figcraft/docs/control-protocol.md

FigCraft control protocol

The desktop app can be driven by other programs (tests, scripts, figcraft-cli mcp --attach, sibling Crafting Apps) through a loopback socket:

cargo run -p figcraft -- --control 7979 [file.figcraft|file.svg|image]

The app listens on 127.0.0.1:<port> only. Each connection carries newline-delimited JSON: one request object per line, one reply line per request, in order.

request:  {"id": 1, "method": "engine.execute", "params": {...}}
reply:    {"id": 1, "ok": true, "result": ...}
          {"id": 1, "ok": false, "error": "message"}

Requests are queued and run on the UI thread at the start of the next frame, so replies reflect the real UI state. A request that the UI doesn't answer within 60 s gets a timeout error. A line that is not JSON gets {"id": null, "ok": false, "error": "bad JSON: …"}. The implementation is in crates/ui-egui/src/control.rs (methods) and apps/figcraft/src/control_server.rs (socket).

Coordinates are page units, y down. x/y in node.set/node.create are parent-relative.

Methods

engine.execute {command, params?}

Run any command (the same ids as the menus, shortcuts, Actions palette, CLI and MCP). Programmatic calls never open dialogs: dialog requests are dropped. Returns the command's result (creation commands return {"id": N}).

{"id":1,"method":"engine.execute","params":{"command":"shape.rectangle","params":{"x":10,"y":10,"width":100,"height":50}}}
{"id":1,"ok":true,"result":{"id":2}}
{"id":2,"method":"engine.execute","params":{"command":"node.set","params":{"ids":[2],"props":{"fill":"#FF6B6B","cornerRadius":8}}}}
{"id":3,"method":"engine.execute","params":{"command":"command.batch","params":{"label":"Tidy","commands":[{"command":"align.left","params":{"ids":[2,3]}},{"command":"distribute.vertical","params":{"ids":[2,3]}}]}}}

engine.commands {}

Every command: [{id, label, params, menu, shortcut, undoable}].

document.inspect {depth?, allPages?}

The active document as JSON: name, path, dirty, currentPage, selection, pages (the current page's layer tree to depth, default 8; others summarised unless allPages), styles, variable count, images, the text being edited and the undo history.

node.get {id, depth?}

One layer (children to depth, default 1). Fields: id, name, type, x, y, width, height, rotation, absoluteBounds, visible, locked, opacity, blend, fills, strokes, strokeWeight, effects, cornerRadius, layout (including grid columnTracks / rowTracks), sizing, minWidth / maxWidth / minHeight / maxHeight, absolute, constraints (children of plain frames and absolute children), gridCell (a grid auto layout child's column, row and spans), clip, text properties, component/instance links, overrides, boundVariables, interactions, exports. The auto layout props that node.set accepts are listed in mcp.md.

ui.inspect {}

UI state: panels and tabs, current tool, canvas rect, dialogs, view, timings.

ui.pointer {kind, x, y | sx, sy, to?, steps?, shift?, alt?, cmd?, ctrl?}

A mouse gesture with the current tool. kind: down, move, up, click, double, hover, drag (needs to: [x, y]; steps intermediate moves, default 8). Position in page coordinates (x, y) or screen pixels (sx, sy). Returns {selection, tool}.

{"id":4,"method":"engine.execute","params":{"command":"tool.set","params":{"tool":"ellipse"}}}
{"id":5,"method":"ui.pointer","params":{"kind":"drag","x":200,"y":200,"to":[300,260],"shift":true}}

ui.key {key, shift?, alt?, cmd?, ctrl?}

A key as if typed on the canvas (in vector edit mode the edit keys come first: V P Q X Shift+B tools, arrows nudge points, Delete/Shift+Delete, Cmd+A, Cmd+J): a menu shortcut ({"key":"G","cmd":true} groups), a tool key, Escape, Delete/Backspace, Enter, arrows (nudge; shift = big nudge). While text is being edited, keys go to the text editor. Returns {"command": id} for shortcuts.

ui.type {text}

Type into the text being edited (start editing with a double-click, text.edit.begin {id}, or the text tool).

ui.menu.invoke {id}

Invoke a menu item exactly like clicking it (this may open a dialog). Returns {ok, lastError}.

ui.set {rightTab?, leftTab?, theme?, home?, palette?, mainMenu?}

Switch panels and modes: rightTab design|prototype|inspect, leftTab file|assets, theme, home (bool), palette (bool: the Actions palette), mainMenu (bool). Returns ui.inspect.

ui.screenshot {path}

PNG of the whole window, UI included (needs a visible window). Returns {path, width, height}.

ui.render {path, scale?, ids?}

PNG of the document without UI chrome (works without a window). Without ids it exports the selection, else the page's top-level layers. Returns the export result.

app.quit {}

Close the app.

Example session (shell)

port=7979
figcraft --control $port &
exec 3<>/dev/tcp/127.0.0.1/$port
echo '{"id":1,"method":"engine.execute","params":{"command":"frame.create","params":{"preset":"iPhone 16","x":0,"y":0}}}' >&3
read -r reply <&3; echo "$reply"
echo '{"id":2,"method":"ui.screenshot","params":{"path":"/tmp/figcraft.png"}}' >&3
read -r reply <&3; echo "$reply"

For agents, prefer the MCP server (figcraft-cli mcp --attach 7979, see docs/mcp.md): it wraps these methods in documented tools and returns images inline.