PostHog Common Errors

Overview

Diagnosis and solutions for the most common PostHog integration errors. Covers event capture failures, authentication issues, feature flag problems, identity fragmentation, and SDK initialization errors.

Prerequisites

• PostHog SDK installed (posthog-js or posthog-node)
• Access to browser console or server logs
• PostHog project API key (phc_...) available

Instructions

Tool discipline

Use Read to inspect the relevant configuration and implementation before proposing changes. Use Grep to locate initialization, capture, flag, and credential boundaries.

Error 1: Events Not Appearing in Dashboard

Symptoms: posthog.capture() calls execute without error but events never show in PostHog Activity tab.

Diagnoses and fixes:

// Problem A: Not flushing in serverless/scripts
// posthog-node queues events — they're lost if process exits before flush
const posthog = new PostHog('phc_...');
posthog.capture({ distinctId: 'user-1', event: 'test' });
// FIX: Always flush before exit
await posthog.shutdown(); // or await posthog.flush()

// Problem B: Wrong API host
posthog.init('phc_...', {
  api_host: 'https://app.posthog.com', // WRONG — this is the UI
});
// FIX: Use the ingest endpoint
posthog.init('phc_...', {
  api_host: 'https://us.i.posthog.com', // CORRECT for US Cloud
  // api_host: 'https://eu.i.posthog.com', // For EU Cloud
});

// Problem C: Ad blocker blocking posthog-js requests
// FIX: Set up a reverse proxy (see posthog-sdk-patterns)
// next.config.js rewrites: /ingest/* → us.i.posthog.com/*
// Then: posthog.init('phc_...', { api_host: '/ingest' });

Error 2: Feature Flag Returns undefined or Wrong Value

// Problem: Checking flag before flags are loaded
const value = posthog.isFeatureEnabled('my-flag'); // undefined — flags not ready

// FIX: Wait for flags to load
posthog.onFeatureFlags(() => {
  const value = posthog.isFeatureEnabled('my-flag'); // Now has correct value
});

// Problem (server): no feature flags secure key, so local definitions are unavailable
const ph = new PostHog('phc_...');
const flag = await ph.getFeatureFlag('my-flag', 'user-1'); // Slow, may fail

// FIX: Pass the server-only feature flags secure key through the SDK's option
const ph = new PostHog('phc_...', {
  personalApiKey: process.env.POSTHOG_FEATURE_FLAGS_SECURE_API_KEY,
});
// Now evaluates locally — faster and more reliable

Error 3: 401 Unauthorized on API Calls

set -euo pipefail
# Symptom: 401 when calling admin endpoints
curl "https://us.posthog.com/api/projects/$POSTHOG_PROJECT_ID/feature_flags/" \
  -H "Authorization: Bearer phc_wrong_key_type"
# Returns: {"detail": "Authentication credentials were not provided."}

# FIX: Use Personal API Key (phx_...) for admin endpoints, not project key (phc_...)
curl "https://us.posthog.com/api/projects/$POSTHOG_PROJECT_ID/feature_flags/" \
  -H "Authorization: Bearer $POSTHOG_PERSONAL_API_KEY"  # Must be phx_...

# Check which key type you have:
echo "$POSTHOG_PERSONAL_API_KEY" | head -c 4
# phx_ = Personal API Key (correct for admin API)
# phc_ = Project API Key (only for event capture)

Error 4: 429 Rate Limited

// Symptom: HTTP 429 on analytics endpoints
// PostHog rate limits: 240 req/min and 1200 req/hour for analytics endpoints
// Feature flag local eval: 600 req/min

