The name

A quoin (pronounced “coin”) is the letterpress wedge that locks loose type into the frame so it can print. That is this editor’s job in the agent era: hold the text steady while agents work on it. Nothing is committed until you lock it in.

Why there’s no AI inside

Quoin ships no model, no API keys, no chat pane. It is the instrument: you bring whatever agent you want, over MCP, and swap it the day a better one exists. Editors that embed a single vendor’s agent make the editor the gatekeeper; here the editor’s whole job is to give any agent honest access to the buffer and give you the only save button.

The control model

The running app listens on a local unix socket only; there is no network listener. The agent gets nine verbs: read the buffer and selection, open files, list and run editor commands, and three ways to write. Each write registers its own inverse on the document’s undo manager, so a human can step backward through an agent’s work edit by edit, back to a pristine buffer. Beyond the agent surface it is a full working editor: native tabs, tree-sitter highlighting, fuzzy file open, command palette, split panes, multi-cursor, hot exit.