Production-Grade PostHog Integration for Next.js 15 (App Router

Production-Grade PostHog Integration for Next.js 15 (App Router) Role You are a Senior Next.js Architect & Analytics Engineer with deep expertise in Next.js 15, React 19, Supabase Auth, Polar.sh billing, and PostHog. You design production-grade, privacy-aware systems that handle the strict Server/Client boundaries of Next.js 15 correctly. Your output must be code-first, deterministic, and suitable for a real SaaS product in 2026.

Goal Integrate PostHog Analytics, Session Replay, Feature Flags, and Error Tracking into a Next.js 15 App Router SaaS application with:

• Correct Server / Client separation (Providers Pattern)
• Type-safe, centralized analytics
• User identity lifecycle synced with Supabase
• Accurate billing tracking (Polar)
• Suspense-safe SPA navigation tracking

Context

• Framework: Next.js 15 (App Router) & React 19
• Rendering: Server Components (default), Client Components (interaction)
• Auth: Supabase Auth
• Billing: Polar.sh
• State: No existing analytics
• Environment: Web SaaS (production)

Core Architectural Rules (NON-NEGOTIABLE)

1. PostHog must ONLY run in Client Components.

2. No PostHog calls in Server Components, Route Handlers, or API routes.

3. Identity is controlled only by auth state.

4. All analytics must flow through a single abstraction layer (lib/analytics.ts).

5. Architecture & Setup (Providers Pattern)

• Create app/providers.tsx.
• Mark it as 'use client'.
• Initialize PostHog inside this component.
• Wrap the application with PostHogProvider.
• Configuration:
  • Use NEXT_PUBLIC_POSTHOG_KEY and NEXT_PUBLIC_POSTHOG_HOST.
  • capture_pageview: false (Handled manually to avoid App Router duplicates).
  • capture_pageleave: true.
  • Enable Session Replay (mask_all_text_inputs: true).

2. User Identity Lifecycle (Supabase Sync)

• Create hooks/useAnalyticsAuth.ts.
• Listen to Supabase onAuthStateChange.
• Logic:
  • SIGNED_IN: Call posthog.identify.
  • SIGNED_OUT: Call posthog.reset().
  • Use appropriate React 19 hooks if applicable for state, but standard useEffect is fine for listeners.

3. Billing & Revenue (Polar)

• PostHog distinct_id must match Supabase User ID.
• Set polar_customer_id as a user property.
• Track events: CHECKOUT_STARTED, SUBSCRIPTION_CREATED.
• Ensure SUBSCRIPTION_CREATED includes {{ revenue: number, currency: string }} for PostHog Revenue dashboards.

4. Type-Safe Analytics Layer

• Create lib/analytics.ts.
• Define strict Enum AnalyticsEvents.
• Export typed trackEvent wrapper.
• Check if (typeof window === 'undefined') to prevent SSR errors.

5. SPA Navigation Tracking (Next.js 15 & Suspense Safe)

• Create components/PostHogPageView.tsx.
• Use usePathname and useSearchParams.
• CRITICAL: Because useSearchParams causes client-side rendering de-opt in Next.js 15 if not handled, you MUST wrap this component in a <Suspense> boundary when mounting it in app/providers.tsx.
• Trigger pageviews on route changes.

6. Error Tracking

• Capture errors explicitly: posthog.capture('$exception', {{ message, stack }}).

Deliverables (MANDATORY) Return ONLY the following files:

1. package.json (Dependencies: posthog-js).
2. app/providers.tsx (With Suspense wrapper).
3. lib/analytics.ts (Type-safe layer).
4. hooks/useAnalyticsAuth.ts (Auth sync).
5. components/PostHogPageView.tsx (Navigation tracking).
6. app/layout.tsx (Root layout integration example).

🚫 No extra files. 🚫 No prose explanations outside code comments.