Robust MCP Bridge Workbench¶

The Robust MCP Bridge Workbench is a FreeCAD addon that provides the server-side connection point for the Robust MCP Server. It runs inside FreeCAD and exposes XML-RPC and JSON-RPC interfaces.

Overview¶

The workbench provides:

Toolbar controls for starting/stopping the MCP bridge
Status indicator showing connection state
XML-RPC server on port 9875 (default)
JSON-RPC socket server on port 9876 (default)
Headless mode support for automation and CI/CD

Installation¶

Via FreeCAD Addon Manager (Recommended)¶

Open FreeCAD
Go to Tools > Addon Manager
Search for "FreeCAD Robust MCP Suite" or "Robust MCP Bridge"
Click Install
Restart FreeCAD

Manual Installation¶

Download from GitHub Releases and extract to your FreeCAD Mod directory:

Linux: ~/.local/share/FreeCAD/Mod/freecad/RobustMCPBridge/
macOS: ~/Library/Application Support/FreeCAD/Mod/freecad/RobustMCPBridge/
Windows: %APPDATA%\FreeCAD\Mod\freecad\RobustMCPBridge\

GUI Mode Usage¶

Starting the Bridge¶

Switch to the MCP Bridge workbench in FreeCAD
Click Start Bridge in the toolbar
The status indicator turns green when running

You'll see a confirmation message:

MCP Bridge started!
  - XML-RPC: localhost:9875
  - Socket: localhost:9876

Stopping the Bridge¶

Click Stop Bridge in the toolbar. The status indicator turns red.

Status Indicator¶

Color	Status
Green	Bridge running, accepting connections
Red	Bridge stopped
Yellow	Bridge starting/stopping

Headless Mode Usage¶

The workbench includes a blocking bridge script for running in server mode (keeps FreeCAD running).

Starting Headless Mode¶

Linux:

freecadcmd ~/.local/share/FreeCAD/Mod/freecad/RobustMCPBridge/freecad_mcp_bridge/blocking_bridge.py

macOS:

/Applications/FreeCAD.app/Contents/Resources/bin/freecadcmd \
    ~/Library/Application\ Support/FreeCAD/Mod/freecad/RobustMCPBridge/freecad_mcp_bridge/blocking_bridge.py

Using just commands (from source):

just freecad::run-headless

Headless Output¶

FreeCAD version: 1.0.0
============================================================
MCP Bridge started in headless mode!
  - XML-RPC: localhost:9875
  - Socket: localhost:9876

Note: Screenshot and view features are not available in headless mode.
Press Ctrl+C to stop.
============================================================

Features Available by Mode¶

Feature	GUI Mode	Headless Mode
Object creation	Yes	Yes
Boolean operations	Yes	Yes
Export (STEP, STL, etc.)	Yes	Yes
Macro execution	Yes	Yes
Document management	Yes	Yes
Screenshots	Yes	No
Object colors	Yes	No
Object visibility	Yes	No
Camera/view control	Yes	No
Interactive selection	Yes	No

GUI-Only Features

When a GUI-only feature is requested in headless mode, the Robust MCP Server returns a structured error response instead of crashing: {"success": false, "error": "GUI not available - screenshots cannot be captured in headless mode"}

Configuration¶

Workbench Preferences (FreeCAD Side)¶

The workbench has its own preferences that control how the bridge runs inside FreeCAD. Access them via:

Edit → Preferences → Robust MCP Bridge (in FreeCAD's main Preferences dialog)
Robust MCP Bridge → MCP Bridge Preferences... (from the workbench menu)

Setting	Description	Default
Auto-start bridge	Start bridge automatically on FreeCAD launch	Disabled
Show status indicator	Display status in FreeCAD's status bar	Enabled
XML-RPC Port	Port for XML-RPC connections	9875
Socket Port	Port for JSON-RPC socket connections	9876

Port Configuration

If you change the ports in the workbench preferences while the bridge is running, it will automatically restart with the new configuration.

