Account

PostHog — Know how people use your app

Feature flags, A/B tests, funnels, and product analytics. Marnix auto-creates insights as you build.

What you get

PostHog is a product analytics platform — every click, view, signup, and conversion gets captured so you stop guessing and start knowing. Once connected, Marnix can auto-create feature flags, launch A/B tests, and build dashboards as you prompt for them during generation.

When you'd reach for this

Hide incomplete features — “put the new onboarding flow behind a feature flag called new-onboarding so I can flip it on for 10% of users first.”
A/B test copy / colors — “split-test the hero headline between ‘Build apps with your voice’ and ‘Your voice, your app, in minutes.’”
Track signup funnel — “create an insight showing how many people landed → clicked Sign Up → completed onboarding. I want to see where people drop off.”
Gradual rollout — “roll out the new pricing to 25% of users this week and ramp up only if conversion holds.”
Session replay — “turn on session recording for users who hit a 500 error so I can watch what led to it.”
Cohort segmentation — “track weekly retention for users who invited at least one friend vs those who didn't.”

How to connect

1. PostHog account (free up to 1M events/mo)

Sign up at app.posthog.com/signup. Pick US Cloud (default) or EU Cloud during signup — pick EU if you're GDPR-sensitive or your users are mostly European.

2. Create a Personal API key

Go to app.posthog.com/settings/user-api-keys → Create personal API key. Use the MCP Server preset — it grants exactly the scopes Marnix needs (feature flags, insights, dashboards, annotations, session recordings). Copy the phx_... string.

3. Paste in Marnix

Open Integrations → Know how people use your app, paste the key, pick your region (US or EU), click Connect. Marnix registers an MCP pointing at mcp.posthog.com (or mcp-eu.posthog.com for EU).

Example prompts

“Add PostHog tracking. Capture pageviews, signups, and the ‘upgrade to Pro’ button click.”
“Create a feature flag called dark-mode. Default off for everyone except me.”
“A/B test the pricing page — variant A shows $20/mo, variant B shows $19/mo. Run for 500 users per variant.”
“Build a signup funnel insight — Landing → Sign Up → Confirmed Email → First Build.”
“Make a weekly dashboard showing DAU, signups, churn, and feature-flag exposure counts.”

Troubleshooting

“401 Unauthorized” from PostHog — the key expired or you picked the wrong region. Re-create the key using the MCP Server preset and verify the region matches your PostHog instance.
Events not showing up in dashboard — PostHog has a 60-second ingestion lag. Also verify your NEXT_PUBLIC_POSTHOG_KEY env var is set in the generated code.

When you'd reach for this

Hide incomplete features — “put the new onboarding flow behind a feature flag called new-onboarding so I can flip it on for 10% of users first.”

A/B test copy / colors — “split-test the hero headline between ‘Build apps with your voice’ and ‘Your voice, your app, in minutes.’”

Track signup funnel — “create an insight showing how many people landed → clicked Sign Up → completed onboarding. I want to see where people drop off.”

Gradual rollout — “roll out the new pricing to 25% of users this week and ramp up only if conversion holds.”

Session replay — “turn on session recording for users who hit a 500 error so I can watch what led to it.”

Cohort segmentation — “track weekly retention for users who invited at least one friend vs those who didn't.”

How to connect

1. PostHog account (free up to 1M events/mo)

Sign up at app.posthog.com/signup. Pick US Cloud (default) or EU Cloud during signup — pick EU if you're GDPR-sensitive or your users are mostly European.

2. Create a Personal API key

3. Paste in Marnix

Example prompts

“Add PostHog tracking. Capture pageviews, signups, and the ‘upgrade to Pro’ button click.”

“Create a feature flag called dark-mode. Default off for everyone except me.”

“A/B test the pricing page — variant A shows $20/mo, variant B shows $19/mo. Run for 500 users per variant.”

“Build a signup funnel insight — Landing → Sign Up → Confirmed Email → First Build.”

“Make a weekly dashboard showing DAU, signups, churn, and feature-flag exposure counts.”

Troubleshooting

“401 Unauthorized” from PostHog — the key expired or you picked the wrong region. Re-create the key using the MCP Server preset and verify the region matches your PostHog instance.

Events not showing up in dashboard — PostHog has a 60-second ingestion lag. Also verify your NEXT_PUBLIC_POSTHOG_KEY env var is set in the generated code.