Subject-line A/B testing is the kind of thing you usually rent. You sign up for an ESP, drop your contacts into their list, pick "split test" from a dropdown, and let their black box decide a winner — on their definition of "winner," with their attribution, on their schedule. That's fine if email is your whole product. It's a lot of ceremony if you have an agent that already sends mail and you just want to know which of two subjects gets more humans to reply.
Here's the pivot: the experiment is just two sends and a counter. If your agent owns a mailbox, it can run the split itself — pick a variant per recipient, send from its own address, watch replies land back in its own threads, and tally the result. No ESP, no list import, no separate analytics product. The whole thing lives inside the same mailbox the agent already uses.
I work on the Nylas CLI, so the terminal commands below are the exact ones I reach for, and I'll show the matching HTTP call for each one so you can wire it into whatever your agent is actually built on. The data plane is the same either way.
What you actually get
An Agent Account is a real, sending-and-receiving mailbox that your code owns outright — no human, no OAuth handshake, no token to refresh. Under the hood it's just a Nylas grant with a grant_id, which is the important part: every grant-scoped endpoint you already know (Messages, Threads, Folders, Webhooks) works against it unchanged. There's nothing new to learn on the data plane. You send with POST /v3/grants/{grant_id}/messages/send, you read threads with GET /v3/grants/{grant_id}/threads/{thread_id}, and replies arrive as message.created webhooks. That's the entire surface area this experiment needs.
