Testing Cross-Sell Subject Lines in Windsurf
Cross-sell campaigns live or die on the subject line. With marginal connected as an MCP server, you can draft, test, and measure cross-sell subject lines in Windsurf — using the same AI agent flow you already use to write code.
Why run cross-sell subject line tests from Windsurf?
Cross-sell emails ask an existing customer to add a complementary product. That intent is different from acquisition or win-back, so the subject line has to feel relevant rather than promotional. The fastest way to find what resonates is to test several angles against each other and let opens and clicks decide.
Because marginal is a hosted email MCP server, Windsurf can call its tools directly. You stay in your editor, describe the cross-sell offer, and the agent handles variant generation, the send, and the results — no separate marketing dashboard to babysit.
- Keep campaign work next to the codebase that triggers your sends
- Let an AI agent for email marketing draft and compare angles for you
- Measure within-test lift before committing a winner to the full list
Connecting marginal to Windsurf
Add marginal as an MCP server in Windsurf using the hosted endpoint at https://marginal.sh/mcp and a Bearer API key (marg_live_...). Nothing is self-hosted — once the key is in place, the four tools become available to the agent.
The tools you'll lean on for a subject line test are generate_variants, launch_test, get_results, and recommend_next. See the docs at https://marginal.sh/docs/ for the full configuration.
- Endpoint: https://marginal.sh/mcp
- Auth: Bearer API key in the marg_live_ format
- Free tier covers 100 experiments per month to get started
A cross-sell subject line workflow
Start by giving Windsurf the context that matters for cross-sell: the product the customer already owns, the complementary item you're offering, and the tone you want. Ask the agent to call generate_variants to produce a spread of subject lines — pair a benefit-led line against a curiosity line, and a 'goes well with' framing against a discount-free reminder.
Then launch the subject line A/B test against a sample of your list and watch open and click tracking decide the winner with real lift metrics.
- generate_variants — produce cross-sell subject line candidates
- launch_test — start a managed send as a subject line test
- get_results — pull open/click rates and within-test lift
- recommend_next — get the suggested winner and follow-up angle
Reading results and iterating
Once get_results reports back, look at lift rather than raw opens alone — a subject line that wins opens but loses clicks isn't a true cross-sell winner. marginal surfaces within-test lift so the comparison is apples to apples.
Use recommend_next to decide the follow-up. For cross-sell, the highest-clicking angle often becomes the template for related product lines, so you can re-run a tighter subject line A/B test on the next segment without rebuilding the workflow in Windsurf.
- Prioritize click lift for cross-sell intent, not just opens
- Promote the winning angle into your next cross-sell campaign
- Re-run quick tests per segment to keep relevance high
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