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)

10 Meta-Tools

Tool	Purpose	Safety
civil3d_health	Check connection, Civil 3D version, active drawing	✅ Read-only
civil3d_status	Real-time operation progress and queue state	✅ Read-only
civil3d_discover	Inventory drawing objects without writing C#	✅ Read-only
civil3d_audit	Read audit log of past operations	✅ Read-only
civil3d_security	Read sandbox mode, export paths, confirmation policy	✅ Read-only
civil3d_query	Execute C# code read-only	✅ No side effects
civil3d_execute	Execute C# code with write access	⚠️ Modifies drawing
civil3d_command	Execute native Civil 3D command strings	⚠️ May modify drawing
civil3d_session	Multi-step transactions (begin/execute/commit/abort)	⚠️ May modify drawing
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
├── corridors/          # Corridor listing and info
├── pipe_networks/      # Pipe network listing and details
├── parcels/            # Sites and parcels
├── profiles/           # Profile listing and elevation queries
├── sections/           # Sample lines and cross-sections
├── labels/             # Label style inventories
├── styles/             # Object style inventories
├── export/             # CSV, LandXML export patterns
├── quantity/           # Volumes and corridor quantities
├── drawing/            # Drawing info
└── workflows/          # Multi-step civil engineering workflows

Script Globals

Code executed via civil3d_execute or civil3d_query has access to:

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

Helper methods: GetSurfaceByName, GetAlignmentByName, GetProfileByName, GetCogoPointByNumber, GetObjectIdByHandle, ListSurfaces, ListAlignments, ListCogoPoints, ToRef, ToPoint

All Civil 3D namespaces are auto-imported. Return values are auto-serialized (ObjectId → handle, Point3d → xyz).

Setup

Quick setup (recommended)

npm run setup

This interactive script will:

1. Detect installed Civil 3D versions (scans C:, D:, and other drives)
2. Configure plugin DLL references automatically
3. Build the MCP server and plugin
4. Print MCP config for Cursor

Civil 3D on D: or custom folder? Use one of these:

# Option A — pass the path directly
npm run setup -- -InstallPath "D:\Program Files\Autodesk\AutoCAD 2026"

# Option B — environment variable (picked up automatically during scan)
$env:CIVIL3D_INSTALL_PATH = "D:\Program Files\Autodesk\AutoCAD 2026"
npm run setup

# Option C — interactive: when the menu appears, press C and paste the path
npm run setup

The path must be the AutoCAD 20XX folder that contains C3D\AeccDbMgd.dll.

Manual setup

1. Build MCP Server

npm install && npm run build

2. Build Plugin

# References are in plugin/Civil3dMcpPlugin/Civil3dMcpPlugin.References.props
# Regenerate with: npm run setup
# Custom install path: npm run setup -- -InstallPath "D:\...\AutoCAD 2026"
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 (set on both MCP server and Civil 3D process)
CIVIL3D_DEFAULT_TIMEOUT_MS	120000	Default operation timeout (ms)
CIVIL3D_EXECUTE_TIMEOUT_MS	120000	C# script execution timeout
CIVIL3D_COMMAND_TIMEOUT_MS	300000	Native command timeout
CIVIL3D_DISCOVER_TIMEOUT_MS	60000	Discover inventory timeout
CIVIL3D_AUDIT_LOG	%LOCALAPPDATA%\Civil3dMcp\audit.jsonl	Audit log file path
CIVIL3D_SANDBOX_MODE	professional	strict | professional | unlocked
CIVIL3D_REQUIRE_CONFIRMATION	true	Require confirmed: true for destructive ops
CIVIL3D_ALLOWED_EXPORT_PATHS	(see below)	Extra export folders, semicolon-separated
LOG_LEVEL	info	Log level

Development Roadmap

See ROADMAP.md for the phased plan to reach professional full access.

Security

Three sandbox modes via CIVIL3D_SANDBOX_MODE:

Mode	File IO	Civil 3D API
strict	Blocked — use WriteExportFile() only via blocked raw File.*	Full
professional (default)	Only under allowed folders	Full
unlocked	Unrestricted (delete still needs confirmation)	Full

Default allowed export paths: %LOCALAPPDATA%\Civil3dMcp\Exports, Desktop, Documents.
Add more with CIVIL3D_ALLOWED_EXPORT_PATHS.

Safe file helpers in scripts: WriteExportFile(path, content), WriteExportLines(path, lines), ReadImportFile(path), AllowedExportPaths.

Destructive operations (erase, purge, delete) require confirmed: true when CIVIL3D_REQUIRE_CONFIRMATION=true (default). The tool returns CIVIL3D.CONFIRMATION_REQUIRED with the list of detected risks.

Always blocked: process execution, network, registry, P/Invoke, dynamic assembly loading.

Call civil3d_security to inspect the active policy before exports or destructive writes.

License

MIT