I updated to macOS Sequoia and to Xcode 16 to make use of the new SwiftData features for my Swift Vapor backend. However, using #Unique requires me to annotate my schemas with @available, resulting in me needing to apply it everywhere else. Instead of doing that, I tried to update my package to support the latest platform version but .v15 is not available.

What are my options?

You may need to update your Swift tools version to 6.0. It’s the first line of Package.swift.

Oh, thank you. I didn't realize this commented line actually mattered.