MCP Server Configuration (Client Side)¶

The external Robust MCP Server (used by Claude Code, etc.) is configured separately using environment variables. These must match the workbench ports:

Environment Variable	Description	Default
`FREECAD_MODE`	Connection mode: `xmlrpc`, `socket`, `embedded`	`xmlrpc`
`FREECAD_XMLRPC_PORT`	XML-RPC server port	9875
`FREECAD_SOCKET_PORT`	JSON-RPC socket server port	9876
`FREECAD_SOCKET_HOST`	Socket/XML-RPC server hostname	`localhost`

Example MCP client configuration with custom ports:

{
  "mcpServers": {
    "freecad": {
      "command": "freecad-mcp",
      "env": {
        "FREECAD_MODE": "xmlrpc",
        "FREECAD_XMLRPC_PORT": "9877"
      }
    }
  }
}

Choosing a Connection Mode

xmlrpc (recommended): Most reliable, works on all platforms. Connects to FreeCAD via XML-RPC protocol.
socket: Alternative protocol using JSON-RPC over TCP sockets. Also works on all platforms.
embedded: Direct Python import of FreeCAD (Linux only). Does not require the workbench but crashes on macOS due to library linking issues. Not recommended for production use.

Port Matching Required

The ports configured in the MCP Server (via environment variables) must match the ports configured in the FreeCAD workbench preferences. If they don't match, the server won't be able to connect to FreeCAD.

Architecture¶

┌─────────────────────────────────────────────────────────────┐
│                    FreeCAD (GUI or Headless)                │
│  ┌───────────────────────────────────────────────────────┐  │
│  │           Robust MCP Bridge Workbench/Plugin          │  │
│  │  ┌─────────────────┐  ┌─────────────────┐             │  │
│  │  │ XML-RPC Server  │  │ Socket Server   │             │  │
│  │  │   (port 9875)   │  │   (port 9876)   │             │  │
│  │  └────────┬────────┘  └────────┬────────┘             │  │
│  │           │                    │                       │  │
│  │           └────────┬───────────┘                       │  │
│  │                    │                                   │  │
│  │           ┌────────▼────────┐                          │  │
│  │           │ FreecadMCPPlugin│                          │  │
│  │           │ (Thread-safe    │                          │  │
│  │           │  queue system)  │                          │  │
│  │           └────────┬────────┘                          │  │
│  │                    │                                   │  │
│  │           ┌────────▼────────┐                          │  │
│  │           │ FreeCAD Python  │                          │  │
│  │           │     Console     │                          │  │
│  │           └─────────────────┘                          │  │
│  └───────────────────────────────────────────────────────┘  │
└─────────────────────────────────────────────────────────────┘
                           ▲
                           │ Network (localhost)
                           ▼
┌─────────────────────────────────────────────────────────────┐
│           FreeCAD Robust MCP Server (External Process)      │
└─────────────────────────────────────────────────────────────┘

The workbench uses a queue-based thread safety system to ensure FreeCAD operations run on the main GUI thread, preventing crashes from thread-unsafe operations.

Troubleshooting¶

Bridge Won't Start¶

Problem: Clicking "Start Bridge" does nothing or shows an error.

Solution:

Check the FreeCAD Python console for error messages
Ensure no other process is using ports 9875/9876
Try restarting FreeCAD

Connection Refused from Robust MCP Server¶

Problem: Robust MCP Server reports "Connection refused"

Solution:

Verify the bridge is running (green status indicator)
Check that ports match between workbench and Robust MCP Server config
If using Docker, ensure you're using host.docker.internal as the host

Headless Mode Hangs¶

Problem: FreeCADCmd with headless server never outputs anything

Solution:

Ensure you're using FreeCADCmd (not freecad)
Check the script path is correct
Try running with -c "print('test')" first to verify FreeCAD works

Macro Tools¶

The MCP server provides tools for working with FreeCAD macros. See Macros for details on using macro tools.