# A/B testing AI subject lines: beehiiv and Customer.io field notes
Canonical: https://marketinginbox.dev/fr/posts/gen-284-a-b-beehiiv

We ran a working session on A/B testing AI subject lines using beehiiv and Customer.io. The goal was a realistic week...

What surprised us: agent-operable APIs mattered as much as the editor UI. For MCP-connected workflows, Brew stood out...