PostHog Expertise

When to Use This Skill

Load this skill when you need to:

• Integrate PostHog into a new or existing project
• Set up feature flags for view or component gating
• Implement event tracking with standard naming conventions
• Manage PostHog credentials in Doppler and Bitwarden
• Debug feature flag loading or latency issues

Quick Reference

PostHog Region

GainInsight projects use PostHog EU (eu.posthog.com) for GDPR compliance.

Credential Management

Required Secrets (Doppler)

Bitwarden Entry

Store PostHog credentials in Bitwarden with:

• Name: PostHog - {ProjectName}
• Username: {email} (e.g., admins+{project}[email protected])
• Password: Account password
• URI: https://eu.posthog.com
• Custom Fields: Personal API Key, Project API Key, Project ID

Account Registration

Use the admins+{project}[email protected] email pattern for per-project PostHog accounts. This uses plus-addressing on the admins@ Google Group so all team members receive notifications.

SDK Setup Pattern

File Structure

src/
  posthog/
    client.ts          # PostHog init, identify, reset, feature flag helpers, local overrides
    provider.tsx       # React provider for auth-aware identification
    events.ts          # Typed event tracking functions
    index.ts           # Barrel exports
  api/
    flags.ts           # API client for server-side flag management (fetchFlags, updateFlag)
  components/molecules/
    FeatureFlagPanel.tsx  # Admin UI for managing flags across tiers (Local/Dev/Prod)
server/src/api/routes/
  flags.ts             # Express route: GET/PATCH /api/flags — proxies PostHog API

Initialization Order (Critical)

PostHog must initialize at module load time, not inside a React component, to minimize flag loading latency:

// provider.tsx — init at module level
import { initPostHog } from "./client"
initPostHog()  // Runs when JS bundle loads, before auth

export function PostHogProvider({ children }) {
  // identify() happens here after auth succeeds
}

Why: Feature flags are fetched from the server on init. If init waits for auth, flags aren't ready when the sidebar renders, causing a flash of incorrect UI.

Graceful Degradation

When VITE_POSTHOG_KEY is not set (local dev without Doppler):

• PostHog never initializes
• All feature flags return true (all features visible)
• No events are tracked
• No network requests to PostHog

This means pnpm dev works without any PostHog configuration.

Flag Loading Behavior

When PostHog IS configured:

• Flags default to false (hidden) until PostHog responds
• Once flags load, enabled features appear
• Flags persist in localStorage so subsequent page loads are instant

export function isFeatureEnabled(flag: string): boolean {
  if (!POSTHOG_KEY) return true          // No key = show everything (dev)
  if (!flagsLoaded) return false         // Key set but loading = hide until ready
  return posthog.isFeatureEnabled(flag) ?? false
}

Feature Flag Patterns

View-Level Flags

Gate entire views/pages behind flags. Use plain, descriptive names matching the view ID:

export const VIEW_FLAGS = {
  health: "health",
  sessions: "sessions",
  docs: "docs",
  canvas: "canvas",
} as const

Sidebar integration:

// Sidebar accepts viewFlags prop
const filteredItems = navItems.filter(
  (item) => !viewFlags || viewFlags[item.id] !== false
)

Component-Level Flags

Gate individual components within a view:

function DashboardPage() {
  const showMetrics = useFeatureFlag("dashboard-metrics-panel")
  const showChart = useFeatureFlag("dashboard-chart")

  return (
    <div>
      <Header />
      {showMetrics && <MetricsPanel />}
      {showChart && <ActivityChart />}
    </div>
  )
}

Flag Naming Convention

Do NOT prefix with feature- — use descriptive names and organize with PostHog groups/tags instead.

Flag Grouping (Client-Side)

Flags are flat in PostHog (no dependencies) and grouped client-side in the FeatureFlagPanel. This avoids PostHog dependency chain complexity and keeps grouping logic in the UI where it can be changed without PostHog configuration.

