Cold Email Subject Line A/B API
If you're searching for a cold email subject line A/B API, you want one call to spin up variants, send them, and tell you which line wins. marginal exposes exactly that through an email MCP server your AI agent can drive directly.
What a subject line A/B API should do
Cold email lives and dies on the subject line. The hard part isn't writing one option — it's testing several against a real segment and measuring open lift without stitching together send infrastructure and analytics yourself.
marginal collapses that workflow into four MCP tools so a subject line A/B test is a few structured calls rather than a multi-tool integration project.
- generate_variants — produce several subject line candidates for a cold email
- launch_test — split your list and send the variants
- get_results — pull open and click metrics with within-test lift
- recommend_next — get a data-backed call on the winning line
Why use the MCP endpoint instead of raw HTTP
marginal is a hosted email MCP server, so the same A/B capabilities are available to any MCP-capable client without you running infrastructure. Point your agent at the endpoint, authenticate, and the subject line tools show up as callable functions.
That means an AI agent doing cold email marketing can decide to test subject lines mid-conversation — generate variants, launch the test, and report lift — instead of you copying results between dashboards.
- Endpoint: https://marginal.sh/mcp
- Auth: Bearer API key (marg_live_...)
- Registry id: sh.marginal/mcp
- Works with Cursor, Claude Code, Claude Desktop, Windsurf, Cline, Continue, Codex, and Zed
A typical cold email A/B flow
A practical loop looks like this: ask generate_variants for a handful of subject lines on your offer, call launch_test to send them across a sample of recipients, then poll get_results once data accrues. Lift is computed within the test, so you compare variants against each other rather than guessing at an absolute baseline.
When a clear leader emerges, recommend_next tells you which subject line to roll out to the rest of your list — closing the loop from idea to decision in one API surface.
- Generate 3–5 subject line variants per cold email
- Launch against a representative sample segment
- Read open/click tracking and within-test lift
- Promote the winner to your full send
Getting started
The free tier covers 100 experiments per month, which is enough to validate subject lines across several cold email campaigns before committing. Managed sends and tracking are included, so you don't bolt on a separate analytics layer.
See the docs at https://marginal.sh/docs/ for the full tool schemas and example payloads for each call in the A/B workflow.
- Free tier: 100 experiments/month
- Managed sends with open/click tracking built in
- 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