Fix: Some build scripts expect newer SDKs. Install legacy SDKs manually or modify your Podfile to target MACOSX_DEPLOYMENT_TARGET=10.13.

If you use CocoaPods, install version 1.8.4 (not the latest). Latest CocoaPods requires Ruby 2.7+, but High Sierra uses Ruby 2.3.

sudo gem install cocoapods -v 1.8.4

Do not double-click the .xip file directly. This can cause a "broken archive" error. Instead:

cd /Applications
xip -x Xcode_10.1.xip

Why? The xip command-line tool handles corruption better than the Archive Utility. Extraction will take 10–20 minutes depending on your drive speed.

Xcode 10.1 supports iOS 12.1 by default. To test on older devices:

If you need newer Xcode:

Fix: Right-click (or Control-click) Xcode.app and select Open. Then click Open in the dialog. This bypasses Gatekeeper for the first launch.