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Architecture

SoundCraft is a Cargo workspace of small crates with enforced layering (cargo xtask layers). Nothing below the UI knows about egui, so the interface can be replaced.

L0  time        audio-io        midi            (standalone: no workspace deps)
L1  dsp         clap-host                       (plugins; clap-host is the only unsafe crate)
L2  model                                       (the session document)
L3  mix                                         (the mix engine)
L4  engine      playback                        (commands, undo, I/O · audio devices)
L5  automation                                  (MCP server, control-channel client)
L6  ui-egui                                     (the user interface)
    apps: soundcraft (desktop) · soundcraft-cli · soundcraft-web (wasm)

The document

soundcraft_model::Session is plain data (serde): tracks with playlists of clips, mixer state (inserts, sends, routing, automation lanes), tempo and meter maps, memory locations, groups, busses, and EditState (selection, tools, modes, view flags). Decoded audio lives in a SourcePool of Arcs that is not serialised; cloning a session is cheap. Sessions save as .scraft JSON plus an Audio Files/ folder.

Commands and undo

Every user action is a CommandSpec (id, label, menu path, shortcut, params doc, enabled, run) registered in crates/engine/src/cmd/. Engine::execute(id, params) runs it, catches any escaped panic, and pushes an undo snapshot (the previous Arc<Session>) when the document changed. Continuous gestures use execute_merged so a fader drag is one undo step. Menus are built from the incumbent's menu catalog (crates/engine/catalog/menus.txt); a menu item lights up when a command has the same menu path and label (or an alias maps it), which also drives the parity report.

Audio

soundcraft_mix::MixEngine::render(session, pos, frames, out) renders one block: clips (gain, fades, clip effects) → trim → inserts → pre-fader sends → fader/mute (automation, VCA, trim automation) → post-fader sends → pan → busses → aux inputs → master faders. Independent strips process in parallel; plugin delay compensation aligns every path. The same engine renders bounces offline (render_range) and realtime playback (soundcraft_playback::Player, which owns it on the audio thread and receives new session snapshots through a channel).

Agent control

The desktop app's control channel (JSON lines over TCP) exposes engine commands, inspection, synthetic input and screenshots; soundcraft-cli mcp wraps either a headless engine or a running app as an MCP server. See control-protocol.md and mcp.md.

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