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

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();
  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:

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

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