PostHog Analytics & Feature Flags Setup

Quick Guide: One-time setup for PostHog analytics and feature flags. Covers posthog-js client provider, posthog-node server client, and environment variables. PostHog handles both analytics AND feature flags with a generous free tier (1M events + 1M flag requests/month).

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<critical_requirements>

CRITICAL: Before Using This Skill

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST initialize posthog-js only in a client/browser context - it requires browser APIs like window and localStorage)

(You MUST call posthog.shutdown(), posthog.flush(), or use captureImmediate() after server-side event capture to prevent lost events)

(You MUST use defaults: '2026-01-30' for automatic SPA page tracking and latest recommended behaviors)

</critical_requirements>

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Auto-detection: PostHog setup, posthog-js, posthog-node, PostHogProvider, analytics setup, feature flags setup, event tracking setup, posthog.init

When to use:

• Initial PostHog setup in a project
• Configuring PostHogProvider for client-side analytics
• Setting up posthog-node for server-side/API route event capture
• Configuring environment variables for PostHog

When NOT to use:

• Event tracking patterns after setup (use analytics event tracking skill)
• Feature flag usage patterns (use feature flags skill)
• Complex multi-environment setups with separate staging/production projects

Key patterns covered:

• Client-side setup with PostHogProvider or framework initialization hook
• Server-side setup with posthog-node
• Environment variables (client vs server prefix)
• User identification and reset flows
• Serverless flush patterns (captureImmediate vs flush)

Detailed Resources:

• examples/core.md - Provider setup, layout integration, user identification, env vars
• examples/server.md - Server client singleton, API routes, serverless patterns
• reference.md - Decision frameworks

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<philosophy>

Philosophy

PostHog is a product analytics + feature flags platform that consolidates multiple tools into one. It's open-source, can be self-hosted, and has a generous free tier. For solo developers and small teams, PostHog eliminates the need for separate analytics and feature flag services.

Core principles:

1. One platform for analytics + feature flags - Reduces tool sprawl and cost
2. Usage-based pricing - Pay for what you use, not per-project
3. Autocapture by default - Automatic event tracking reduces manual instrumentation
4. Server and client SDKs - Full coverage for SSR and client-side apps

When to use PostHog:

• Need both analytics and feature flags in one platform
• Want generous free tier (1M events + 1M flag requests/month)
• Prefer open-source with self-host option
• Building product analytics (funnels, retention, sessions)

When NOT to use PostHog:

• Need advanced A/B testing with statistical rigor
• Require real-time event streaming
• Already have established analytics + flag tools

</philosophy>

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<patterns>

Core Patterns

Pattern 1: PostHog Project Structure

Use a single PostHog organization for your apps. One org pools billing. Use separate projects per app, or one project with custom properties to filter.

PostHog Organization: "Your Company"
├── Project: "Main App" (or separate per app)
│   ├── API Key: phc_xxx
│   └── Host: https://us.i.posthog.com (or eu.i.posthog.com)

Why good: Single org pools billing across all projects, usage-based pricing, 6 projects included on paid tier

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Pattern 2: Client-Side Setup

Install posthog-js and configure a provider or use your framework's client-side initialization hook.

Key config options: defaults: "2026-01-30" enables recommended behaviors, person_profiles: "identified_only" reduces costs.

See examples/core.md for full implementation of both approaches.

Why good: defaults date enables automatic SPA page/leave tracking, person_profiles: "identified_only" reduces event costs, debug mode in development aids troubleshooting

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Pattern 3: Server-Side Setup with posthog-node

Install posthog-node and create a singleton for server-side event capture.

Serverless flush options:

• captureImmediate() - simplest, awaits HTTP request directly (one request per event)
• capture() + await flush() - batched, requires explicit flush before response returns

See examples/server.md for singleton setup, API route usage, and the flush anti-pattern.

Why good: Singleton prevents multiple client instances, flushInterval/flushAt configure batching, captureImmediate simplifies serverless usage

</patterns>

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<red_flags>

RED FLAGS

• Initializing posthog-js on the server (requires browser APIs - will crash)
• No flush() or captureImmediate() after server-side capture in serverless environments (events silently lost)
• Client-side env vars not exposed to the browser bundle (check your framework's prefix convention)
• Hardcoding API keys in source code instead of environment variables
• Missing posthog.reset() on sign out (user identity bleeds to next session)
• Not using defaults date option (manual pageview tracking required, misses recommended behaviors)
• Not calling posthog.identify() after authentication (anonymous and authenticated sessions remain unlinked)
• No person_profiles: 'identified_only' option (unnecessary anonymous profiles created, higher costs)
• Not wrapping app with PostHogProvider when using hooks (hooks return null)
• Forgetting to add environment variables to deployment platform (events fail silently)
• Using different PostHog projects for dev/prod without realizing (separate data)

Gotchas & Edge Cases:

• posthog-js must be initialized after window is available (hence useEffect or a client-side initialization hook)
• Server-side SDK does NOT auto-flush like the client - you must explicitly call flush(), shutdown(), or use captureImmediate()
• captureImmediate() is simpler for serverless but sends one HTTP request per event (no batching)
• Free tier resets monthly (1M events then stops capturing until next month)
• person_profiles: 'identified_only' reduces costs but means no anonymous user profiles are created
• When using auto-initialization hooks, config values remain fixed for the session - bootstrapping only works if flags are evaluated on the server before render

</red_flags>

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<critical_reminders>

CRITICAL REMINDERS

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST initialize posthog-js only in a client/browser context - it requires browser APIs like window and localStorage)

(You MUST call posthog.shutdown(), posthog.flush(), or use captureImmediate() after server-side event capture to prevent lost events)

(You MUST use defaults: '2026-01-30' for automatic SPA page tracking and latest recommended behaviors)

Failure to follow these rules will cause lost analytics events, broken tracking, or security vulnerabilities.

</critical_reminders>

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Sources

• PostHog JavaScript SDK
• PostHog JavaScript Configuration
• PostHog Node.js SDK