An agent-callable email API for subject line testing
An agent-callable email API lets your AI assistant run real email experiments without you stitching together SDKs, webhooks, and analytics dashboards. marginal exposes exactly that through a hosted email MCP server — your agent calls a tool, marginal sends and measures.
What 'agent-callable' actually means
A traditional email API expects a human developer to read docs, write request code, and handle auth manually. An agent-callable email API is shaped for autonomous use: the operations are discrete, typed tools that an AI agent can discover and invoke on its own.
marginal delivers this over the Model Context Protocol. Once connected, an agent sees a small, purpose-built toolset rather than a sprawling REST surface, so it can reason about email work step by step.
- generate_variants — draft multiple subject lines for a campaign
- launch_test — start a subject line A/B test with managed sends
- get_results — pull open/click tracking and within-test lift metrics
- recommend_next — get the data-backed next action
Why agents make subject line A/B tests easier
Subject line testing is repetitive and metrics-heavy — a good fit for AI agent email marketing. The agent can brainstorm variants, launch the split, wait for signal, and read the lift without a human babysitting each step.
Because marginal handles the send infrastructure and tracking, the agent never touches deliverability plumbing. It reasons in terms of experiments and outcomes, which keeps prompts and tool calls simple.
- Generate and test subject variants in one conversation
- Open/click tracking returned as structured data the agent can compare
- Within-test lift metrics so winners are clear, not guessed
- Free tier of 100 experiments/month to validate the workflow
Connecting the API to your agent
marginal is hosted at https://marginal.sh/mcp, so there's nothing to deploy or self-host. You authenticate with a Bearer API key (marg_live_...) and point a supported MCP client at the endpoint.
Clients include Cursor, OpenAI Codex, Claude Desktop, Claude Code, Windsurf, Cline, Continue, and Zed. Add the server config once and the email tools become available to whatever agent runs inside that client.
- Endpoint: https://marginal.sh/mcp
- Auth: Bearer API key (marg_live_...)
- Registry id: sh.marginal/mcp
- Full reference: 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