@mastra/posthog

PostHog AI Observability exporter for Mastra applications.

Installation

npm install @mastra/posthog

Usage

Zero-Config Setup

The exporter automatically reads credentials from environment variables:

# Required
POSTHOG_API_KEY=phc_...

# Optional
POSTHOG_HOST=https://us.i.posthog.com  # or eu.i.posthog.com

import { PosthogExporter } from '@mastra/posthog';

const mastra = new Mastra({
  ...,
  observability: {
    configs: {
      posthog: {
        serviceName: 'my-service',
        exporters: [new PosthogExporter()],
      },
    },
  },
});

Explicit Configuration

You can also pass credentials directly:

import { PosthogExporter } from '@mastra/posthog';

const mastra = new Mastra({
  ...,
  observability: {
    configs: {
      posthog: {
        serviceName: 'my-service',
        exporters: [
          new PosthogExporter({
            apiKey: 'phc_...',
            host: 'https://us.i.posthog.com', // optional, defaults to US region
          }),
        ],
      },
    },
  },
});

Features

AI Tracing

• Event-based architecture: Captures LLM calls and operations as PostHog events
• LLM analytics: Automatic tracking of token usage, latency, and costs
• Streaming support: Handles streaming LLM responses with MODEL_CHUNK events
• Privacy mode: Optional exclusion of input/output data for sensitive applications
• Serverless optimized: Auto-configures batching for serverless environments

Supported Event Types

• $ai_generation: LLM model calls (MODEL_GENERATION, MODEL_STEP)
• $ai_span: Operations like tool calls, workflows, and streaming chunks
• Hierarchical traces with parent-child relationships via $ai_parent_id
• Session grouping with $ai_session_id

Configuration

Basic Configuration

new PosthogExporter({
  apiKey: process.env.POSTHOG_API_KEY,
});

Advanced Configuration

new PosthogExporter({
  // Required
  apiKey: process.env.POSTHOG_API_KEY,

  // Optional: Region/Host
  host: 'https://eu.i.posthog.com', // EU region
  // or
  host: 'https://your-instance.com', // Self-hosted

  // Optional: Batching (defaults: flushAt=20, flushInterval=10000)
  flushAt: 20, // Batch size before auto-flush
  flushInterval: 10000, // Flush interval in milliseconds

  // Optional: Serverless mode (auto-configures smaller batches)
  serverless: true, // Sets flushAt=10, flushInterval=2000

  // Optional: User identification
  defaultDistinctId: 'anonymous', // Fallback if no userId in metadata

  // Optional: Privacy
  enablePrivacyMode: false, // Set to true to exclude input/output from LLM events
});

Serverless Environments

When deploying to serverless environments (Lambda, Vercel Functions, etc.), enable serverless mode:

new PosthogExporter({
  apiKey: process.env.POSTHOG_API_KEY,
  serverless: true, // Auto-configures for serverless
});

Important: Always call await mastra.shutdown() before your serverless function exits to flush remaining events.

Privacy Mode

To exclude sensitive input/output data while still tracking token usage and latency:

new PosthogExporter({
  apiKey: process.env.POSTHOG_API_KEY,
  enablePrivacyMode: true, // Excludes $ai_input and $ai_output_choices
});

Note: Privacy mode only applies to $ai_generation events. Span events (tool calls, etc.) still include input/output state.

Metadata

Include metadata in your Mastra spans to enrich PostHog events:

// User identification
{
  metadata: {
    userId: 'user-123',      // → distinctId in PostHog
    sessionId: 'session-abc', // → $ai_session_id for grouping

    // Custom properties (passed through to PostHog)
    environment: 'production',
    version: '1.0.0',
  }
}

Cost Tracking

PostHog automatically calculates costs from:

• Model name + token counts (uses OpenRouter pricing data)
• Or you can send pre-calculated costs in span attributes

Viewing Data in PostHog

1. Navigate to Product Analytics → Events
2. Filter for events starting with $ai_
3. Use the AI Observability dashboard (if available in your PostHog plan)
4. Query events by:
   • $ai_trace_id: Group all events in a trace
   • $ai_session_id: Group traces in a session
   • $ai_model: Filter by model (e.g., "gpt-4o")
   • $ai_provider: Filter by provider (e.g., "openai")

Environment Variables

# Required
POSTHOG_API_KEY=phc_...

# Optional
POSTHOG_HOST=https://us.i.posthog.com  # or eu.i.posthog.com

Links

• PostHog LLM Analytics Documentation
• Mastra Observability Documentation