What is the FAS file format?
.fas files are AutoCAD Fast-Load AutoLISP files - compiled bytecode produced by the Visual LISP (VLISP) compiler built into AutoCAD. Introduced with AutoCAD Release 13 in 1994, the format lets developers distribute AutoLISP routines in a form that loads significantly faster than raw .lsp source files and conceals source code from end-users through symbol-name obfuscation.
Each .fas file contains exactly one compiled routine. The bytecode is interpreted by AutoCAD's internal VLISP engine; both (load "routine.fas") and (vlload "routine.fas") commands accept .fas files. Because symbol names and string literals are partially encrypted by the compiler, .fas files cannot be reliably decompiled back to readable LISP.
For applications bundling multiple routines or including DCL dialog definitions, Autodesk recommends the .vlx package format, which supersedes .fas for complex deployments. Performance-critical code instead targets ObjectARX (.arx) DLLs. The binary format is proprietary with no published magic-byte signature - AutoCAD identifies the file by extension alone.
.fas files are supported by AutoCAD for Windows and, with limited coverage, AutoCAD for Mac. Third-party CAD platforms such as BricsCAD and GstarCAD also load .fas files.
Security & safety
RISK: MEDIUMFAS files execute arbitrary AutoLISP code within AutoCAD. Malicious FAS files could delete or modify drawing files, run OS commands via AutoLISP's (shell) or (startapp) functions, or exfiltrate data. Only load FAS files from trusted sources. Autodesk does not sign or sandbox FAS files. Source code is hidden, so inspect the .lsp original if possible before compiling or loading.
Format details
in a nutshellPrograms that open FAS files
Windows3 apps
AutoCAD
Paid Type APPLOAD at the AutoCAD command line → browse to the .fas file → Load. Or type (load "C:/path/file.fas") directly at the command line. 
macOS1 app
AutoCAD for Mac
Paid Limited AutoLISP support; FAS files compiled on Windows may not run. Use (load) command to attempt loading. Technical details
deep spec| Encoding | Binary (proprietary VLISP bytecode) |
| Byte order | Little-endian (x86 Windows convention) |
| Routines per file | Exactly one compiled AutoLISP routine |
| Compiled from | AutoLISP source files (.lsp) |
| Bytecode interpreter | VLISP interpreter built into AutoCAD |
| Load commands | (load "file.fas") and (vlload "file.fas") both accepted by AutoCAD |
| Source protection | Symbol names and string literals obfuscated/encrypted by VLISP compiler; file cannot be decompiled back to readable LISP |
| Compression | None (bytecode is compact but uncompressed) |
| Typical file size | 1 KB - 500 KB; most single-routine files under 50 KB |
| Platform support | AutoCAD for Windows (primary); limited support in AutoCAD for Mac; also BricsCAD, GstarCAD |
| Superseded by | .vlx for multi-routine packages; ObjectARX .arx DLLs for performance-critical code |
| Related formats | .lsp (source), .vlx (multi-routine package), .dcl (dialog definition), .mnl (menu LISP) |
| Released | 1994 (AutoCAD Release 13, Visual LISP / VLISP compiler) |
| Latest version | Format unchanged since introduction; supported in AutoCAD 2026 |
| Specification | www.autodesk.com |
FAS conversions
Community Q&A
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