Subject Line A/B Testing in Cline
Cline is an autonomous coding agent that lives in your editor — and with marginal's email MCP server connected, it can also generate, launch, and evaluate subject line A/B tests without leaving your workflow.
Connect marginal to Cline
marginal is a hosted MCP server, so there's nothing to deploy or self-host. You point Cline at the endpoint, supply your Bearer API key, and the four marketing tools become available to the agent.
Add the following to your Cline MCP configuration. Replace the placeholder with your live key (it starts with marg_live_).
- Endpoint: https://marginal.sh/mcp
- Registry: sh.marginal/mcp
- Auth: Bearer API key (marg_live_...)
- Free tier covers 100 experiments per month
MCP config for Cline
Drop this into Cline's MCP servers settings. Once saved, Cline can discover the generate_variants, launch_test, get_results, and recommend_next tools.
{
"mcpServers": {
"marginal": {
"url": "https://marginal.sh/mcp",
"headers": {
"Authorization": "Bearer marg_live_your_key_here"
}
}
}
}A Cline agent workflow for subject line A/B tests
The strength of running a subject line A/B test through Cline is that the whole loop — drafting variants, sending, and reading results — happens via natural-language prompts the agent executes as tool calls.
A typical pass looks like this:
- Ask Cline to generate_variants for a campaign idea and audience tone
- Have it launch_test to send the variants and start open/click tracking
- Pull get_results once the test has run to compare within-test lift
- Let recommend_next suggest the winning subject line and the follow-up step
Why pair Cline with marginal
Cline already handles multi-step tasks autonomously, which makes it a natural fit for AI agent email marketing. Instead of switching to a separate dashboard, you keep subject line experimentation in the same agent you use to ship code.
Because marginal manages the sends and tracking, Cline doesn't need raw access to a downstream email platform — it works entirely through the email MCP server's tools and the metrics they return.
- Managed sends with open and click tracking
- Within-test lift metrics for clear winners
- Same tools available across Cursor, Claude Code, Windsurf, Continue, Zed and more
- Full reference 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