vcv-rack-mcp-server

vcv-rack-mcp-server lets AI clients control a live VCV Rack 2 patch through a dedicated Rack module that exposes both HTTP and MCP endpoints. The screenshot above shows the MCP Server module running alongside a simple ambient drone patch created through the server and saved back to Rack.

Connect AI clients to VCV Rack 2 through a module that exposes Rack over HTTP and MCP. With the MCP Server module loaded in a patch, an MCP client can inspect the patch, add modules, connect cables, set parameters, and save or load .vcv files.

What this plugin does

The plugin adds one VCV Rack module:

• MCP Server (RackMcpServer)

When the module is enabled, it starts a local server on 127.0.0.1 using the configured port (default 2600) and exposes:

• POST /mcp for MCP JSON-RPC requests
• GET /status and other REST endpoints for scripts and debugging

Typical uses:

• control a Rack patch from Claude Desktop, Cursor, or another MCP client
• script patch-building from the terminal
• inspect installed plugins and module slugs before creating patches programmatically
• save and load patches from automation tools

How it works

Claude Desktop / Cursor / any MCP client
        |
        | JSON-RPC 2.0  (POST http://127.0.0.1:2600/mcp)
        v
  MCP Server module inside VCV Rack
        |
        | Rack engine API
        v
  Current patch: modules, cables, params, save/load

The server only works while:

• VCV Rack is open
• the MCP Server module is present in the patch
• the module is switched on

Installation

From VCV Library

Once the plugin is approved in the VCV Library:

1. Open VCV Rack 2.
2. Sign in to your VCV account.
3. Open the Library and subscribe to MCP Server by Neural Harmonics.
4. Restart Rack if needed.
5. Add the module from the browser under Utility.

From GitHub Releases

1. Open the GitHub Releases page.
2. Download the .zip file for your platform.
3. Unzip it. You should get a folder named VCVRackMcpServer.
4. Move that folder into your Rack 2 plugins folder:

macOS Apple Silicon: ~/Library/Application Support/Rack2/plugins-mac-arm64/
macOS Intel: ~/Library/Application Support/Rack2/plugins-mac-x64/
Linux x64: ~/.local/share/Rack2/plugins-lin-x64/

5. Restart Rack and add the module from the browser.

Build your own release package

If you are building from source and want the same installable artifact used on GitHub Releases:

make dist

This creates a .zip archive in dist/, for example:

dist/VCVRackMcpServer-2.1.0-mac-arm64.zip

Unzip that file and move the extracted VCVRackMcpServer folder into your Rack plugins directory.

Quick start in Rack

1. Open VCV Rack 2.
2. Create or open a patch.
3. Add the MCP Server module.
4. Leave the port at 2600 unless you need a different port.
5. Toggle ON/OFF until the status LED turns green.
6. Keep this patch open while your client connects.

The server will now listen at:

http://127.0.0.1:2600

First patch walkthrough

If you want a quick smoke test, build this minimal patch:

Fundamental VCO -> Core AudioInterface2

1. Start VCV Rack and add MCP Server.
2. Turn the module on and confirm the LED is green.
3. Add a Core AudioInterface2 module in Rack so audio has somewhere to go.
4. Configure your audio driver and output device manually in Rack if you have not done that already.
5. Use one of the commands below to add a Fundamental VCO.
6. Connect the VCO output to the left and right inputs on AudioInterface2.
7. Lower your system volume before testing, then adjust the oscillator frequency to taste.

Using the included CLI helper, the flow looks like this:

python3 skills/vcvrack_client.py status
python3 skills/vcvrack_client.py library Fundamental
python3 skills/vcvrack_client.py library Core
python3 skills/vcvrack_client.py add Fundamental VCO
python3 skills/vcvrack_client.py add Core AudioInterface2
python3 skills/vcvrack_client.py modules
python3 skills/vcvrack_client.py connect <vco_id> 0 <audio_id> 0
python3 skills/vcvrack_client.py connect <vco_id> 0 <audio_id> 1

How to work with this plugin

A reliable workflow is:

Start Rack -> add MCP Server -> turn it on -> verify /status -> connect your AI client -> search library -> add modules -> inspect module IDs and params -> connect cables

Two practical rules matter a lot for AI-driven patch building:

• Rack edits are executed on Rack's UI thread. If Rack is busy, hidden behind a modal, or otherwise not stepping normally, MCP calls can time out even when the request is valid.
• Parameter values are often raw knob positions, not musical units. Always inspect a module's parameter metadata before trying to set "65 Hz", "200 ms", "saw", or similar concepts directly.

