Provider Coverage

The unified API is honest about what each provider can represent without provider-specific surprises.

SendGrid

Best fit for transactional sends and marketing contacts where custom fields must be created on first use.

Mailchimp

Best fit for audience-backed contact management and campaign analytics. Requires an audience/list id.

Klaviyo

Best fit for profile properties, campaign reads, and Klaviyo-native lifecycle data. Transactional sends stay flow-bound.

Read Klaviyo notes →

Matrix

Tool	SendGrid	Mailchimp	Klaviyo
`upsert_contact`	Yes	Yes	Yes
`get_contact`	Yes	Yes	Yes
`get_connection_status`	Yes	Yes	Yes
`send_transactional`	Yes	No*	No**
`get_campaign_stats`	Yes	Yes	Yes
`list_templates`	Yes	Yes	Yes
`list_campaigns`	Yes	Yes	Yes

* Mailchimp transactional email is Mandrill, a separate product. Phase 2 returns TOOL_NOT_SUPPORTED per D34.

** Klaviyo transactional email is Flow-triggered and template-bound. Vorlek returns TOOL_NOT_SUPPORTED per D19 instead of pretending it is a synchronous send.

SendGrid

Authentication uses a SendGrid API key.
upsert_contact auto-creates missing custom fields when needed.
data.contact_id is normalized to md5(lowercase(trim(email))) per D33.
The underlying import job id is available at meta.provider_metadata.import_job_id, and accepted async imports surface meta.job.status: "pending".

Mailchimp

Authentication uses a Marketing API key with a data-center suffix such as -us7.
An audience/list is required; Vorlek can auto-detect it when exactly one audience exists.
data.contact_id is the Mailchimp subscriber hash: md5(lowercase(trim(email))).
send_transactional returns TOOL_NOT_SUPPORTED in Phase 2 because Mandrill is not shipped.

Klaviyo

Authentication uses a private API key starting with pk_.
Requests use pinned API revision 2026-04-15.
Klaviyo accepts arbitrary profile properties, so data.fields_auto_created is always [].
data.contact_id is the Klaviyo profile id.