Trial Conversion Subject Line Tests in Windsurf
Connect Windsurf to marginal's email MCP server and let your AI agent draft, test, and optimize trial conversion subject lines without leaving your editor.
Why test trial conversion subject lines from Windsurf
Trial-to-paid is one of the highest-leverage moments in any SaaS funnel, and the subject line decides whether your conversion email gets opened at all. When you wire Windsurf to marginal, you can iterate on those subject lines in the same place you build the product — describing the test in plain language and letting the agent handle the mechanics.
marginal is a hosted email MCP server, so there's nothing to deploy or maintain. Windsurf talks to the marginal endpoint over MCP, and you keep your focus on the messaging that moves trial users to upgrade.
- Draft and compare subject lines for day-7 and end-of-trial nudges
- Run a proper subject line A/B test instead of guessing
- Track opens and clicks to tie wording back to conversions
- Stay inside Windsurf — no separate ESP dashboard switching
Connect Windsurf to the marginal MCP server
Add marginal as an MCP server in Windsurf and authenticate with your Bearer API key (the marg_live_... value from your dashboard). Once connected, the four marginal tools — generate_variants, launch_test, get_results, and recommend_next — become available to the agent.
Point the configuration at https://marginal.sh/mcp and Windsurf will surface the tools automatically. Full setup details live in the docs at https://marginal.sh/docs/.
- Endpoint: https://marginal.sh/mcp
- Auth: Bearer API key (marg_live_...)
- Registry name: sh.marginal/mcp
- Free tier covers 100 experiments per month
A trial conversion workflow with your AI agent
AI agent email marketing works best when you describe the goal and let the tools chain together. For trial conversion, prompt Windsurf with the audience and the upgrade angle, then walk through generate, launch, and measure.
- Ask Windsurf to call generate_variants for subject lines aimed at trial users in their final days
- Use launch_test to send the variants and split traffic for the subject line test
- Pull get_results to read open and click rates with within-test lift metrics
- Call recommend_next so the agent suggests the winning line or a follow-up variant
Reading subject line A/B test results
marginal reports within-test lift so you can see how each trial conversion subject line performed relative to the others, not against some stale baseline. That makes it straightforward to decide which line to ship to the rest of your trial cohort.
Because get_results and recommend_next are MCP tools, Windsurf can summarize the outcome and propose the next subject line A/B test in the same conversation — turning a one-off experiment into an ongoing optimization loop.
- Compare open and click lift across variants in one place
- Promote the winner to your broader trial audience
- Queue the next test directly from the agent's recommendation
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