The sh.marginal MCP Registry Entry Explained
If you found marginal through the sh.marginal MCP registry, here's exactly what it does, which tools it exposes, and how an AI agent connects to run subject line A/B tests end to end.
What the sh.marginal MCP registry identifier means
The registry name sh.marginal/mcp is how clients and directories reference marginal as a discoverable Model Context Protocol server. It maps to a single hosted endpoint — https://marginal.sh/mcp — so there's nothing to install or self-host.
Authentication is a Bearer API key in the form marg_live_..., passed by your MCP client. Once connected, an agent sees a small, purpose-built toolset focused on email experimentation rather than a generic mail dump.
- Registry: sh.marginal/mcp
- Endpoint: https://marginal.sh/mcp
- Auth: Bearer API key (marg_live_...)
- Hosted only — no local MCP server to run
Tools exposed by this email MCP server
marginal is an email MCP server built so an AI agent can design, ship, and read an experiment without leaving the conversation. The four tools form a tight loop for any subject line A/B test.
- generate_variants — produce candidate subject lines to test
- launch_test — start a managed send across variants
- get_results — pull open/click tracking and within-test lift
- recommend_next — suggest the follow-up experiment to run
Using it for AI agent email marketing
Because the registry entry surfaces clean tool schemas, agents handle the full workflow: draft variants, launch a test, wait for data, then act on lift metrics. This makes marginal a practical building block for AI agent email marketing where a human reviews decisions rather than copy-pasting between dashboards.
The free tier covers 100 experiments per month, which is enough to wire up a real subject line testing pipeline before committing.
- Managed sends with open and click tracking
- Within-test lift metrics for clear winners
- 100 experiments/month on the free tier
- Works in Cursor, Claude Desktop, Claude Code, Codex, Windsurf, Cline, Continue, and Zed
Connecting from the registry entry
Point your MCP client at the endpoint listed in the registry and add your key. From there the agent can call generate_variants and launch_test directly. Full setup instructions for each supported client live in the docs at https://marginal.sh/docs/, and the canonical endpoint is always https://marginal.sh/mcp.
- Add https://marginal.sh/mcp to your client config
- Supply your marg_live_... Bearer key
- Verify the four tools appear in your agent
- Run a first subject line A/B test to confirm tracking
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