1. Verify the server is alive

From a terminal:

curl -s http://127.0.0.1:2600/status | python3 -m json.tool

You should get a JSON response with sample rate and module count.

2. Discover available modules before building

Ask the server what plugins and modules are installed:

curl -s "http://127.0.0.1:2600/library?q=oscillator" | python3 -m json.tool
curl -s "http://127.0.0.1:2600/library/Fundamental" | python3 -m json.tool

This is important because VCV Rack automation depends on exact plugin and module slugs.

3. Add modules and inspect them

Example: add a VCO and inspect its details.

curl -s -X POST http://127.0.0.1:2600/modules/add \
  -H "Content-Type: application/json" \
  -d '{"plugin":"Fundamental","slug":"VCO"}' | python3 -m json.tool

curl -s http://127.0.0.1:2600/modules | python3 -m json.tool
curl -s http://127.0.0.1:2600/modules/1 | python3 -m json.tool

Use the returned module IDs when wiring cables or setting parameters.

4. Connect cables and tune parameters

Example requests:

curl -s -X POST http://127.0.0.1:2600/cables \
  -H "Content-Type: application/json" \
  -d '{"outputModuleId":1,"outputId":0,"inputModuleId":2,"inputId":0}' | python3 -m json.tool

curl -s -X POST http://127.0.0.1:2600/modules/1/params \
  -H "Content-Type: application/json" \
  -d '{"params":[{"id":0,"value":0.0}]}' | python3 -m json.tool

Important notes for parameter automation:

• Call GET /modules/:id/params first and use the returned name, min, max, value, displayValue, and optional options fields as the source of truth.
• Do not assume parameter 0 means frequency in Hz or that a value like 65 means 65 Hz. Many modules expose normalized or module-specific control ranges.
• Prefer small POST /modules/:id/params batches, then read the params again to confirm the change before continuing.
• If a write times out, first confirm Rack is responsive and the MCP Server module is still enabled, then retry with a smaller step.

Use with MCP clients

Claude Desktop

Add this to ~/Library/Application Support/Claude/claude_desktop_config.json on macOS, or the equivalent config path on your platform:

{
  "mcpServers": {
    "vcvrack": {
      "type": "http",
      "url": "http://127.0.0.1:2600/mcp"
    }
  }
}

Then:

1. restart Claude Desktop
2. open VCV Rack with the MCP Server module enabled
3. ask Claude to inspect or build a patch

Example prompts:

• List the modules currently in my Rack patch.
• Search the installed Rack library for oscillators and add a good starting VCO.
• Build a simple subtractive synth with VCO, VCF, VCA, ADSR, and Audio.
• Build a simple ambient drone. Inspect each module's params before setting them, and treat displayed param values as authoritative instead of guessing in Hz.

Troubleshooting MCP timeouts

If an MCP tool such as vcvrack_set_params reports a timeout:

1. Make sure VCV Rack is still open, the patch is active, and the MCP Server module LED is green.
2. Check that Rack is responsive and not blocked by a dialog, browser search field, menu, or file picker.
3. Retry the workflow in this order: vcvrack_get_status -> vcvrack_get_module -> vcvrack_get_params -> vcvrack_set_params.
4. Write one or two params at a time and keep values inside the reported min and max range.
5. Re-read params after each write instead of assuming the model guessed the control mapping correctly.

Cursor or another MCP client

Use the same server URL:

{
  "servers": {
    "vcvrack": {
      "type": "http",
      "url": "http://127.0.0.1:2600/mcp"
    }
  }
}

Use with the included CLI helper

This repo includes skills/vcvrack_client.py, a small Python CLI for talking to the local Rack server.

