Win-back subject line tests in Claude Code
Re-engaging dormant subscribers starts at the inbox, and the subject line decides whether your win-back email gets opened at all. With marginal connected as an email MCP server, Claude Code can generate variants, launch a subject line test, and report results without leaving your editor.
Why test win-back subject lines from Claude Code
Win-back campaigns target people who haven't opened or clicked in a while, so subject line tone and framing carry outsized weight. A line that works for active subscribers may fall flat for someone who's gone cold. Testing is the only reliable way to learn what pulls them back.
Claude Code keeps you in a single agentic workflow: describe the audience, ask for win-back angles, and have the agent run a real subject line A/B test through marginal's MCP tools. The model proposes copy; marginal handles the send, tracking, and within-test lift math.
- Iterate on win-back angles conversationally instead of copy-pasting between tools
- Generate and compare several subject lines in one prompt
- Let the agent launch the test and fetch open/click results
The marginal MCP tools you'll use
marginal exposes four tools to any MCP client, including Claude Code. For a win-back subject line test, you'll typically chain them in order.
- generate_variants — draft multiple win-back subject lines for the segment
- launch_test — start the managed send and split traffic across variants
- get_results — pull open and click tracking plus within-test lift metrics
- recommend_next — get a data-backed suggestion for the follow-up subject line
A win-back workflow, step by step
Point Claude Code at your win-back goal and let the AI agent email marketing loop do the rest. A typical session looks like this:
- Prompt: "Generate 4 win-back subject lines for subscribers inactive 90+ days."
- Review the variants and ask Claude Code to refine tone — urgency, incentive, or curiosity.
- Launch the subject line A/B test against your dormant segment via launch_test.
- Check get_results once opens accumulate to see which line lifts engagement.
- Use recommend_next to plan the second touch in the win-back series.
Getting connected
marginal is hosted, so there's nothing to deploy. Add the endpoint at https://marginal.sh/mcp to Claude Code with a Bearer API key (marg_live_...), and the four tools become available to the agent immediately.
The free tier covers 100 experiments per month, which is plenty for running and iterating on win-back subject line tests. Full setup details for Claude Code and other clients live in the docs at 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