Quoin

the agent edits, you decide.

Above: a real session, captured live. An agent connected over MCP applies three edits to the open buffer (a grammar fix, a typo, a missing section); each lands selected and unsaved. Then plain undo, the same step Cmd+Z triggers, peels the agent's edits off one at a time until the original text is back and the buffer is clean. One agent edit, one undo step; disk is never touched until you save.

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.

Quoin is a macOS text editor where AI agents are first-class users. An agent connected over MCP reads and edits the live buffer, unsaved changes included. Every agent edit lands as one undoable step on the normal undo stack, and nothing touches disk until you press Cmd+S: the agent proposes, you dispose.

Beyond the agent surface, it is a Sublime-class editor: native tabs, tree-sitter highlighting, Goto Anything, command palette, split panes, multi-cursor, hot exit, and Sublime's menu vocabulary (line, comment, case, sort and permute commands, selection tools, syntax and indentation switches), every item of which is also an agent verb. Swift + AppKit, pure SwiftPM, macOS 14+, no Xcode.app required.

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. It is one of a pair of agent-native instruments built on the same principle (bounded verbs for the agent, commit rights for the human); the other is CBCTScope, a CBCT viewer for medical imaging. Both at rezamotaghi.com.

Quickstart

Scripts/test.sh          # swift build + swift test: 113 unit tests
Scripts/bundle-app.sh    # -> build/Quoin.app
open build/Quoin.app

Full guide: QUICKSTART.md, also in-app via Help > Quickstart Guide. Architecture and invariants: ARCHITECTURE.md.

Connect an AI agent (MCP)

The running app listens on a local unix socket (no network listener, local-only by construction). The bundled shim exposes that socket as an MCP server:

claude mcp add quoin -- "$(pwd)/build/Quoin.app/Contents/MacOS/QuoinMCP"

The same line is under Agent > Copy MCP Setup Command in the app. For Claude Desktop, each release carries quoin-<version>.mcpb, a one-click MCP Bundle of the shim (Apple silicon).

Eleven verbs, each self-describing (annotations and an output schema):

• Read: quoin_list_open_documents, quoin_read_buffer (the live buffer, unsaved edits included), quoin_read_lines (a window of lines), quoin_get_selection, quoin_list_commands.
• Act: quoin_open_file (path and line), quoin_run_command (any menu command by id).
• Write: quoin_replace_selection, quoin_replace_lines, quoin_apply_edit (offset range), quoin_set_text (whole buffer).

Each write is one step on the normal undo stack, so Cmd+Z peels agent edits off one at a time, back to a pristine buffer, whether or not you were at the keyboard. Edits mark the document dirty like typing does; the file on disk changes only when you save. The commit fence makes that a mechanism rather than a sentence: save, save as, revert, close, and quit are refused over the agent surface, so the only save button is yours.

Beyond verbs: the open buffers are MCP resources (quoin://buffer and quoin://selection for the front document, quoin://buffer/<absolute path> and quoin://selection/<absolute path> for any open file, quoin://documents), and a host can subscribe to a buffer and be told the moment it changes instead of polling. Three prompts package the choreographies (proofread-selection, review-buffer, undo-tour), and skills/quoin-editing/SKILL.md teaches any host the verbs and the contract. Turn the whole surface off with "agent_server": false in settings, or in the Agent menu.

Open files from a terminal

ln -s "$(pwd)/Scripts/quoin" /usr/local/bin/quoin   # once, from the repo root
quoin notes.md                                       # open a file
quoin src/main.swift:42                              # open at line 42

quoin drives the quoin://open?file=...&line=... URL scheme; anything else on the machine can use those links directly.

Editing features

Left: tree-sitter highlighting (Mariana scheme) on the code that makes agent edits undoable. Right: markdown coloring beside the rendered preview, Cmd+Shift+M. Both are the real app, captured over its own MCP surface.

Key	Action
Cmd+P	Goto Anything (fuzzy file open in the project folder)
Cmd+Shift+P	Command Palette
Cmd+D	Select word, then add next occurrence (multi-cursor)
Ctrl+Cmd+G	Select all occurrences
Escape	Collapse to one caret
Cmd+Alt+2	Toggle split editor (two views, one buffer)
Cmd+F	Find; Cmd+Alt+F find and replace
Cmd+Shift+D / Ctrl+Shift+K	Duplicate / delete line; Ctrl+Cmd+Up/Down swap lines
Cmd+/	Toggle comment (per-language token)
Cmd+Shift+L	Split selection into lines; Ctrl+Shift+Up/Down add a caret on the previous / next line
Ctrl+G	Goto line
Cmd+Shift+M	Markdown preview
Cmd+Shift+[ / ]	Previous / next tab
Cmd+,	Settings (your settings.jsonc, in the editor)

Ten menus carry Sublime's vocabulary: Edit > Line, Comment, Convert Case, Sort Lines, Permute Lines; Selection; Find; View > Word Wrap, Line Numbers, Whitespace, Syntax, Indentation, Font, Theme; Goto; and an Agent menu (status, the setup command, the server switch, Undo Last Agent Edit). Every item is also a palette command and an agent verb; the View toggles write your settings file in place, comments kept. Tree-sitter highlighting ships for Swift, Python, JSON, and Markdown; JSONC uses a hand-rolled lexer, and other file types open unhighlighted as plain text (View > Syntax overrides per document). Hot exit is on by default: quit and relaunch restores every tab, including unsaved buffers (graceful quit; a force-kill loses them). If anything rewrites a file you have unsaved edits in, a banner offers Reload From Disk / Keep My Edits.

Settings

Sublime key names, JSONC, hot-reloaded on save:

• Defaults (documented): Settings/default-settings.jsonc (ships in the app)
• Your overrides: ~/Library/Application Support/Quoin/settings.jsonc
• Color schemes: Settings/schemes/*.jsonc (mariana dark, breakers light, "theme": "auto" follows macOS)

Contributing

Small, tested, one-concern PRs are welcome: CONTRIBUTING.md has the build steps and conventions. Security reports go through private advisories, not issues: SECURITY.md. Release history: CHANGELOG.md.

License

MIT, see LICENSE.

Built by Dr. Reza Motaghi, an oral and maxillofacial radiologist who trains imaging models on his own reads and tests them on cases they have never seen. More at rezamotaghi.com.

Sublime Text is a trademark of Sublime HQ Pty Ltd. Quoin is an independent project, not affiliated with or endorsed by Sublime HQ.