AbletonMCP Companion (Extensions SDK add-on)

An optional add-on for Live 12.4.5+ Suite (Ableton's Extensions SDK, public beta) that gives the ableton-live-mcp server three things its Remote Script cannot reach:

In Live	For the MCP server
Share selection / clips / clip / track with AbletonMCP in the right-click menus of the Arrangement time selection, clip slots, clips and tracks	get_live_selection: which part of the song the user means (the Live Object Model has no time selection)
A dialog with your question and OK/Cancel	confirm_in_live: a yes/no from the musician, in Live
Rendering an audio track's clips to a file before its devices	render_track_audio: audio without playing the set in real time

Status: not yet run inside Live. The Trial this was written on cannot load extensions (they need Suite and the beta). It type-checks against the SDK's own declarations (@ableton-extensions/sdk 1.0.0-beta), and its tests run the built bundle in a vm context set up like the Extension Host's. Please report what happens in Live.

Build and install

1. Join Ableton's beta program and download the Extensions SDK (Centercode).
2. In this folder:

   npm ci
   npm install --no-save <path>/ableton-extensions-sdk-1.0.0-beta.N.tgz
   npm run build            # → dist/extension.js
   

3. In Live: Settings › Extensions, add this folder (the one with manifest.json), and enable it.
4. Ask the assistant to run doctor: the companion line should say it answers.

How it talks to the server

It listens on 127.0.0.1:9878 (ABLETON_MCP_COMPANION_PORT changes it) with the Remote Script's framing: one JSON object per line each way, replies echo the request's id, and the first request is hello with a per-launch token. The token and port go to ~/.ableton-mcp/companion-session.json (mode 600); where Live only lets extensions write in their storage folder, the file goes there and the server searches Ableton's folders for it. If the server still cannot find it, set ABLETON_MCP_COMPANION_TOKEN (the token is in Live's log, [AbletonMCP Companion] session file: …).

Commands: hello, song, get_selection, confirm, render_audio.

The Extension Host, as other projects found it in Live

These shaped the code (see build-options.mjs and the tests); they come from projects that ran extensions in Live 12.4.5–12.4.15 betas, not from this one:

• The bundle runs in a vm context: globalThis but no global, few web globals (no URL, TextEncoder, crypto), require for node: modules. So the build is one CommonJS file with global defined as globalThis.
• Integer getters (colors, pitches, positions) arrive as bigint even where the types say number: values are converted with Number().
• Errors from node: modules come from another realm (instanceof Error is false); messages are read by shape.
• File access may be limited to the extension's storage and temp folders.

Development

npm run typecheck       # against types/ableton-sdk.d.ts (no SDK needed)
npm run typecheck:sdk   # against the installed SDK
npm test                # unit tests + the bundle in a simulated host
npm run build:check     # bundles with the SDK left out