Instrument Feature Flags — Agent Skill by PostHog

From the author

Add PostHog feature flags to gate new functionality. Use after implementing features or reviewing PRs to ensure safe rollouts with feature flag controls. Also handles initial PostHog SDK setup if not yet installed.

— the description by PostHog

AutoGineer summary

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

This skill provides a structured approach to instrumenting PostHog feature flags within your codebase. It handles the end-to-end process, including detecting your platform, setting up the PostHog SDK if necessary, configuring environment variables, and implementing the code required to gate features. It emphasizes best practices such as using environment variables for configuration, preferring server-side evaluation to prevent UI flicker, and ensuring minimal, non-intrusive code changes.

What it does

Detects your project's platform and language automatically.
Initializes the PostHog SDK if it is not already present in the project.
Configures required environment variables securely without hardcoding.
Provides platform-specific code patterns for implementing feature flag checks.
Guides the integration of boolean and multivariate feature flags.
Ensures PostHog configuration does not break application boot or runtime stability.

When to use it

When you need to gate new functionality for controlled releases.
After implementing a new feature that requires a safe rollout strategy.
When reviewing pull requests that introduce features needing toggle controls.
When you need to set up PostHog instrumentation in a new or existing project.

Advantages

Supports a wide range of languages, frameworks, and platforms.
Promotes safe, production-ready instrumentation patterns.
Prevents common issues like UI flicker through server-side evaluation.
Maintains clean code by avoiding hardcoded configuration keys.

Usage examples

“Add a PostHog feature flag to gate the new checkout button in my React app.”: The agent will analyze your React project, verify PostHog setup, and provide the code snippet using posthog.isFeatureEnabled to conditionally render the button.
“I need to instrument a new feature flag for my Node.js backend.”: The agent will detect the Node.js environment, ensure the SDK is initialized, and guide you through adding the flag check logic to your server-side code.

Install

Download the skill as a ZIP:
https://autogineer.com/agent-skills/posthog/instrument-feature-flags/download

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

mkdir -p .agents/skills && unzip instrument-feature-flags.zip -d .agents/skills

Documentation by PostHog

Add PostHog feature flags

Use this skill to add PostHog feature flags that gate new or changed functionality. Use it after implementing features or reviewing PRs to ensure safe rollouts with feature flag controls. If PostHog is not yet installed, this skill also covers initial SDK setup. Supports any platform or language.

Supported platforms: React, Next.js, React Native, Web (JavaScript), Node.js, Python, PHP, Ruby, Go, Java, Rust, .NET, Elixir, Android, iOS, Flutter, and the REST API.

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, go.mod, Gemfile, composer.json, mix.exs, etc.) to determine the language and framework.

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 (SDK initialization, env vars, etc.). If PostHog is already installed and initialized, skip to STEP 3.

STEP 2: Research instrumentation. (Skip if PostHog is already set up.) 2.1. Find the reference file below that matches the detected platform — it is the source of truth for SDK initialization, flag evaluation methods, and framework-specific 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: Create or find the feature flag.

Check if a PostHog MCP server is connected. If available, use its tools to search for an existing feature flag the user wants to instrument, or create a new one.
If no MCP server is available, instruct the user to create the flag in the PostHog dashboard.

STEP 4: Plan release conditions.

Determine the rollout strategy (percentage rollout, user targeting, group targeting, etc.).
Plan how the feature flag will gate the new functionality in code.

STEP 5: Instrument the feature.

Add the feature flag code following the platform-specific reference patterns.
Use server-side evaluation when possible to avoid UI flicker.
Do not alter the fundamental architecture of existing files. Make additions minimal and targeted.
You must read a file immediately before attempting to write it.

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 project token 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 token instead.
For the PostHog host URL: check the projects-get MCP response for a region field — US maps to https://us.i.posthog.com, EU maps to https://eu.i.posthog.com. If the region is not available from the MCP response or from existing project configuration, ask the user: "Are you on PostHog US Cloud or EU Cloud?" Do not assume US Cloud.
Write these values to the appropriate env file using the framework's naming convention.
Reference these environment variables in code instead of hardcoding them.

