What is the VLX file format?
A .vlx file is a compiled AutoLISP/Visual LISP application for AutoCAD, produced by the Visual LISP IDE (VLIDE) bundled with AutoCAD. VLX is the highest-level compiled distribution format in the AutoLISP toolchain: .lsp (source) → .fas (compiled single file) → .vlx (compiled multi-module application package).
A .vlx file begins with the ASCII signature VRTLIB-, the Visual LISP runtime library header. It bundles compiled bytecode from one or more .lsp or .fas modules together with optional resources - .dcl dialog definition files and text help files - plus an export table specifying which functions AutoCAD can call.
Key capabilities of the .vlx format:
- Source protection: the VLIDE's "protect source" compile option obfuscates the bytecode, making the original
.lspsource difficult to reverse-engineer. - Multi-module bundling: multiple script files and dialog resources are packaged in a single
.vlxfor straightforward distribution. - Backward compatibility: newer AutoCAD versions generally load older
.vlxfiles through a compatible runtime.
Load a .vlx into AutoCAD using the APPLOAD command or the expression (load "myapp.vlx") at the LISP console. The format remains current through AutoCAD 2025 and is the standard way to distribute AutoLISP tools commercially or across teams.
Security & safety
RISK: MEDIUMVLX is executed inside AutoCAD's LISP runtime and has access to AutoCAD's drawing API, file I/O functions and shell-execute capabilities. A malicious VLX from an untrusted source could modify DWG files, delete files, or execute shell commands. Only load VLX files from trusted sources (official Autodesk App Store, reputable CAD vendors). The 'VRTLIB-' magic bytes can be checked to confirm it is a genuine VLX before loading. AutoCAD does not sandbox LISP file system access.
Format details
in a nutshellPrograms that open VLX files
Windows3 apps
AutoCAD
Paid In AutoCAD, type APPLOAD at the command prompt, browse to the .vlx file and click Load. Or type '(load "C:/path/to/app.vlx")' in the LISP console. Custom commands defined in the VLX are now available. 
AutoCAD LT
Paid Note: AutoCAD LT does NOT support AutoLISP or VLX files - the APPLOAD command is absent. Full AutoCAD is required. 
Visual LISP IDE (within AutoCAD)
Built-in In AutoCAD, type VLIDE to open the Visual LISP IDE. Use it to compile .lsp/.fas files into .vlx: Application > Make Application Wizard. Technical details
deep spec| Magic signature | ASCII "VRTLIB-" at offset 0 (Visual LISP runtime library header) |
| Encoding | Binary, compiled bytecode |
| Byte order | Little-endian (Windows conventions) |
| Container | Proprietary Autodesk Visual LISP application package (not a PE/EXE format) |
| Compiled from | One or more .lsp (AutoLISP source) or .fas (compiled single-file) modules |
| Bundled resources | Optional .dcl dialog definition files and text help or resource files |
| Source protection | Optional bytecode obfuscation via VLIDE protect-source compile option |
| Export table | Entry-point table controls which functions are exposed to AutoCAD |
| Loading method | APPLOAD command or (load "myapp.vlx") expression from AutoCAD LISP console |
| Compression | Possible; not publicly documented by Autodesk |
| Typical file size | 5 KB - 5 MB (small utility scripts to large application suites) |
| Toolchain position | LSP (source) → FAS (compiled single file) → VLX (multi-module application) |
| Associated OS | Windows |
| AutoCAD compatibility | Newer AutoCAD versions generally load older VLX files via backward-compatible runtime |
| Released | 1998-1999 (Visual LISP IDE introduced in AutoCAD Release 14 / AutoCAD 2000) |
| Latest version | N/A (format is stable; produced by the current AutoCAD Visual LISP compiler in AutoCAD 2025+) |
| Specification | help.autodesk.com |
VLX conversions
Community Q&A
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