Instrument Integration

Add PostHog SDK integration

Use this skill to add the PostHog SDK to an application. Use it when setting up PostHog for the first time, or reviewing PRs that need PostHog initialization. Covers SDK installation, provider setup, and basic configuration. Supports any framework or language.

Supported frameworks and languages: Next.js, React, React Router, Vue, Nuxt, TanStack Start, SvelteKit, Astro, Angular, Django, Flask, FastAPI, Laravel, PHP, Ruby on Rails, Go, Elixir, Android, iOS, Swift, Flutter, React Native, Expo, Node.js, and vanilla JavaScript.

Instructions

Follow these steps IN ORDER:

STEP 1: Analyze the codebase and detect the platform.

Look for dependency files (package.json, pubspec.yaml, Podfile, Package.swift, requirements.txt, Gemfile, composer.json, go.mod, mix.exs, etc.) to determine the framework and language.

Look for lockfiles (pnpm-lock.yaml, package-lock.json, yarn.lock, bun.lockb, go.sum, pubspec.lock, Podfile.lock, Package.resolved, mix.lock) to determine the package manager.

• Check for existing PostHog setup. If PostHog is already installed and initialized, do not modify its code. Inform the user and skip to verification.

STEP 2: Research integration. 2.1. Find the reference file below that matches the detected framework — it is the source of truth for SDK initialization, provider setup, and configuration patterns. Read it now. 2.2. If no reference matches, fall back to your general knowledge and web search. Use posthog.com/docs as the primary search source.

STEP 3: Install the PostHog SDK.

• Add the PostHog SDK package for the detected platform. Do not manually edit package.json — use the package manager’s install command.
• Always install packages as a background task. Don’t await completion; proceed with other work immediately after starting the installation.

STEP 4: Initialize PostHog.

• Follow the framework reference for where and how to initialize. This varies significantly by framework (e.g., instrumentation-client.ts for Next.js 15.3+, AppConfig.ready() for Django, create_app() for Flask).
• Set up the PostHog provider/wrapper component if the framework requires one.

STEP 5: Identify users.

• Add PostHog identify() calls on the client side during login and signup events.
• If both frontend and backend exist, pass the client-side session and distinct ID using X-POSTHOG-DISTINCT-ID and X-POSTHOG-SESSION-ID headers to the server-side code.

STEP 6: Set up environment variables.

• Check if the project already has PostHog environment variables configured (e.g. in .env, .env.local, or framework-specific env files). If valid values already exist, skip this step.
• If the PostHog API key is missing, use the PostHog MCP server’s projects-get tool to retrieve the project’s api_token. If multiple projects are returned, ask the user which project to use. If the MCP server is not connected or not authenticated, ask the user for their PostHog project API key instead.
• For the PostHog host URL, use https://us.i.posthog.com for US Cloud or https://eu.i.posthog.com for EU Cloud.
• Write these values to the appropriate env file (e.g. .env.local for Next.js, .env for others) using the framework’s naming convention.
• Reference these environment variables in code instead of hardcoding them.

STEP 7: Verify and clean up.

• Check the project for errors. Look for type checking or build scripts in package.json.
• Ensure any components created were actually used.
• Run any linter or prettier-like scripts found in the package.json.

Reference files

• references/EXAMPLE-next-app-router.md - next-app-router example project code
• references/EXAMPLE-next-pages-router.md - next-pages-router example project code
• references/EXAMPLE-react-react-router-6.md - react-react-router-6 example project code
• references/EXAMPLE-react-react-router-7-framework.md - react-react-router-7-framework example project code
• references/EXAMPLE-react-react-router-7-data.md - react-react-router-7-data example project code
• references/EXAMPLE-react-react-router-7-declarative.md - react-react-router-7-declarative example project code
• references/EXAMPLE-react-vite.md - react-vite example project code
• references/EXAMPLE-nuxt-3-6.md - nuxt-3-6 example project code
• references/EXAMPLE-nuxt-4.md - nuxt-4 example project code
• references/EXAMPLE-vue-3.md - vue-3 example project code
• references/EXAMPLE-react-tanstack-router-file-based.md - react-tanstack-router-file-based example project code
• references/EXAMPLE-react-tanstack-router-code-based.md - react-tanstack-router-code-based example project code
• references/EXAMPLE-tanstack-start.md - tanstack-start example project code
• references/EXAMPLE-sveltekit.md - sveltekit example project code
• references/EXAMPLE-astro-static.md - astro-static example project code
• references/EXAMPLE-astro-view-transitions.md - astro-view-transitions example project code
• references/EXAMPLE-astro-ssr.md - astro-ssr example project code
• references/EXAMPLE-astro-hybrid.md - astro-hybrid example project code
• references/EXAMPLE-angular.md - angular example project code
• references/EXAMPLE-javascript-node.md - javascript-node example project code
• references/EXAMPLE-javascript-web.md - javascript-web example project code
• references/EXAMPLE-django.md - django example project code
• references/EXAMPLE-flask.md - flask example project code
• references/EXAMPLE-fastapi.md - fastapi example project code
• references/EXAMPLE-python.md - python example project code
• references/EXAMPLE-laravel.md - laravel example project code

Each framework reference contains SDK-specific installation, initialization, and usage patterns. Find the one matching the user’s stack.

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.
• Analytics contract: Treat event names, property names, and feature flag keys as part of an analytics contract. Reuse existing names and patterns found in the project. When introducing new ones, make them clear, descriptive, and consistent with existing conventions.