Reference files

references/react.md - React feature flags installation
references/react-native.md - React native feature flags installation
references/web.md - Web feature flags installation
references/nodejs.md - Node.js feature flags installation
references/python.md - Python feature flags installation
references/django.md - Django
references/flask.md - Flask
references/php.md - Php feature flags installation
references/laravel.md - Laravel
references/ruby.md - Ruby feature flags installation
references/ruby-on-rails.md - Ruby on rails
references/go.md - Go feature flags installation
references/java.md - Java feature flags installation
references/rust.md - Rust feature flags installation
references/dotnet.md - .net feature flags installation
references/dotnet.md - .net
references/elixir.md - Elixir feature flags installation
references/android.md - Android feature flags installation
references/ios.md - Ios feature flags installation
references/usage.md - Ios SDK usage
references/flutter.md - Flutter feature flags installation
references/api.md - API feature flags installation
references/next-js.md - Next.js
references/adding-feature-flag-code.md - Adding feature flag code
references/best-practices.md - Best practices for production-ready flags
references/COMMANDMENTS.md - Framework-specific rules the integration must follow

Each platform reference contains SDK-specific installation, flag evaluation, and code examples. Find the one matching the user's stack. If unlisted, use the API reference as a fallback.

Key principles

Environment variables: Always use environment variables for PostHog keys. Never hardcode them.
Minimal changes: Add feature flag code alongside existing logic. Don't replace or restructure existing code.
Boolean flags first: Default to boolean flag checks unless the user specifically asks for multivariate flags.
Server-side when possible: Prefer server-side flag evaluation to avoid UI flicker.

Files

ATTRIBUTION.mdAdded by AutoGineer958 B
LICENSELicence file added by AutoGineer1.0 KiB
SKILL.md5.7 KiB
references/COMMANDMENTS.md1.5 KiB
references/adding-feature-flag-code.md114.9 KiB
references/android.md4.6 KiB
references/api.md6.2 KiB
references/best-practices.md21.2 KiB
references/django.md14.9 KiB
references/dotnet.md25.8 KiB
references/elixir.md1.9 KiB
references/flask.md8.1 KiB
references/flutter.md8.2 KiB
references/go.md6.4 KiB
references/ios.md4.4 KiB
references/java.md2.9 KiB
references/laravel.md13.1 KiB
references/next-js.md18.3 KiB
references/nodejs.md6.6 KiB
references/php.md5.3 KiB
references/python.md6.2 KiB
references/react-native.md5.2 KiB
references/react.md9.2 KiB
references/ruby-on-rails.md19.7 KiB
references/ruby.md6.0 KiB
references/rust.md7.8 KiB
references/usage.md29.2 KiB
references/web.md7.5 KiB

references/elixir.md

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

Elixir Feature Flags installation

Canonical URL: https://posthog.com/docs/feature-flags/installation/elixir

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).

Install with the wizard

Run this command in your project directory to add PostHog to your Elixir 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.

This library was built by the community but it's being maintained by the PostHog core team since v1.0.0. Thank you to Nick Kezhaya for building it originally. Thank you to Alex Martsinovich for contributing v2.0.0.

The package can be installed by adding posthog to your list of dependencies in mix.exs:

def deps do
  [
    {:posthog, "~> 2.0"}
  ]
end

Configuration

config/config.exs

config :posthog,
  enable: true,
  api_host: "https://us.i.posthog.com",
  api_key: "<ph_project_token>",
  in_app_otp_apps: [:my_app]

You can see all the available configuration options in the PostHog.Config module.

Optionally, you might want to enable the Plug integration to attach request metadata and tracing context in Plug-based applications including Phoenix. You still need to capture events explicitly with PostHog.capture/2 or PostHog.capture/3.

Development/Test mode

For a test environment, you can pass in test_mode: true value to the config. This causes events to be dropped instead of sent to PostHog.

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, Needs Python, Needs Node.js

Security

What our scanner noticed· Low risk

Instructions that use the network

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

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Download ZIP

The ZIP holds the skill’s files, its licence files and an ATTRIBUTION.md. Download instrument-feature-flags.zip