Using QCAD with Claude Desktop

Introduction

QCAD contains an MCP (Model Context Protocol) server. MCP is an open standard that lets AI assistants use tools provided by other applications. With the QCAD MCP server, the Claude Desktop app can work directly with the drawing that is currently open in QCAD. For example, you can ask Claude to:

describe an unfamiliar drawing, its layers, blocks, dimensions and text
check a drawing for problems such as unclosed polylines, duplicate entities or geometry on layer 0
measure distances, lengths and areas
create a bill of materials from block references
add layers, lines, arcs, circles, polylines, text, dimensions and hatches
move, rotate, scale, trim, round or offset existing entities
export the drawing as PNG, PDF, SVG, DXF or DWG

This tutorial explains how to connect QCAD to Claude Desktop under Windows or macOS.

Requirements

QCAD with the MCP plugin, installed and running on the same computer as Claude Desktop
The Claude Desktop app for Windows or macOS

The QCAD extension for Claude Desktop comes with QCAD. It is the file qcad.mcpb in the folder mcp of your QCAD installation, for example:

Windows: C:\Program Files\QCAD\mcp\qcad.mcpb
macOS: /Applications/QCAD-Pro.app/Contents/Resources/mcp/qcad.mcpb

Under macOS, the folder is inside the application package and is not shown by the Finder when you open the Applications folder. Misc > MCP > Install in Claude Desktop in QCAD opens or shows the file for you (see below).

Node.js does not have to be installed separately. Claude Desktop comes with everything that is needed to run the extension.

Starting the MCP Server in QCAD

The MCP server in QCAD does not start by itself. Until it is started, QCAD does not accept any connections.

Start QCAD.
Choose Misc > MCP > Start.

The command line at the bottom of the QCAD window confirms that the server is running:

MCP: MCP server running on port 3907 (http://127.0.0.1:3907/mcp)

The server keeps running until you choose Misc > MCP > Stop or quit QCAD. It has to be started again every time QCAD is started.

The menu Misc > MCP also offers:

Install in Claude Desktop: helps you install the QCAD extension in Claude Desktop (see below)
Status: shows on the command line whether the server is running and on which port
Restart: stops and restarts the server, for example after changing its port

The server only accepts connections from the same computer (address 127.0.0.1). It cannot be reached from other computers on your network.

Installing the QCAD Extension in Claude Desktop

The extension is installed only once. Make sure Claude Desktop is installed and you are logged in before you start.

The QCAD extension is a local extension: Claude Desktop runs it on your computer, where it connects to the MCP server in QCAD. Do not use Add custom connector in Claude for this. Custom connectors only accept servers that are reachable from the internet through an https address, so the address http://127.0.0.1:3907/mcp of QCAD is not accepted there.

Windows

In QCAD, choose Misc > MCP > Install in Claude Desktop. QCAD shows the file qcad.mcpb selected in the File Explorer. The command line in QCAD also shows the steps to install it.
In Claude Desktop, open the menu at the top left of the window and choose File > Settings.
Go to Extensions, open Advanced settings and click Install Extension.
Choose the file qcad.mcpb, for example C:\Program Files\QCAD\mcp\qcad.mcpb.

Under Windows, do not double-click the file qcad.mcpb and do not drag it into the Claude Desktop window. Neither installs the extension: Claude Desktop does not register itself for this file type, so a double-click only makes Windows ask which app to use to open the file.

macOS

In QCAD, choose Misc > MCP > Install in Claude Desktop. QCAD opens the file qcad.mcpb directly in Claude Desktop.
If Claude Desktop cannot be found, QCAD shows the file in the Finder instead. Drag it into the Claude Desktop window, or choose Claude > Settings from the menu bar in Claude Desktop, go to Extensions, open Advanced settings, click Install Extension and choose the file.

Completing the Installation

Claude Desktop shows the details of the QCAD extension. Click Install and confirm.

