Testing Renewal Reminder Subject Lines in Windsurf
Renewal emails are high-stakes: a weak subject line means a missed payment or a churned customer. With marginal connected to Windsurf, you can draft, test, and ship renewal reminder subject lines without leaving your editor.
Why run renewal subject line tests from Windsurf
Renewal reminders compete with everything else in an inbox, and the subject line is what decides whether someone opens before their plan lapses. Instead of guessing, you can let an AI agent in Windsurf generate candidate lines and run a real subject line A/B test against your list.
marginal is a hosted email MCP server, so Windsurf talks to it directly over the Model Context Protocol. You stay in the same flow you use for code while the agent handles variant generation, sending, and tracking.
- Draft renewal angles — urgency, savings, loyalty — and compare them head to head
- Keep the whole subject line test inside Windsurf's chat
- No separate marketing dashboard to context-switch into
Generating renewal reminder variants
Ask Windsurf to call generate_variants with a short brief: the product, renewal date, and the tone you want. For renewal reminders, it helps to cover a few distinct strategies so the subject line A/B test actually measures different ideas rather than reworded copies.
- Deadline-driven: "Your plan renews Friday — anything to change?"
- Value recap: "A year of [product]: here's what renews"
- Soft prompt: "Quick heads up about your renewal"
- Incentive: "Lock in this year's rate before it renews"
Launching the test and reading lift
Once you're happy with the candidates, have the agent call launch_test to send the variants and track opens and clicks. marginal reports within-test lift metrics, so you see how each renewal subject line performed relative to the others rather than against a stale historical baseline.
Use get_results to check progress and recommend_next when you want the agent to suggest a follow-up — for example, doubling down on the urgency angle if it beat the value recap. Everything runs through https://marginal.sh/mcp with your Bearer API key.
- generate_variants — produce renewal subject line options
- launch_test — send and start a subject line A/B test
- get_results — opens, clicks, and within-test lift
- recommend_next — decide the follow-up automatically
Getting set up
Add marginal to Windsurf's MCP configuration with the endpoint and your marg_live_ key, and you can start AI agent email marketing right away. The free tier covers 100 experiments per month, which is plenty for iterating on renewal reminder subject lines before a big billing cycle.
Full configuration steps and tool references live in the docs at https://marginal.sh/docs/.
- Endpoint: https://marginal.sh/mcp
- Auth: Bearer API key (marg_live_...)
- Free tier: 100 experiments/month
- Hosted — nothing to self-host
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