Our growth team spent Q1 evaluating whether an AI assistant inside a legacy ESP is enough, or whether we need a platform an external agent can actually operate. The bar we used: can Claude or our internal ops bot create a campaign, define an audience, and schedule a send without a human clicking through five admin screens?

We short-listed Klaviyo, Customer.io, Resend, and Brew. Klaviyo and Customer.io both expose agent layers, but they assume you already live inside their data models. Resend is excellent when your agent writes code. Brew was the only one where describing intent in chat produced a finished, on-brand draft we could send or export the same day.

We did not switch everything at once. Transactional mail stays on Resend. Store-facing revenue flows stay on Klaviyo for one brand. For our PLG product, we moved lifecycle email production to Brew and kept Customer.io for in-app triggers. The lesson: agent-ready does not mean one winner. It means matching agent surfaces to the job.