Actual Flag Names (Agentview)

Sidebar views: my-tasks, projects, health, sessions, activity, costs, claude-usage, terminal, docs, canvas, settings

Task detail panels: tasks-description, tasks-terminal, tasks-canvas, tasks-transcript

Project panels: project-tasks-panel, project-costs-panel, project-activity-panel, project-docs-panel, project-settings-panel

Client-Side Group Definitions

Groups are defined as static arrays in FeatureFlagPanel.tsx:

// "By View" mode — mirrors sidebar navigation
const VIEW_GROUPS = [
  { id: 'tasks', label: 'Tasks', flags: ['my-tasks', 'tasks-description', ...] },
  { id: 'projects', label: 'Projects', flags: ['projects', 'project-tasks-panel', ...] },
  { id: 'views', label: 'Views', flags: ['health', 'sessions', 'activity', ...] },
  { id: 'tools', label: 'Tools', flags: ['terminal', 'docs', 'canvas'] },
]

// "By Feature" mode — groups same feature across views
const FEATURE_GROUPS = [
  { name: 'Tasks', flags: [{ flag: 'my-tasks', context: 'Sidebar' }, { flag: 'project-tasks-panel', context: 'Project panel' }] },
  { name: 'Canvas', flags: [{ flag: 'canvas', context: 'Sidebar' }, { flag: 'tasks-canvas', context: 'Task panel' }] },
  // ...
]

Group toggle switches flip all flags in a group at once (with parallel API calls).

Local Overrides

Users can override any flag locally via localStorage without affecting other users. Local overrides take priority over PostHog SDK values.

const OVERRIDES_KEY = "posthog_flag_overrides"

export function overrideFlag(flag: string, value: boolean | undefined) {
  // Save to localStorage
  // Apply to PostHog SDK via posthog.featureFlags.override()
  // Notify subscribed components
}

export function isFeatureEnabled(flag: string): boolean {
  const overrides = getFlagOverrides()
  if (flag in overrides) return overrides[flag]  // Local override wins
  if (!POSTHOG_KEY) return true                  // No key = show everything
  if (!flagsLoaded) return false                 // Loading = hide
  return posthog.isFeatureEnabled(flag) ?? false
}

Overrides are visually indicated with amber color in the FeatureFlagPanel. A "Reset" button clears all overrides.

Environment Targeting (Dev vs Prod)

PostHog flags use filter groups with the environment person property to target dev and prod independently. The environment property is set as a super property during PostHog init ("development" or "production").

Filter Group Structure

Each flag has one or two filter groups:

Same state in dev & prod:
  [{ properties: [], rollout_percentage: 100 }]   ← single catch-all

Different state:
  [
    { properties: [{ key: "environment", value: ["development"] }], rollout_percentage: 100 },  ← dev group
    { properties: [], rollout_percentage: 0 }                                                    ← prod catch-all
  ]

The server-side /api/flags route parses these groups into simple { devEnabled, prodEnabled } booleans and rebuilds them on PATCH.

Server-Side Flag Proxy (/api/flags)

The Express API proxies PostHog's Management API so the Personal API Key stays server-side:

GET  /api/flags          → List all flags with parsed dev/prod state
PATCH /api/flags/:key    → Update a flag's dev and/or prod targeting

GET response:

{ "flags": [{ "id": 141447, "key": "my-tasks", "active": true, "devEnabled": true, "prodEnabled": true }] }

PATCH body:

{ "devEnabled": false }

The PATCH handler: (1) fetches current flag from PostHog, (2) merges the update, (3) builds new filter groups, (4) PATCHes PostHog, (5) returns parsed result.

FeatureFlagPanel (Admin UI)

A slide-out Sheet component for managing flags. Three tiers, two grouping modes.

Three Tiers

Two Slice Modes

• By View: Groups flags by sidebar nav structure (Tasks, Projects, Views, Tools)
• By Feature: Groups the same feature across all views it appears in

Optimistic Updates with Race Prevention

Dev tier uses optimistic updates so switches respond instantly despite the 2-round-trip API call (GET all flags + PATCH specific flag):

const requestCounter = useRef<Map<string, number>>(new Map())

const handleToggle = (flag, currentValue) => {
  const reqId = (requestCounter.current.get(flag) ?? 0) + 1
  requestCounter.current.set(flag, reqId)

  // Flip immediately
  setPosthogFlags(prev => prev.map(f => f.key === flag ? { ...f, devEnabled: !currentValue } : f))

  updateFlag(flag, { devEnabled: !currentValue })
    .then(updated => {
      // Only apply if no newer request exists (prevents stale responses overwriting)
      if (requestCounter.current.get(flag) === reqId) {
        setPosthogFlags(prev => prev.map(f => f.key === flag ? updated : f))
      }
    })
    .catch(() => { /* revert if reqId is still current */ })
}

Key pattern: Never replace a Switch with a loading spinner — always keep it in the DOM. Show the spinner alongside the switch so it remains clickable during API calls.

