# Robust MCP Bridge Workbench - FreeCAD Robust MCP Suite
FreeCAD Robust MCP Suite - MCP Server, Bridge Workbench, and Macros for AI assistant integration

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# 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)

1. Open FreeCAD
2. Go to Tools > Addon Manager
3. Search for "FreeCAD Robust MCP Suite" or "Robust MCP Bridge"
4. Click Install
5. Restart FreeCAD

### Manual Installation

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

- Linux: `~/.local/share/FreeCAD/Mod/FreecadRobustMCPBridge/`
- macOS: `~/Library/Application Support/FreeCAD/Mod/FreecadRobustMCPBridge/`
- Windows: `%APPDATA%\FreeCAD\Mod\FreecadRobustMCPBridge\`

## GUI Mode Usage

### Starting the Bridge

1. Switch to the MCP Bridge workbench in FreeCAD
2. Click Start Bridge in the toolbar
3. 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/FreecadRobustMCPBridge/freecad_mcp_bridge/blocking_bridge.py

macOS:

    /Applications/FreeCAD.app/Contents/Resources/bin/freecadcmd \
        ~/Library/Application\ Support/FreeCAD/Mod/FreecadRobustMCPBridge/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:

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

### Connection Refused from Robust MCP Server

Problem: Robust MCP Server reports "Connection refused"

Solution:

1. Verify the bridge is running (green status indicator)
2. Check that ports match between workbench and Robust MCP Server config
3. 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:

1. Ensure you're using `FreeCADCmd` (not `freecad`)
2. Check the script path is correct
3. 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.

2026-01-18
2026-01-06