effectcraft/docs/control-protocol.md

Control protocol

The desktop app exposes a control channel for agents, tests and the MCP bridge:

effectcraft --control 9877        # or EFFECTCRAFT_CONTROL_PORT=9877

• Transport: TCP on 127.0.0.1 only, one JSON object per line in each direction. Connections are persistent; requests on one connection are answered in order.
• Request: {"id": 1, "method": "engine.execute", "params": {...}} (id is echoed back, params defaults to {}).
• Reply: {"id": 1, "ok": true, "result": ...} or {"id": 1, "ok": false, "error": "message"}.
• Requests run on the UI thread between frames. Input methods (ui.click, ui.key and similar) reply after the synthetic input has been processed. Methods that need an element that isn't drawn yet are retried for a few frames. A request times out after 60 s.

Quick test:

printf '{"id":1,"method":"engine.execute","params":{"command":"comp.new","params":{"name":"A"}}}\n' | nc 127.0.0.1 9877

Engine

Method	Params	Result
engine.execute	{command, params?}	The command's result. Every engine command (effectcraft-cli commands) runs exactly as it does headless, so it never opens a dialog: comp.new {} creates a default comp. UI-only ids (tool.*, view.*, window.*, playback.*, timeline.*, app.*) fall through to the menu dispatcher. Unknown top-level params are rejected, and the error lists the accepted keys.
ui.menu.invoke	{id, params?}	Like a menu click: commands with empty params may open their dialog (New Composition, Solid Settings, file pickers).
engine.commands	{filter?, enabledOnly?}	[{id, label, menu, shortcut, params, enabled, why}]
ui.menu.list	{}	The menu tree with enablement.
render.frame	{comp?, time?, max_side?, path?, base64?}	Renders a comp frame through the session, independent of the viewer zoom or resolution. comp is an id or name (default: the active comp), time is in comp seconds (default: the CTI), max_side caps the longest side (0 = full size). Writes a PNG to path (default: a temp file) and returns {comp, time, width, height, path}. With base64: true it returns {…, png: "<base64>"} and writes no file.

Useful query commands for engine.execute (they aren't journaled or undoable): project.summary, comp.info {comp?}, layer.tree {layer, comp?, depth?, time?} (every node has a path), prop.get {layer, path, comp?, time?}, editor.state, command.list {filter?, enabledOnly?}.

UI state

Method	Params	Result
ui.inspect	{}	Window size, pixels per point, fps, serialized UiState, playback, time, active comp, selection, open dialog, last render ms.
ui.elements	{prefix?}	[{id, label, rect: [x, y, w, h]}]: the widgets drawn in the last frame, in points.
ui.set	{tool?, workspace?, theme?, focused?, viewer?, timeline?, menuBar?, home?}	viewer: {zoom (null = fit), res: Full|Half|Third|Quarter|Auto, pan: [x,y], grid, rulers, safeMargins, transparencyGrid}; timeline: {pps (null = fit), start, graphEditor, showModes, openLayers: [ids], openGroups: [uids]}.
ui.panel.show / ui.panel.close	{panel}	Panels: Project, Composition, Timeline, EffectControls, EffectsPresets, Info, Preview, Character, Paragraph, Align and others.
ui.playback	{action: play|stop|toggle|status}	{playing, time, audio, levelsDb, peaksDb} (audio: an audio preview is running; L/R meter levels and held peaks in dBFS)
ui.resize	{width, height}	Resizes the window (points).
ui.focus	{}	Activates the window and takes keyboard focus. Agents normally avoid this, because the app is started without stealing focus.
app.quit	{}	Closes the app.

Input

Points are logical window coordinates. A target is {id} (the element's centre; add fx/fy between 0 and 1 to pick a spot inside it) or {x, y}. modifiers is {shift, alt, command, ctrl}, where command is ⌘ on macOS and Ctrl elsewhere.

Method	Params
ui.click	{id | x,y, button?: left|right|middle, count?, modifiers?}
ui.move	{id | x,y}
ui.drag	{from: target, to: target, steps? (12), modifiers?}: press, move, release
ui.scroll	{id | x,y, dx, dy}
ui.key	{key}, for example Space, F9, Cmd+Z, Cmd+Shift+Z, PageDown, Escape
ui.type	{text}: text input into the focused field

Locating things

Method	Params	Result
ui.viewer.locate	{x, y} (comp pixels)	{x, y} window point
ui.viewer.hit	{id | x,y} (window point)	{x, y} comp pixel
ui.timeline.locate	{layer (id), time?}	{x, y} window point on the layer bar

Screenshots

Method	Params	Result
ui.screenshot	{path?, panel?, id?}	{path, width, height}: a PNG of the window, or cropped to one panel or element.

Element ids are stable, for example tools.Selection, panel.Timeline, panel.tab.EffectControls, header.workspace.Animation, project.item.<id>, viewer.comp, viewer.handle.<layer>.<i>, timeline.layer.<id>.bar, timeline.layer.<id>.twirl, timeline.prop.<uid>.stopwatch, timeline.key.<uid>.<frame>, timeline.cti, effectControls.prop.<uid>.value and effects.item.<name>. Use ui.elements to see what is on screen.

The MCP server's bridge mode (effectcraft-cli mcp --bridge 9877) is a thin client of this protocol; see agents.md.