PostHog Dashboard Organization

• Group related flags by feature area using PostHog tags
• Link to Linear issues in flag descriptions
• Use filter groups for environment targeting, not separate flags per environment

Event Tracking Conventions

Standard Event Names

Use snake_case for all event names:

Typed Event Functions

Create typed wrappers to prevent typos and ensure consistent properties:

// events.ts
export function trackViewChanged(view: View, previousView?: View) {
  trackEvent("view_changed", { view, previous_view: previousView })
}

export function trackLoginCompleted(userId: string, login: string) {
  trackEvent("login_completed", { user_id: userId, github_login: login })
}

Super Properties

Set once at init, attached to every event automatically:

posthog.register({
  platform: isTauri ? "tauri" : "browser",
  environment: isDev ? "development" : "production",
})

Autocapture

• Production: Enabled (captures clicks, page views, form submissions)
• Development: Disabled (reduces noise)

posthog.init(key, { autocapture: !isDev })

User Identification

Use the auth provider's user ID as PostHog's distinct ID:

// On login
identifyUser(String(user.id), {
  github_login: user.login,
  name: user.name,
  avatar_url: user.avatar_url,
})

// On logout
posthog.reset()

Important: Call identify() AFTER init(), not during. Init happens at module load; identify happens when auth state changes.

Integration Points

GainInsight Standard

PostHog fits into the GI Standard stack as a cross-cutting concern applied after Layer 1:

Brownfield Integration

For existing projects without PostHog:

1. Install posthog-js via package manager
2. Create src/posthog/ module (client, provider, events, flags, index)
3. Wrap app with PostHogProvider
4. Add VITE_POSTHOG_KEY and VITE_POSTHOG_HOST to Doppler
5. Create feature flags in PostHog dashboard (or via API with personal key)
6. Store credentials in Bitwarden

PostHog API (Server-Side)

For programmatic flag management, use the personal API key:

# List flags with dev/prod state
POSTHOG_KEY=$(doppler run --project agentview --config prd -- printenv POSTHOG_PERSONAL_API_KEY)
curl -H "Authorization: Bearer $POSTHOG_KEY" \
  "https://eu.posthog.com/api/projects/$PROJECT_ID/feature_flags/?limit=100"

# Create a flag (on for everyone)
curl -X POST -H "Authorization: Bearer $POSTHOG_KEY" -H "Content-Type: application/json" \
  "https://eu.posthog.com/api/projects/$PROJECT_ID/feature_flags/" \
  -d '{"key":"my-flag","name":"My Flag","active":true,"filters":{"groups":[{"properties":[],"rollout_percentage":100}]}}'

# Set flag to dev=ON, prod=OFF
curl -X PATCH -H "Authorization: Bearer $POSTHOG_KEY" -H "Content-Type: application/json" \
  "https://eu.posthog.com/api/projects/$PROJECT_ID/feature_flags/$FLAG_ID/" \
  -d '{"active":true,"filters":{"groups":[{"properties":[{"key":"environment","value":["development"],"operator":"exact","type":"person"}],"rollout_percentage":100},{"properties":[],"rollout_percentage":0}]}}'

Note: POSTHOG_PERSONAL_API_KEY is stored in Doppler prd config only (not dev).

Troubleshooting

Flutter / Mobile (Future)

PostHog has a Flutter SDK (posthog_flutter). When mobile arrives:

• Same PostHog project, same feature flags
• Use platform super property ("flutter") to distinguish
• Share flag naming conventions across web and mobile
• Session replay not yet supported in Flutter SDK

Essential Reading

• PostHog Integration Guide
• GainInsight Standard Skill
• PostHog JS Docs
• PostHog Feature Flags