Still present with macOS 15.3 Beta (24D5034f), Xcode Version 16.2 (16C5032a), and iPadOS 18.2 (22C150)

I had the same ('IBPlatformToolFailureException', reason: 'The tool closed the connection (AssetCatalogSimulatorAgent)). The cause was an SVG I had created in Sketch. It was a simple shape with a fill and an inner shadow. I removed the inner shadow, re-exported the SVG, and re-added the SVG to my asset catalog, and the app now builds. As @benjamin_g mentioned, other effects (gaussian filter) in SVGs might trigger this build error as well.