Check connectivity:

python3 skills/vcvrack_client.py status

Command format:

python3 skills/vcvrack_client.py [--port PORT] <command> [args...]

Common commands:

Command	Example	What it does
status	python3 skills/vcvrack_client.py status	Check server health
modules	python3 skills/vcvrack_client.py modules	List modules in the patch
module <id>	python3 skills/vcvrack_client.py module 42	Inspect one module
library	python3 skills/vcvrack_client.py library	List installed plugins
library <plugin>	python3 skills/vcvrack_client.py library Fundamental	List modules in one plugin
add <plugin> <slug>	python3 skills/vcvrack_client.py add Fundamental VCO	Add a module
remove <id>	python3 skills/vcvrack_client.py remove 42	Remove a module
params <id>	python3 skills/vcvrack_client.py params 42	List parameters
set-param <id> <paramId> <value> ...	python3 skills/vcvrack_client.py set-param 42 0 0.0	Set parameters
cables	python3 skills/vcvrack_client.py cables	List cable connections
connect <outMod> <outPort> <inMod> <inPort>	python3 skills/vcvrack_client.py connect 1 0 2 0	Create a cable
disconnect <cableId>	python3 skills/vcvrack_client.py disconnect 7	Remove a cable

Ready-made walkthroughs live in skills/examples/.

REST API reference

All responses use one of these envelopes:

{ "status": "ok", "data": ... }

{ "status": "error", "message": "..." }

Method	Endpoint	Body / Query	Description
GET	/status	-	Server info, sample rate, module count
GET	/modules	-	List all modules
GET	/modules/:id	-	Inspect one module
POST	/modules/add	{plugin, slug, x?, y?, nearModuleId?}	Add a module
DELETE	/modules/:id	-	Remove a module
GET	/modules/:id/params	-	List parameter values
POST	/modules/:id/params	{params:[{id,value}]}	Set parameters
GET	/cables	-	List cables
POST	/cables	{outputModuleId, outputId, inputModuleId, inputId}	Create a cable
DELETE	/cables/:id	-	Remove a cable
GET	/sample-rate	-	Return sample rate
GET	/library	?q=&tags=	Search installed plugins/modules
GET	/library/:plugin	-	List one plugin's modules
POST	/mcp	JSON-RPC body	MCP endpoint
GET	/mcp	-	SSE stream

Building from source

Prerequisites

• CMake 3.21+ or GNU Make
• a C++17 compiler
• curl or wget if you use the Makefile path
• jq and zip for make dist

Build with Make

make
make install
make dist

Use an existing Rack SDK if you already have one:

make RACK_DIR=/path/to/Rack-SDK

Build with CMake

cmake -B build
cmake --build build --parallel
cmake --install build

Releasing

1. Update plugin.json to the new version.
2. Commit and push.
3. Build release artifacts or let CI build them.
4. Create a GitHub release for that version.

If you tag releases from git, keep the tag aligned with plugin.json, for example:

git tag v2.1.0
git push origin v2.1.0

Publishing to the VCV Library

For an open-source plugin, the normal path is through the VCVRack/library repository.

1. Open an issue in VCVRack/library.
2. Use the plugin slug as the issue title: VCVRackMcpServer.
3. Include the source repository URL: https://github.com/Neural-Harmonics/vcv-rack-plugin-mcp-server.
4. Wait for the maintainer to review and add the plugin.
5. For later updates, comment on the same issue with the new version and commit SHA.

The metadata used for approval comes primarily from plugin.json, so keep its URLs, version, and module information current.

Notes and limitations

• The plugin controls the Rack patch, not Rack application settings.
• Audio driver and device selection still need to be configured manually in Rack.
• The server is local-only by default and intended for use on the same machine.
• Exact plugin/module slugs depend on what is installed in your Rack library.

License

MIT. See LICENSE.

This project also uses cpp-httplib, which is MIT-licensed.