Testing referral program subject lines in Cline
Cline can already read your codebase and run tasks — with marginal's email MCP server it can also draft, test, and ship referral program subject lines without leaving your editor.
Why run a referral subject line test from Cline
Referral program emails live or die on the subject line. A clear incentive in the inbox ('Give $20, get $20') almost always beats a vague nudge, but you can't know your audience's preference until you run a subject line A/B test. With marginal connected to Cline, you describe the campaign in plain language and the agent handles variant generation, launch, and result analysis.
Because Cline operates inside your workflow, you can wire referral copy decisions right alongside the code that triggers the send — feature-flag a referral milestone, then ask Cline to test which subject line drives the most invites.
- Iterate on referral subject lines without context-switching to a separate tool
- Let the AI agent email marketing loop generate, send, and report in one thread
- Tie subject line tests to product events your team already tracks
Connect marginal to Cline
marginal is a hosted email MCP server, so there's nothing to self-host. Add the endpoint and your Bearer API key (marg_live_...) to Cline's MCP configuration and the four tools become available immediately.
- Endpoint: https://marginal.sh/mcp
- Auth: Bearer API key beginning with marg_live_
- Tools exposed: generate_variants, launch_test, get_results, recommend_next
- Free tier covers 100 experiments per month
A referral program workflow in Cline
Once connected, prompt Cline with the goal of your referral campaign and the agent chains the marginal tools together. A typical run looks like this:
- Call generate_variants to draft 4–6 referral program subject lines around your incentive
- Use launch_test to start a managed send with even traffic across variants
- Poll get_results for open and click tracking plus within-test lift metrics
- Ask recommend_next to surface the winning subject line and a follow-up idea
Tips for stronger referral variants
When you ask Cline to generate referral subject lines, give it the reward amount, the audience segment, and the action you want. Concrete inputs produce sharper variants and a cleaner subject line A/B test.
- Test incentive-forward vs. social-proof angles ('Your friends are earning credits')
- Vary specificity: exact dollar amount vs. percentage vs. 'free month'
- Keep enough send volume per variant for the lift metrics to be meaningful
- Check the docs at https://marginal.sh/docs/ for tool parameters and result fields
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