Discount Percentage Subject Line Test
Does "Save 25%" beat "Get $15 off" in your subject line? A discount percentage subject line test answers that with data instead of guesswork. With marginal, an AI agent can spin up the variants, launch the send, and report the lift — all through MCP tools.
Why discount framing is worth testing
Subscribers react differently to the same offer depending on how it's framed. A percentage discount often feels larger on lower-priced items, while a flat dollar amount can read as more concrete on higher-ticket products. The only reliable way to know which wins for your list is to run a controlled subject line A/B test.
- Percentage framing: "25% off your next order"
- Dollar framing: "$15 off — today only"
- Urgency overlays: adding "ends tonight" to either variant
- Number prominence: leading with the discount vs the product
Running the test with marginal
marginal is a hosted email MCP server that exposes four tools to your AI agent: generate_variants, launch_test, get_results, and recommend_next. For a discount percentage subject line test, you describe the offer and let the agent draft framing variants, then launch the split.
Because marginal handles managed sends and open/click tracking, you don't wire up a separate sending pipeline. The within-test lift metrics tell you whether the percentage or dollar variant pulled ahead, and by how much.
- generate_variants — produce percentage vs dollar subject lines
- launch_test — split your audience and send
- get_results — open and click rates per variant
- recommend_next — suggest the follow-up test to run
AI agent email marketing in practice
AI agent email marketing shines on repetitive optimization like discount framing. Instead of manually building each variant in an ESP UI, you prompt your agent inside Cursor, Claude Desktop, Claude Code, Windsurf, or any MCP client, and it calls marginal's endpoint at https://marginal.sh/mcp to execute the test.
A typical loop: generate four discount framings, launch the test, wait for statistically meaningful opens and clicks, then ask the agent for the recommended next experiment — perhaps testing the winning percentage against a free-shipping angle.
Getting started
Connect any supported MCP client with a Bearer API key (marg_live_...) and point it at the marginal endpoint. The free tier covers 100 experiments per month, which is plenty to validate discount framing across several segments before scaling.
- Endpoint: https://marginal.sh/mcp
- Auth: Bearer API key
- 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