// FIX: Implement backoff with Retry-After header
async function postHogRequest(url: string, options: RequestInit) {
  const response = await fetch(url, options);

  if (response.status === 429) {
    const retryAfter = parseInt(response.headers.get('Retry-After') || '60');
    console.warn(`PostHog rate limited. Retrying in ${retryAfter}s`);
    await new Promise(r => setTimeout(r, retryAfter * 1000));
    return postHogRequest(url, options); // Retry
  }

  return response;
}

// FIX: Cache insight results instead of polling
let cachedInsights: any = null;
let cacheExpiry = 0;

async function getInsights() {
  if (cachedInsights && Date.now() < cacheExpiry) return cachedInsights;

  const res = await postHogRequest(
    `https://us.posthog.com/api/projects/${PROJECT_ID}/insights/trend/`,
    { headers: { Authorization: `Bearer ${PERSONAL_KEY}` } }
  );
  cachedInsights = await res.json();
  cacheExpiry = Date.now() + 300000; // Cache 5 minutes
  return cachedInsights;
}

Error 5: Identity Fragmentation (Duplicate Users)

// Problem: Same user appears as multiple persons in PostHog
// Cause: Different distinct_id on frontend vs backend

// Frontend captures with anonymous ID: "anon-abc123"
posthog.capture('page_viewed'); // distinct_id = "anon-abc123"

// Backend captures with user ID: "user-456"
serverPosthog.capture({
  distinctId: 'user-456', // Different from frontend!
  event: 'api_called',
});

// FIX: Call identify on frontend to merge anonymous → known
posthog.identify('user-456'); // Merges anon-abc123 → user-456

// FIX: Use same distinct_id on server as frontend
serverPosthog.capture({
  distinctId: 'user-456', // Same as what frontend uses after identify
  event: 'api_called',
});

Error 6: posthog-js Not Loading or Initializing

// Problem: posthog.capture is not a function
// Cause: SDK not initialized or called server-side

// FIX: Guard all browser calls
if (typeof window !== 'undefined') {
  posthog.init('phc_...', { api_host: 'https://us.i.posthog.com' });
}

// FIX: Check if initialized before capturing
if (typeof posthog.capture === 'function') {
  posthog.capture('my_event');
}

// Problem: CSP (Content Security Policy) blocking PostHog
// FIX: Add PostHog domains to your CSP header
// connect-src: https://us.i.posthog.com https://us-assets.i.posthog.com

Quick Diagnostic Commands

set -euo pipefail
# 1. Check PostHog API reachability
curl -s -o /dev/null -w "HTTP %{http_code}\n" https://us.i.posthog.com/healthz

# 2. Verify the project token without adding an event.
curl -s -X POST "$POSTHOG_PUBLIC_HOST/flags?v=2" \
  -H 'Content-Type: application/json' \
  -d "{\"api_key\":\"$NEXT_PUBLIC_POSTHOG_KEY\",\"distinct_id\":\"diagnostic-check\"}" | jq '{flags, errorsParsingFlags}'

# 3. Verify personal API key works
curl -s "$POSTHOG_PRIVATE_HOST/api/projects/" \
  -H "Authorization: Bearer $POSTHOG_PERSONAL_API_KEY" | jq '.[0].name'

# 4. Check installed SDK versions
npm list posthog-js posthog-node 2>/dev/null || echo "No PostHog SDK found"

# 5. Check environment variables
env | grep -i posthog | sed 's/=.*/=***/'

Error Handling

Error	HTTP Code	Cause	Solution
Events missing	N/A	Not flushed	await posthog.shutdown()
Auth failed	401	Wrong key type	Use phx_ for admin, phc_ for capture
Rate limited	429	Too many API calls	Backoff, cache results
Flag undefined	N/A	Flags not loaded	Use onFeatureFlags callback
CSP blocked	N/A	Missing CSP entry	Add us.i.posthog.com to connect-src
Duplicate users	N/A	Identity mismatch	Call posthog.identify() consistently

Output

• Root cause identified for PostHog integration errors
• Fix applied with verification steps
• Diagnostic output confirming resolution

Examples

Given “server-side flags return undefined after deploy,” verify the region host, project token, feature-flags secure key, initialization lifetime, and fallback behavior. Return the first failed boundary, the evidence that proves it, and the smallest reversible correction.

Resources

See official PostHog references for current authority and verification boundaries.

• PostHog Troubleshooting
• PostHog Status Page
• PostHog API Overview

Next Steps

For comprehensive debugging, see posthog-debug-bundle.