Convertkit

REST API v3 at api.convertkit.com/v3/. Subscriber creation: POST /v3/forms/{form_id}/subscribe with email and optional fields — note that subscribers must be added through a form, not directly. Subscriber listing: GET /v3/subscribers. Tag management: POST /v3/tags/{tag_id}/subscribe to tag a subscriber. Sequence subscription: POST /v3/sequences/{sequence_id}/subscribe. Broadcast creation: POST /v3/broadcasts. Custom fields store additional subscriber data. The form-centric subscriber creation is unusual — agents must know a form ID to add subscribers, which adds an indirection layer. Tag-based operations (list tagged subscribers, tag/untag) are the primary segmentation mechanism. Pagination uses page parameter. The API is functional but some design choices (form-required subscriber creation) create friction.

API errors return JSON with error and message fields. Standard HTTP status codes. Rate limits are per-account and not publicly documented with specific numbers — agents should implement reasonable backoff. Subscriber creation through forms can fail silently if the form is archived or invalid. Duplicate subscriber detection is by email — adding an existing email updates their information rather than creating a duplicate. Webhook support is limited — ConvertKit provides subscriber event webhooks but with fewer event types than Customer.io or Loops. The main reliability consideration: ConvertKit's API is not designed for high-volume automation — it's built for creator-scale operations (thousands, not millions of subscribers). For agents operating at scale, the API performance and rate limits may be restrictive.

ConvertKit is purpose-built for individual creators — bloggers, podcasters, course creators, and newsletter writers. For agents, the API covers subscriber management (add, tag, list, search), form and landing page management, sequence (autoresponder) management, broadcast sending, and purchase/product tracking. The subscriber model uses tags for segmentation rather than lists — subscribers can have multiple tags, enabling flexible categorization. Visual automations handle complex subscriber journeys. Commerce features enable selling digital products and paid newsletters. The API is adequate for standard CRM operations but limited for complex marketing automation — many automation features are dashboard-only. For agents building creator-focused tools or managing subscriber lists programmatically, ConvertKit's API covers the essentials.

Two auth methods: API key (passed as api_key query parameter for read operations, api_secret for write operations) and OAuth 2.0. The API key/secret split is unusual: some endpoints require the API key, others require the API secret, and the distinction isn't always obvious. OAuth is available for third-party application integrations. API key and secret are found in ConvertKit account settings. No fine-grained scoping. No key expiry. For agents, the dual-key model (api_key vs. api_secret) adds unnecessary complexity — agents must know which credential each endpoint requires. OAuth follows the standard authorization code flow. The auth model works but the key/secret distinction is a minor usability issue.

API documentation at developers.convertkit.com provides endpoint reference with parameters and examples. The documentation is organized by resource (subscribers, forms, tags, sequences, broadcasts). OAuth documentation covers the authorization flow for third-party apps. The documentation is adequate for the API's scope but could be more detailed on error handling and edge cases. No official SDKs — agents use raw HTTP. The community has created wrapper libraries for various languages. The documentation reflects ConvertKit's audience: creators, not enterprise developers. The getting started guide assumes familiarity with ConvertKit's product concepts (forms, tags, sequences). For agents, understanding ConvertKit's product model (form-based subscriber creation, tag-based segmentation) is prerequisite to effective API use.