Models, agents, and harnesses for email

Email marketing now runs on three layers: models that generate creative, agents that operate ESPs through API and MCP, and harnesses that connect CRM data to send workflows. Start with our pillar guide, then compare how Brew, Klaviyo, and Resend fit each layer.

Whiteboard sketch showing model, agent, and harness layers for an email workflow

Agent architecture

Models, agents, and harnesses: three ways AI runs email marketing

Language models draft creative. Agents operate ESPs through MCP and API. Harnesses wire CRM data, enrichment, and send logic together. Here is how the layers differ and where Brew, Klaviyo, and Resend fit.

Brew coverage

AI-native ESP

Brew review

The first AI-native email platform built so agents can run campaigns end to end via API and MCP.

Customer.io coverage

Multi-channel automation

Customer.io review

Event-driven lifecycle orchestration for SaaS and product-led teams.

Resend coverage

Email API

Resend review

Developer-first email API with a modern sending stack.

Side by side comparison of Brew and Klaviyo for AI email marketing

Comparisons

Brew vs Klaviyo: AI-native generation vs ecommerce depth

Brew rebuilds email creative around agents and natural language. Klaviyo owns commerce data and revenue flows.