Testing Loyalty Reward Subject Lines in Zed
Rewards emails only earn their open if the subject line lands. With marginal connected to Zed as an MCP server, you can generate and A/B test loyalty reward subject lines right where you already write code — and let the AI agent handle the experiment lifecycle.
Why pair Zed with marginal for loyalty emails
Loyalty reward campaigns are sensitive to tone. "You've earned 500 points" reads very differently from "Your reward is waiting" — and the only way to know which wins with your list is to test. marginal is a hosted email MCP server, so Zed's assistant can call its tools directly instead of you bouncing between an ESP dashboard and your editor.
Because marginal is fully hosted, there's no self-hosted server to maintain. You add the endpoint, drop in a Bearer API key, and the four core tools become available to the agent inside Zed.
- generate_variants — draft reward-focused subject line options
- launch_test — send a subject line A/B test to your audience
- get_results — pull open/click and within-test lift metrics
- recommend_next — get a data-backed pick for the next send
A loyalty reward subject line test, end to end
Start by describing the campaign to the agent in Zed: a points-milestone email, a tiered-status upgrade, or a redeem-by-date nudge. Ask it to generate variants that play with value framing, urgency, and personalization. From there the workflow is a short chain of tool calls.
A typical AI agent email marketing loop for a loyalty reward looks like this:
- Generate 4–6 subject lines emphasizing the reward value and member status
- Launch a subject line A/B test split across a sample of your loyalty segment
- Track opens and clicks as marginal records within-test lift
- Call recommend_next to roll the winner out to the rest of the segment
Connecting marginal in Zed
Configure marginal as an MCP server in Zed using the registry name sh.marginal/mcp and the endpoint https://marginal.sh/mcp. Authentication is a single Bearer API key in the form marg_live_..., so there are no ESP credentials to wire up in the editor.
Once connected, prompt the assistant naturally — for example, "draft and launch a loyalty reward subject line test for members hitting Gold tier." The agent maps that to the right marginal tools and returns results you can iterate on. See https://marginal.sh/mcp and the docs at https://marginal.sh/docs/ for setup details.
- Endpoint: https://marginal.sh/mcp
- Registry: sh.marginal/mcp
- Auth: Bearer key (marg_live_...)
- Free tier: 100 experiments per month
Tips for sharper reward subject lines
Loyalty audiences already opted in, so your test should isolate phrasing rather than offer mechanics. Keep the reward constant across variants and change one lever at a time — that keeps the lift metric interpretable when you read results back in Zed.
- Test specific value ("$10 reward") against status framing ("Gold perk unlocked")
- Try first-name personalization versus a clean, short subject
- Compare urgency ("expires Sunday") with no deadline at all
- Hold your segment and send size steady so lift comparisons stay fair
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