Email Marketing Without the Klaviyo API
You don't need a Klaviyo API key to run real email experiments. marginal is a hosted email MCP server that lets your AI agent generate variants, launch subject line A/B tests, and read lift metrics — all over a single endpoint.
Why skip the Klaviyo API entirely?
Plenty of teams want subject line testing and managed sends without inheriting a heavyweight ESP integration. Stitching together API keys, list syncs, and webhook handlers is a project of its own — and it's overkill when your goal is simply to test and ship better email.
marginal takes the opposite approach. It's a hosted service that handles sending and tracking for you, so doing email marketing without a Klaviyo API becomes a matter of calling a few MCP tools instead of managing infrastructure.
- No ESP credentials to provision or rotate
- Managed sends with built-in open and click tracking
- Within-test lift metrics computed for you
- Free tier of 100 experiments per month to start
How the email MCP server works
marginal exposes its capabilities as MCP tools that an AI agent can call directly. You describe the campaign in plain language, and the agent orchestrates the test through the email MCP server at https://marginal.sh/mcp.
- generate_variants — draft multiple subject line candidates
- launch_test — start a subject line A/B test across your audience
- get_results — pull open/click rates and within-test lift
- recommend_next — get a data-backed suggestion for the next send
A subject line A/B test from your agent
The point of AI agent email marketing is to compress the loop between idea and result. With marginal, an agent in Cursor, Claude Desktop, Claude Code, Windsurf, Cline, Continue, Zed, or OpenAI Codex can take a campaign brief and run the full experiment.
A typical run looks like this: ask the agent to generate four subject line variants, launch the A/B test against a sample, wait for engagement data, then read the lift metrics and let recommend_next pick the winner to roll out.
- Connect via the registry id sh.marginal/mcp
- Authenticate with a Bearer API key (marg_live_...)
- Let the agent iterate without you touching an ESP dashboard
What you get out of the box
Because marginal is hosted, there's nothing to self-host and no separate sending platform to configure. You point your client at the endpoint, supply your key, and start testing.
See the docs at https://marginal.sh/docs/ for tool schemas and examples covering variant generation, launching tests, and reading results.
- Hosted endpoint — no self-hosted MCP server required
- Subject line A/B tests with clear lift reporting
- Works across every major MCP-capable client
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