AutoCAD LLM Drafting

Use this skill to let LLM generate AutoCAD scripts from natural language descriptions, then AutoCAD executes them to produce drawings.

API Selection Guide

Choose the right API based on the task:

Default to AutoLISP for most LLM-generated scripts. It's the simplest to generate, execute, and debug.

Workflow

1. Understand the drawing intent: Ask the user what they want to draw. Clarify dimensions, layers, units, and coordinate system if not specified.
2. Choose API: Default AutoLISP. Use .NET only if AutoLISP cannot handle the complexity.
3. Generate the script: Write the complete script file. Include all geometry, layers, and annotations.
4. Add error handling: AutoLISP scripts should handle missing layers, locked layers, and cancel gracefully.
5. Deliver with instructions: Provide the script file and clear load/run instructions.

AutoLISP Reference

Basic Structure

(defun c:MyCommand ( / )
  ;; Your drawing logic here
  (princ "\nDone.")
)

c: prefix makes it callable from the AutoCAD command line.

Common Operations

Draw a line:

(command "_.LINE" "0,0" "100,100" "")

Draw a rectangle:

(command "_.RECTANGLE" "0,0" "200,100")

Draw a circle:

(command "_.CIRCLE" "50,50" 25)

Create and set a layer:

(command "_.LAYER" "N" "Walls" "C" "3" "Walls" "S" "Walls" "")

Add text:

(command "_.TEXT" "10,50" 5 0 "Hello AutoCAD")

Insert a block:

(command "_.INSERT" "blockname" "50,50" 1 1 0)

Safe Pattern

(defun c:DrawFloorPlan ( / old_osmode old_cmdecho)
  (setq old_osmode (getvar "OSMODE"))
  (setq old_cmdecho (getvar "CMDECHO"))
  (setvar "OSMODE" 0)
  (setvar "CMDECHO" 0)

  ;; --- Drawing logic here ---

  (setvar "OSMODE" old_osmode)
  (setvar "CMDECHO" old_cmdecho)
  (princ "\nFloor plan drawn.")
)

Always save and restore system variables. Turn off OSMODE and CMDECHO to prevent snap/echo interference.

.NET API Reference (C#)

Use only when AutoLISP is insufficient (complex transactions, custom entities, event handling).

Minimal .NET Command

using Autodesk.AutoCAD.ApplicationServices;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.Geometry;
using Autodesk.AutoCAD.Runtime;

public class DrawingCommands
{
    [CommandMethod("DrawCircle")]
    public void DrawCircle()
    {
        Document doc = Application.DocumentManager.MdiActiveDocument;
        Database db = doc.Database;

        using (Transaction tr = db.TransactionManager.StartTransaction())
        {
            BlockTable bt = (BlockTable)tr.GetObject(db.BlockTableId, OpenMode.ForRead);
            BlockTableRecord btr = (BlockTableRecord)tr.GetObject(bt[BlockTableRecord.ModelSpace], OpenMode.ForWrite);

            Circle circle = new Circle(new Point3d(50, 50, 0), Vector3d.ZAxis, 25);
            btr.AppendEntity(circle);
            tr.AddNewlyCreatedDBObject(circle, true);

            tr.Commit();
        }
    }
}

Compile as DLL, load with NETLOAD, run DrawCircle command.

SCR Script Reference

Simplest format — one command per line, plain text.

_.CIRCLE 50,50 25
_.LINE 0,0 100,100

_.TEXT 10,50 5 0 Hello

Save as .scr, run with SCRIPT command. No variables, no loops. Good for linear batch operations only.

Generation Rules

• Always include units: Specify if drawing uses mm, cm, inches, or meters. AutoCAD is unitless by default.
• Use absolute coordinates unless the user specifies relative. Include (command "_.OSNAP" "OFF") at the start if appropriate.
• Layer discipline: Create named layers with colors before drawing. Group related geometry on the same layer.
• Close polylines: Use (command "_.PLINE" pt1 pt2 pt3 "_C") with _C to close.
• No hardcoded paths: If referencing external files (blocks, images), use relative paths or prompt the user.
• Add comments: LISP uses ;; comment, C# uses // comment. Explain sections of non-trivial scripts.
• Test-friendly: Structure scripts so they can be run multiple times without duplicating geometry. Check if entities exist before creating.

Anti-Patterns

• Do not generate ObjectARX C++ code unless explicitly requested. It requires the SDK, compiler setup, and is error-prone.
• Do not use (entmod) or (entmake) when (command ...) suffices. Command calls are more readable and debuggable.
• Do not assume AutoCAD version. Stick to commands available in AutoCAD 2020+.
• Do not generate scripts that modify system files, registry, or run shell commands.