Run an Email A/B Test Without a Credit Card
You want to test a subject line before you commit any budget. marginal lets you launch a real email A/B test without a credit card — its hosted email MCP server includes 100 experiments per month on the free tier, no card details required to start.
Why no credit card matters for testing
Most email tools gate experimentation behind a paid plan, so even a quick subject line A/B test means a payment form before you see a single result. That friction kills early experiments — the exact moment you most want to learn whether a tweak actually moves opens.
marginal flips that around. You sign up, grab an API key, and start running real tests. The free tier covers 100 experiments a month, which is enough to validate your messaging cadence before you ever think about billing.
- No card on file to begin running an email A/B test without a credit card
- 100 experiments/month included on the free tier
- Managed sends with built-in open and click tracking
- Within-test lift metrics so you know which variant actually won
How the email MCP server fits in
marginal is a hosted email MCP server, which means your AI agent talks to it over the Model Context Protocol instead of you wiring up a dashboard. The endpoint at https://marginal.sh/mcp authenticates with a Bearer API key (the marg_live_ prefix), and there is nothing to self-host.
For AI agent email marketing workflows, this is the simplest path: the agent calls tools directly, drafts variants, ships the test, and reads back results — all without leaving your editor or chat client.
- generate_variants — produce subject line options to test
- launch_test — send a managed subject line A/B test
- get_results — pull open/click and lift metrics
- recommend_next — suggest the follow-up experiment
Running a subject line A/B test free
A typical flow stays inside your free experiment budget. Ask your agent to generate two or three subject line variants, launch the test to your audience, then check results once enough sends have landed. Because tracking and lift calculation are handled by marginal, you don't bolt on a separate analytics layer.
Each launched test counts as one experiment against your monthly allotment, so you can iterate through dozens of subject line ideas before any payment conversation comes up.
- Generate candidate subject lines with one tool call
- Launch a managed send and let marginal split your audience
- Read open, click, and within-test lift to pick a winner
- Use recommend_next to plan the next round
Connect your AI client
marginal works with the MCP clients you already use — Cursor, Claude Desktop, Claude Code, OpenAI Codex, Windsurf, Cline, Continue, and Zed. Point the client at the endpoint, add your key, and your agent can run a subject line A/B test on demand.
Full setup steps and tool references live in the docs at https://marginal.sh/docs/. Start free, stay free for your first 100 experiments a month, and never touch a billing form to learn what your audience opens.
- Endpoint: https://marginal.sh/mcp
- Registry: sh.marginal/mcp
- Auth: Bearer API key (marg_live_...)
- Docs: 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