The email marketing MCP server for AI agents
marginal is a hosted email marketing MCP server that gives your AI agent the tools to draft variants, launch subject line A/B tests, and read back real lift metrics — all over the Model Context Protocol.
What an email marketing MCP server actually does
An email MCP server exposes email campaign actions as MCP tools, so an AI agent can run an experiment end to end without you wiring up a custom integration. Instead of pasting copy into a dashboard, the agent calls structured tools and gets structured results back.
marginal ships four focused tools that map to the natural flow of a subject line A/B test.
- generate_variants — produce candidate subject lines for a campaign
- launch_test — start a managed send split across variants
- get_results — pull open/click tracking and within-test lift
- recommend_next — suggest the winning direction for the next round
Why run subject line A/B tests through an agent
Subject line testing is repetitive, measurable, and easy to script — which makes it an ideal job for AI agent email marketing. The agent can propose variants, launch the test, wait for engagement to accumulate, and then decide what to try next based on actual numbers rather than a guess.
Because marginal handles managed sends and open/click tracking, the agent always works from real within-test lift metrics. That keeps each round grounded in data instead of speculation about what 'sounds better.'
- Generate and compare several subject lines per send
- Measure lift between variants inside the same test population
- Let the agent iterate toward higher open and click rates
- Keep a clean experiment history you can audit later
Connecting marginal to your tools
marginal is fully hosted at https://marginal.sh/mcp — there is no self-hosted server to deploy or maintain. You add the endpoint to any MCP-capable client and authenticate with a Bearer API key in the marg_live_... format.
It works with the editors and agent clients developers already use, so you can drive a subject line A/B test from wherever you write code or chat with your agent.
- Compatible with Cursor, Claude Desktop, Claude Code, Windsurf, Cline, Continue, Zed, and OpenAI Codex
- Registry identifier: sh.marginal/mcp
- Free tier includes 100 experiments per month
- Full setup and tool reference in the docs at https://marginal.sh/docs/
Get started with marginal
marginal is a hosted email marketing MCP server at marginal.sh. Sign up free, create an API key, and connect https://marginal.sh/mcp from Cursor, Codex, or Claude Desktop.
- 100 experiments/month on the free tier
- Four MCP tools: generate_variants, launch_test, get_results, recommend_next
- Listed in the MCP Registry as sh.marginal/mcp