After the installation, QCAD appears in the list of extensions under Settings > Extensions. Make sure that it is enabled.

Extension Settings

The QCAD extension has three settings, which can be changed in Claude Desktop under Settings > Extensions > QCAD. Usually, the defaults do not need to be changed:

QCAD host: the address of the QCAD MCP server. Leave this at 127.0.0.1.
QCAD MCP port: the port of the QCAD MCP server, by default 3907. This must match the port configured in QCAD.
Reply timeout (ms): how long Claude waits for an answer from QCAD, by default 60000 ms (one minute). Increase this value if you work with very large drawings.

Working with Claude and QCAD

Open a drawing in QCAD, or start with a new drawing.
Make sure the MCP server is running (Misc > MCP > Status).
Open a new chat in Claude Desktop and ask for what you need, for example:

Describe the drawing that is open in QCAD.

Check the drawing in QCAD for problems and list what you find.

Create a bill of materials from the block references in the drawing.

Draw a rectangle of 100 x 50 mm on a new layer "Outline" and round all corners with a radius of 5 mm.

Claude decides which QCAD tools to use. The first time a tool is used, Claude Desktop asks for your permission. You can allow the tool once or always.

Claude always works with the drawing that is currently active in QCAD. If several drawings are open, switch to the right one in QCAD before asking.

Changes made by Claude appear immediately in the QCAD window. Tools that modify or delete existing entities, rename layers or purge unused layers and blocks only report what they would change, unless Claude explicitly asks them to apply the change. If you want to review a change first, ask Claude to show you what it would do before applying it. Every change can be undone in QCAD with Edit > Undo, or by asking Claude to undo it.

Claude can also look at the drawing: it renders the drawing as an image to check what it shows or to verify the result of a change.

Voice Mode

MCP is not available in the voice mode of Claude Desktop. The QCAD tools can only be used in text chats. To work with QCAD, type your requests instead of speaking them.

Privacy

The QCAD MCP server and the extension only communicate with each other on your own computer. However, everything Claude reads from your drawing, for example layer names, text, dimensions and coordinates, is sent to Claude as part of the conversation, like any other content you share in a chat. Do not use the MCP server with drawings that must not leave your computer.

Troubleshooting

Windows Asks Which App to Use to Open qcad.mcpb

Claude Desktop for Windows cannot open .mcpb files by double-click. Close the dialog and install the file in Claude Desktop under File > Settings > Extensions > Advanced settings > Install Extension instead, as described above.

Claude Does Not Accept the Address of QCAD as a Connector

Add custom connector in Claude only works with servers that are reachable from the internet. QCAD is connected through the QCAD extension instead, as described above.

Claude Reports “Cannot reach QCAD”

The extension cannot connect to QCAD. Check that:

QCAD is running
the MCP server has been started with Misc > MCP > Start
the port shown by Misc > MCP > Status matches the port in the extension settings in Claude Desktop

QCAD does not have to be started before Claude Desktop, and Claude Desktop does not have to be restarted after starting the MCP server. However, if Claude does not seem to know about the QCAD tools, start QCAD and its MCP server first and then open a new chat in Claude Desktop.

The Server Cannot Be Started

If Misc > MCP > Start reports that the address is already in use, another program, or another QCAD window, is already using port 3907. Only one QCAD window can run the MCP server on a given port. Stop the server in the other QCAD window with Misc > MCP > Stop and start it in the window you want to work in.

Claude Works on the Wrong Drawing

Claude always works with the active drawing of the QCAD instance that runs the MCP server. Click on the tab or window of the drawing you want Claude to work with in QCAD. If several QCAD instances are running, make sure that the MCP server is running in the instance you are looking at.

A Request Takes a Long Time or Fails with a Timeout

QCAD cannot answer while a dialog is open in QCAD. Close any open dialogs in QCAD and ask again. For very large drawings, increase the Reply timeout in the extension settings.