The last error in an Unreal Engine cook log is often a lie: notes from 37 controlled runs

If you've packaged an Unreal Engine project, you've probably seen this at the bottom of a log: AutomationTool exiting with ExitCode=25 (Error_UnknownCookFailure) Or, in the editor, PackagingResults: Error: Unknown Cook Failure . Search for it and you'll find years of forum threads where someone pastes those last lines, someone else asks for the full log, and the thread dies. I wanted to know what…