Customer Io

Customer.io is a behavioral messaging platform — agents send user events and attributes, and Customer.io triggers automated campaigns across email, SMS, push notifications, in-app messages, and webhooks. The platform has two distinct APIs: Track API (ingest user data and events) and App API (manage campaigns, segments, and messages). For agents, the Track API is the primary integration point: identify users with attributes, track events that trigger campaigns, and manage user profiles. The App API provides campaign management, broadcast sending, segment management, and message content operations. Customer.io's strength is sophisticated behavioral segmentation and multi-step workflow automation. For agents managing user lifecycle communications across multiple channels, Customer.io's behavioral model is more powerful than simple email platforms.

Track API uses site_id and API key via HTTP Basic Auth — credentials are workspace-specific. App API uses a separate Bearer token. Transactional API uses its own API key. This separation ensures that track credentials (high-volume, potentially embedded in client-side code) can't access management operations. Track API credentials are designed to be safe for client-side exposure — they can only write data, not read. App API tokens grant management access. No fine-grained scoping beyond the Track/App separation. No OAuth for third-party integrations. For agents, using Track API credentials for event ingestion and App API tokens for management provides clean security boundaries. Token rotation requires generating new credentials.

Documentation at customer.io/docs is comprehensive, covering API reference, SDK guides, campaign setup, and integration patterns. The API documentation clearly separates Track API, App API, and Transactional API with distinct reference pages. Getting started guides walk through user identification, event tracking, and campaign creation. SDKs for JavaScript (browser and Node), React Native, iOS, Android, and Flutter handle identity and event tracking. The documentation covers multi-channel patterns (email + SMS + push orchestration) which helps agents design cross-channel workflows. Webhook event documentation lists all delivery and engagement events. For agents, the Track API quick start and campaign trigger documentation are the essential starting points.

Two API surfaces with different purposes and auth. Track API at track.customer.io/api/v1/: identify (PUT /customers/{identifier} with attributes), track (POST /customers/{identifier}/events with event name and data), delete, and suppress. The Track API is optimized for high-volume event ingestion. App API at api.customer.io/v1/: campaigns, broadcasts, segments, customers, messages, and exports. Campaign API provides listing, metrics, and action management. Broadcast API enables one-time message sends to segments. Export API generates customer and message data exports. The Transactional API at api.customer.io/v1/send sends individual messages triggered by API call rather than automation. Webhooks deliver delivery and engagement events. The dual-API model is clean: Track for data in, App for management and data out.

Track API responses are minimal (200 OK for success) — designed for high-throughput ingestion with minimal overhead. App API errors return JSON with errors array. Rate limits: Track API handles thousands of events per second; App API has stricter per-endpoint limits. The event-to-campaign pipeline is async: events are ingested, evaluated against segment and campaign conditions, and messages are queued — this can take seconds to minutes for complex workflows. Delivery events (bounced, opened, clicked) are available via webhooks and App API. The main reliability consideration: Track API is designed to never block — it accepts events optimistically and processes them async. This means invalid data shapes are accepted without error at ingest time. Agents should validate data shapes before sending.