Testing Welcome Series Subject Lines in Cursor
Your welcome series is the highest-engagement moment in a subscriber's lifecycle — and the subject line decides whether the first email even gets opened. With marginal connected to Cursor, you can generate, launch, and evaluate welcome series subject lines without leaving your editor.
Why run the subject line test from Cursor
Cursor already understands your codebase, your onboarding flows, and the copy living in your templates. Wiring marginal in as an MCP server means the same AI agent that edits your signup logic can also propose and test the subject lines that greet new users.
marginal is a hosted email MCP server, so there's nothing to deploy. Cursor calls the tools over HTTPS at https://marginal.sh/mcp and the sends, tracking, and lift math happen on the server side.
- Keep welcome series copy decisions next to the code that triggers them
- Let an AI agent draft subject line variants grounded in your product context
- No separate dashboard tab-switching during iteration
Generating and launching a subject line test
A typical welcome-series flow in Cursor uses three of marginal's tools in sequence. Ask the agent to call generate_variants for a few distinct subject line angles, then launch_test to start the subject line A/B test against your welcome segment.
Each variant is sent with open and click tracking attached, so you get clean within-test lift metrics rather than guesses.
- generate_variants — produce candidate welcome subject lines (curiosity, benefit, personal tone)
- launch_test — start the managed send and subject line A/B test
- get_results — pull opens, clicks, and within-test lift
- recommend_next — let the AI agent suggest the next iteration
Iterating with an AI agent
Welcome series optimization is rarely one-and-done. After get_results returns, call recommend_next so the agent can reason over what worked and propose a sharper round of variants. This loop — generate, test, read, recommend — turns AI agent email marketing into a measurable practice instead of a one-off prompt.
Because everything runs through the same MCP session, your Cursor chat keeps the full history of which welcome subject lines won and by how much.
- Compare lift across each welcome email in the sequence
- Re-test winning angles with fresh phrasing
- Promote the best-performing subject line to your live welcome series
Getting set up
Add marginal to Cursor's MCP configuration with your Bearer API key (the marg_live_... token from your account) and point it at https://marginal.sh/mcp. The free tier covers 100 experiments per month, which is plenty for refining a welcome series across multiple subject line tests.
Full tool reference and configuration steps 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
- Docs: 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