Back in Stock Subject Line Tests in Claude Code
Restock alerts are time-sensitive — the right subject line can mean the difference between a sellout and a stale inbox. With marginal connected to Claude Code, you can draft, test, and ship back in stock subject lines without leaving your editor.
Why test back in stock subject lines
A back in stock email carries urgency that few other campaigns do: the recipient already wanted the product. That makes the subject line your highest-leverage variable. Small wording changes — naming the product, hinting at limited quantity, or leading with the customer's wishlist — can swing open rates dramatically.
Rather than guessing, run a subject line A/B test. marginal handles the split, the managed send, and the open/click tracking so you can compare variants on real recipients and see within-test lift instead of relying on intuition.
- Name the product directly vs. tease scarcity
- "It's back" vs. "You asked, we restocked"
- Emoji vs. plain text in the subject line
- Personalized first name vs. product-led opener
Using Claude Code with the email MCP server
Claude Code is a supported MCP client for marginal, so you can drive the whole workflow conversationally. Point it at the marginal endpoint at https://marginal.sh/mcp, authenticate with your Bearer API key, and the four core tools become available to the agent.
From a Claude Code session you can ask the agent to generate back in stock subject line candidates, launch the test, and report results — all using natural language while marginal does the email infrastructure work behind the scenes.
- generate_variants — produce restock subject line options
- launch_test — start the subject line A/B test on a managed send
- get_results — pull open and click metrics with within-test lift
- recommend_next — get a data-backed suggestion for the follow-up send
A restock test workflow, step by step
Here's a typical loop for AI agent email marketing around a restock. Each step maps to a single prompt in Claude Code, and marginal keeps the experiment state so you don't have to track it manually.
- Ask Claude Code to generate five back in stock subject lines for the returning product
- Launch the subject line test against your restock-interested segment
- Let the send run and check get_results for open and click lift
- Use recommend_next to pick the winner for a wider follow-up blast
Getting started
marginal is hosted, so there's nothing to deploy or maintain — you connect Claude Code to the registered server and start testing. The free tier covers 100 experiments per month, which is plenty for iterating on restock and other transactional-adjacent campaigns.
Full setup details, tool schemas, and client configuration live in the docs at https://marginal.sh/docs/. Once connected, your first back in stock subject line test is a single Claude Code prompt away.
- Endpoint: https://marginal.sh/mcp
- Registry: sh.marginal/mcp
- Auth: Bearer API key (marg_live_...)
- Free tier: 100 experiments/month
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