Subject Line Testing With a Free API
If you're searching for a subject line testing free API, marginal exposes the whole loop—generate, launch, measure—through an MCP server your AI agent can call directly. The free tier covers 100 experiments per month, enough to validate the workflow before you scale.
What the free API actually does
marginal is a hosted email marketing MCP server. Instead of stitching together a creative tool, an ESP, and an analytics dashboard, you call four MCP tools that handle a subject line A/B test end to end.
- generate_variants — produce candidate subject lines for a given audience and goal
- launch_test — split your list and start a managed send
- get_results — pull open and click data with within-test lift metrics
- recommend_next — get a data-backed suggestion for the winning angle
Why route it through an MCP server
A plain REST endpoint makes you write glue code. The email MCP server model lets an AI agent discover the tools, fill in arguments, and chain calls without hardcoded request bodies. That's what makes AI agent email marketing practical rather than a demo.
Connect from Cursor, Claude Desktop, Claude Code, Codex, Windsurf, Cline, Continue, or Zed. The server is hosted at https://marginal.sh/mcp—there's nothing to deploy and no self-hosted component to maintain.
Running your first subject line A/B test
Authenticate with a Bearer API key (the marg_live_ prefix), point your client at the endpoint, and let the agent walk the loop. A typical run asks generate_variants for three to five options, launches a split with launch_test, then polls get_results until the lift is statistically meaningful.
- Start small: a couple of variants against a holdout segment
- Read within-test lift instead of raw open counts to compare fairly
- Use recommend_next to carry the winning theme into the next campaign
- Stay inside the 100 experiments/month free tier while you iterate
Where to go next
The docs at https://marginal.sh/docs/ cover each tool's arguments and response shape, plus client-specific setup. If you've been hunting for a subject line testing free API that an agent can drive on its own, marginal gives you the registry entry (sh.marginal/mcp) and the endpoint to get started.
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