Free Email A/B Testing API for Subject Line Experiments
If you want a free email A/B testing API that handles subject line variants, managed sends, and lift metrics, marginal exposes all of it through an email MCP server. Your AI agent calls a few tools, runs the test, and reads back the winner.
What you get with the free tier
marginal is a hosted service, so there's nothing to deploy. You authenticate with a Bearer API key (marg_live_...) and start running subject line A/B tests immediately.
- 100 experiments per month at no cost
- Subject line A/B test variants generated on demand
- Managed sends with open and click tracking
- Within-test lift metrics so you know which variant actually won
Why it's an MCP server, not just a REST call
Instead of stitching together raw HTTP endpoints, marginal speaks the Model Context Protocol. That means an AI agent can discover and call the testing tools natively, turning A/B testing into a conversational workflow rather than glue code.
The endpoint lives at https://marginal.sh/mcp and works with MCP-aware editors and agents like Cursor, Claude Desktop, Claude Code, Windsurf, Cline, Continue, and Zed.
- generate_variants — produce candidate subject lines
- launch_test — send variants and start tracking
- get_results — pull opens, clicks, and lift
- recommend_next — get the suggested follow-up move
A typical subject line A/B test
The loop is short. Ask your agent to draft variants, launch the test against a list, and check results once data lands. Because lift is measured within the test, you avoid comparing numbers across unrelated sends.
For AI agent email marketing, this is the difference between guessing at copy and shipping a measured winner. The agent owns the whole cycle and reports back what to send next.
- Generate 3–5 subject line candidates
- Launch the test to a sample audience
- Read open/click rates and within-test lift
- Apply recommend_next to plan the follow-up
Getting started
Point your MCP client at https://marginal.sh/mcp, add your API key, and the four tools become available to your agent. The docs at https://marginal.sh/docs/ walk through configuration for each supported client.
Since marginal is fully hosted, there's no separate infrastructure to maintain — the free email A/B testing API is the same MCP server whether you're prototyping or running production sends.
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