astro