Civil 3D MCP Server — Code Execution Architecture

An MCP server that enables AI assistants to write and execute C# code directly inside Autodesk Civil 3D. Instead of fixed tools, the AI generates code that runs with full API access.

Architecture

┌─────────────────┐     stdio      ┌──────────────────┐     TCP/JSON-RPC    ┌──────────────────┐
│   AI Assistant   │ ◄────────────► │  MCP Server (TS) │ ◄──────────────────► │  Civil 3D Plugin │
│ (Claude, Cline)  │               │   3 meta-tools    │     port 8080       │  Roslyn Engine   │
└─────────────────┘               └──────────────────┘                      └──────────────────┘
                                         │                                         │
                                    Skills Library                           C# Code Execution
                                   (.skill.md files)                      (full Civil 3D API)

3 Meta-Tools

Tool	Purpose	Safety
civil3d_execute	Execute C# code with write access (transaction committed)	⚠️ Modifies drawing
civil3d_query	Execute C# code read-only (no commit)	✅ No side effects
civil3d_skills	Browse/search/read code skill templates	✅ Metadata only

How It Works

1. AI reads a skill → Gets a documented C# code template
2. AI adapts the code → Fills in parameters, combines patterns
3. AI sends code → Via civil3d_execute or civil3d_query
4. Roslyn compiles + runs → Inside Civil 3D with full API access
5. Results return as JSON → Back to the AI

Example Interaction

User: "What surfaces are in my drawing?"

AI: Uses civil3d_query with:
  var surfaces = new List<object>();
  foreach (ObjectId id in CivilDoc.GetSurfaceIds()) {
    var s = Transaction.GetObject(id, OpenMode.ForRead) as TinSurface;
    surfaces.Add(new { s.Name, s.Layer });
  }
  return surfaces;

Result: [{ "Name": "EG", "Layer": "C-TOPO-EG" }, ...]

Skills Library

Skills are documented C# code templates in skills/:

skills/
├── surfaces/           # Surface operations
├── alignments/         # Alignment + station/offset
├── points/             # COGO points
├── geometry/           # Lines, polylines, text
├── drawing/            # Drawing info
└── workflows/          # Complex multi-object operations

Script Globals

Code executed via civil3d_execute or civil3d_query has access to:

Global	Type	Description
Document	Document	Active AutoCAD document
CivilDoc	CivilDocument	Active Civil 3D document
Database	Database	Document database
Transaction	Transaction	Active transaction
Editor	Editor	Document editor

All Civil 3D namespaces are auto-imported.

Setup

1. Build MCP Server

npm install && npm run build

2. Build Plugin

# Copy DLLs from Civil 3D to C_References/ (see C_References/README.md)
cd plugin/Civil3dMcpPlugin
dotnet build

3. Load in Civil 3D

NETLOAD → select Civil3dMcpPlugin.dll
C3DMCPSTATUS → verify running

4. Configure AI

{
  "mcpServers": {
    "civil3d": {
      "command": "node",
      "args": ["/path/to/civil3d-mcp/build/index.js"]
    }
  }
}

Environment Variables

Variable	Default	Description
CIVIL3D_HOST	localhost	Plugin host
CIVIL3D_PORT	8080	Plugin port
CIVIL3D_COMMAND_TIMEOUT	120000	Execution timeout (ms)
LOG_LEVEL	info	Log level

Security

The Roslyn sandbox blocks:

• Process execution (Process.Start)
• File deletion (File.Delete)
• Network requests (HttpClient, Sockets)
• Registry access
• Dynamic assembly loading

All Civil 3D API operations are allowed.

License

MIT