@eskimo When pipe.fileHandleForWriting.closeFile() is called, the process exits with singal 9 (SIGKILL). When I try to acquire the file handle manually, the NSException thrown says [NSConcreteFileHandle fileDescriptor]: Bad file descriptor. So what's up with that last line?

OutlineGroup is a joke IMHO, it neither has the style nor the featureset of a real NSOutlineView.

Enabling com.apple.dt.XCBuild EnableSwiftBuildSystemIntegration is what CAUSED the issue for me. Disabling it solved it.

Its clear that you only work on toy projects. Try working on anything with 100k+ lines, then you will see how your "light" frameworks scale if you need to create 100 of them just for namespacing.

Still not working. "We can't process your request." Can't create new versions.

Issue still persists

Yes, but Xcode 26 had different values set as default. Probably an issue with default migration.