Documentation

Mercur gives a process on your machine a public HTTPS endpoint, and keeps a record of every request that arrived at it. You point a webhook provider, a teammate or an agent at a URL, and you can read exactly what they sent.

What works today

Mercur is in alpha, and the docs only describe what is built:

HTTP endpoints. A public URL per endpoint, with three run modes: record requests, forward them to a local port, or both.
A macOS desktop app that carries traffic to your machine.
An npm CLI (@mercur_dev/cli) that does the same from a terminal, CI, or any OS with Node.js 20+. HTTP needs no account: npx @mercur_dev/cli http 3000. See CLI agent.
The browser console can create endpoints and record requests, but it cannot forward traffic to your machine.
WebSocket and TCP are on Solo and Team. Subscribe from pricing (workspace owners). Guest and Free stay HTTP-only. WebSocket endpoints record connections and frames in the console — see Inspect a WebSocket session.
Remote MCP lets MCP clients inspect request history via OAuth or workspace API keys. See Remote MCP and the marketing page /mcp.
Temporary mailboxes on Solo and Team (@mercur-mail.com) inspect inbound email and can forward parsed JSON to an HTTP endpoint. See Use a temporary mailbox.

There is no Windows or Linux desktop app. Forwarding on those platforms uses the npm CLI.

Ways in

No accountReceive a webhook

Get a public endpoint and read a real request. Nothing to install, nothing to run locally.

No accountExpose a local service

Forward public HTTP traffic to a port on your machine with the npm CLI, or the macOS app.

npmCLI agent

HTTP without an account: npx @mercur_dev/cli http 3000. Token + start is advanced.

Solo / TeamUse a temporary mailbox

Get a @mercur-mail.com inbox, inspect inbound email, and optionally forward parsed JSON to an HTTP endpoint.

Solo / TeamInspect a WebSocket session

Forward a local WebSocket service and read handshake, frames, and connection state in the console.

Reference

Limits — what each plan allows and what happens when you reach a cap.
Security and data retention — what is stored, for how long, and how to restrict who can reach an endpoint.
Replay a stored request — re-send a captured HTTP request with Retry.
Remote MCP — MCP client access to request history.
Troubleshooting — every error the relay returns, and its cause.