What is the ARX file format?
.arx is the file extension for AutoCAD Runtime Extension plug-ins - compiled C++ binaries that extend AutoCAD's capabilities in-process. Introduced in 1996 with AutoCAD Release 13, .arx files are standard Windows DLL files built against the ObjectARX C++ SDK and renamed with the .arx extension rather than .dll.
Because .arx modules load directly into AutoCAD's process, they have full, low-latency access to the drawing database (AcDb), geometry engine (AcGe), graphics subsystem (AcGi), and editor layer (AcEd). This in-process architecture makes ARX the preferred mechanism for performance-critical extensions such as custom entity types, new commands, and integrated UI panels.
Each .arx plug-in is compiled for a specific ObjectARX SDK version and is not binary-compatible across major AutoCAD releases - an .arx built for AutoCAD 2024 will not load in AutoCAD 2025 without recompilation. AutoCAD loads ARX plug-ins via the (arxload "plugin.arx") AutoLISP command or through the APPLOAD dialog. A companion format, .dbx, serves headless ObjectDBX modules that access drawing databases without the AutoCAD UI layer. For managed-code development Autodesk also offers a .NET API that loads standard .dll files instead of .arx.
Security & safety
RISK: HIGHARX files are Windows DLLs that execute inside the AutoCAD process with full user-level privileges - they can read/write files, access the network, and make system calls without any sandboxing. A malicious ARX delivered via email or third-party download can function as malware. Only load ARX files from trusted vendors (verified digital signature preferred). AutoCAD's APPLOAD does not sandbox the code; treat ARX with the same caution as an EXE. Verify Authenticode signatures before loading unknown ARX files.
Format details
in a nutshell- ARX Archive (Arj-based or custom archive) - Some older archiving utilities used .arx for compressed archives; extremely rare today and unrelated to AutoCAD.
Programs that open ARX files
Windows2 apps
AutoCAD
Paid Load via Tools > Load Application (APPLOAD), browse to the .arx file and click Load. Or type (arxload "path\plugin.arx") at the command line. AutoCAD version must match the ARX's compile target. 
ObjectARX SDK (Autodesk Developer Network)
Free For developers: download the SDK matching your AutoCAD version, compile the ARX in Visual Studio against the SDK headers and libs. Technical details
deep spec| Binary type | Windows PE DLL (Portable Executable, compiled .dll renamed to .arx) |
| Magic bytes | 4D 5A - standard Windows PE/DLL header at offset 0 |
| API framework | ObjectARX C++ SDK (Autodesk Developer Network) |
| Execution model | In-process DLL loaded directly inside AutoCAD's host process |
| Required entry point | AcRxEntryPoint() - called by AutoCAD on plug-in load and unload |
| AutoCAD subsystems accessed | AcDb (drawing database), AcGe (geometry), AcGi (graphics), AcEd (editor) |
| Version compatibility | Not binary-compatible across major AutoCAD releases; must be recompiled per major release |
| Load command | (arxload "path/to/plugin.arx") in AutoCAD AutoLISP console, or via APPLOAD dialog |
| Typical file size | 100 KB - 20 MB |
| Platform | Windows only (ObjectARX ARX mechanism is Windows-exclusive) |
| Code signing | Optional Authenticode digital signature (standard Windows PE mechanism) |
| Companion format | .dbx - ObjectDBX module for headless drawing database access without AutoCAD UI |
| Released | 1996 (AutoCAD Release 13; ObjectARX API first public release) |
| Latest version | ObjectARX 2027 SDK (AutoCAD 2027) |
| Specification | www.autodesk.com |
Community Q&A
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