Testing Re-engagement Subject Lines in Windsurf
Win-back campaigns live and die by the subject line. With marginal connected to Windsurf, you can draft, test, and ship re-engagement subject lines without leaving your editor — letting an AI agent run the whole subject line A/B test loop for you.
Why run re-engagement tests from Windsurf
Dormant subscribers are tough to reach: they've stopped opening, and a flat "We miss you" rarely moves the needle. Subject line testing is the fastest lever you have, and Windsurf's agentic workflow is a natural place to iterate on it.
Because marginal is a hosted email MCP server, you connect Windsurf once via the endpoint at https://marginal.sh/mcp and start generating variants. No infrastructure to run, no self-hosted server to maintain.
- Stay in your editor while the agent drafts and ships subject lines
- Reuse Windsurf's context — campaign notes, segment definitions, past copy
- Let an AI agent handle the mechanical parts of email marketing
The re-engagement test workflow
A typical re-engagement subject line test in Windsurf follows marginal's four MCP tools end to end. You describe the dormant segment and the tone you want, and the agent does the rest.
- generate_variants — produce several re-engagement subject lines with distinct angles (curiosity, incentive, blunt honesty)
- launch_test — start the subject line A/B test against your win-back segment with managed sends
- get_results — pull open and click tracking plus within-test lift metrics as data arrives
- recommend_next — get a data-backed call on the winner and the next iteration
Writing prompts that surface better win-back angles
Good re-engagement subject lines need contrast. Ask Windsurf to vary the underlying psychology, not just the wording, so your A/B test learns something real about what wakes your audience up.
Be explicit about constraints — character length, whether to include an offer, and the brand voice — so generate_variants returns options worth shipping.
- "Draft 4 re-engagement subject lines for users inactive 90+ days: one curiosity, one incentive, one social proof, one plain-spoken."
- "Keep each under 45 characters and avoid the words 'miss' and 'back'."
- "Launch a subject line A/B test and report open-rate lift once results stabilize."
Get started
Add marginal to Windsurf, point it at https://marginal.sh/mcp with your Bearer API key, and run your first re-engagement subject line test. The free tier covers 100 experiments per month — enough to validate a win-back campaign before scaling.
Full setup 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
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