The resolution was to move the /Development Sample Code folder to a different location. And in general, you can search for files containing 'ctype.h' and remove them from the project folder.

An Apple engineer resolved the issue for me in response to a FeedbackReport. Here was the problem, and a solution. Xcode 16 appears to have changed the header search behavior. My project was structured something like this: Project.xcworkspace Project.xcproject /Project \App <---- Folder containing all app source code /Development Sample Code <---- Folder containing ctype.h reference On Xcode 15.4, the headers in /Development Sample Code were not search, they appear to be in Xcode 16.

A slightly more complex code snippet works: let session = URLSession.shared let config = URLSessionConfiguration.background(withIdentifier: "SESSION_ID") config.sessionSendsLaunchEvents = true let backgroundSession = URLSession(configuration: config) let response = await withTaskCancellationHandler { try? await session.data(for: request) } onCancel: { let task = backgroundSession.downloadTask(with: request)) task.resume() }