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1. SAC 2025 backward compatibility broken, AGAIN!

Since Civil 3D 2025, people continue to found that custom Sub assemblies saved by Sub Assembly Composer (SAC) 2025 are NOT compatible with SAC 2024, 2023, 2022 or lower any more.

The .PKT  file compatibility broken since SAC 2025 is NOT explicitly announced by Autodesk.

The reason for this error message is: 

Since Civil 3D 2025,  (1) the SAC version name is changed, (2) the .dll file inside .pkt file has been removed and (3) a new .cdmd file has been added.

2. Make SAC 2025 pkt file compatible with SAC 2024

To fix the "Unknown version: ForVail" error and make the 2025 pkt file compatible with SAC 2024, do the following:

1. Change 2025 pkt file extension to zip, extract all to a new folder

2. Explore the folder created in (1), edit the file of .cfg extension by Notepad, replace "ForVail" with "ForMatterhorn"

3. Select all file in folder and create a zip file.  Rename .zip extension to .pkt

3. Remarks

• To continue downgrading 2024 .pkt file, follow  this instruction

• Since SAC 2023, Autodesk has introduced Auxiliary Curve. Though this feature is not supported by SAC 2022 and below,  the 2023 .pkt file with Auxiliary Curve still can be opened in SAC 2022 with some error messages. 

• Please note that this solution may not work if your pkt file is complex, but it is simple and worth a try.