Hi Quinn, Thank you for your answer. I will try bundling each auxiliary binary but this may imply making the macOS version quite different from the versions for other platforms, something I hoped I could avoid. A remaining couple of questions: 1- when myMainApp calls myAuxBin shall it be through a Finder command "open" or can I still use a direct call to the binary within the MacOS folder ? 2- One of these auxiliary binary uses a classical Open dialogue to open a file but also opens with no dialogue a set of other files with the same name but different extensions. Is this compatible with sandboxing or is there a trick I should know ? All the best Linus

I forgot to mention the solution I finally found. I codesign and harden the auxiliary binaries prior adding them to Xcode. In Xcode I do the following:   Product/Archive    Distribute App    Developer ID    Upload    Distribution certificate: select "Developer ID application..."    Upload    Close       wait for Apple's notification    Click button "Export Notarized App"

I reply to myself. Transporter's error message "Failed to decompress the app." with no more explanation is just a shame and was responsible for a big waste of time on my side. Another problem is that command line test tools such as spctl --assess or spctl -a -t execute may give inaccurate results depending on the certificate is use (Developer ID or Apple Distribution) After a lot of efforts, it turned out that: one of the 226 libraries of my App had a missing LCUUID one of the libs had a problem with a sealed ressource. I had to search for it manually to finally identify the problem. In other words "Transporter: Failed to decompress the app." should translate into "Transporter successfully decompress the App, but the bundle is rejected because ..."_

I have not tested it yet but I read that the problem is "productsign" not "Transporter.app". productsign is Okay with Catalina but does not correctly sign .pkg files with Big Sur. If this is the correct answer, I would say that the error message from Transporter is totally non-informative and misleading.