API
Your lists, over HTTPS.
A REST API for audiences, contacts, fields, segments, topics, and suppressions — built so you can migrate from another provider in one script and keep your app in sync afterwards.
Built for migration
Import up to 1,000 contacts per call, address them by email or id, and upsert instead of colliding. Coming from Resend or Mailchimp is a short script, not a project.
Bring your opt-outs
Import contacts already marked unsubscribed, and push your existing suppression list straight in — so nobody who opted out ever hears from you again.
Keys you control
Create and revoke bearer keys on the API keys page. The key is shown once and only its hash is stored; every key is scoped to one organization.
The docs live in the app
Quickstart, endpoint map, and cURL / JavaScript / Python snippets sit under your key list — pre-filled with your own audience id. There's also a copy-paste prompt that hands the whole reference to your AI coding assistant.
Questions
- Does day3 have an API?
- Yes — a REST API at /api/v1 covering audiences, contacts, custom fields, segments, topics, and the suppression list. It uses bearer API keys, JSON with snake_case, cursor pagination, and idempotency keys on writes.
- Can I migrate my list from another provider?
- That's what it's designed for. Batch up to 1,000 contacts per call, upsert by email, carry over custom fields, and import your unsubscribes and suppression list so the opt-outs come with you.
- Is the API on every plan?
- Yes. The free set-up tier's 500-subscriber cap applies to API writes too — an import that would cross it is rejected whole rather than half-applied.