Debug Foundation Models Sessions With Instruments

This article provides a technical guide on debugging Foundation Models sessions with Apple's Instruments tool, as demonstrated in WWDC26 session 243. It covers inspecting prompts, latency, and control flow across multiple LanguageModelSession instances in iOS 27-era app workflows. The author builds a SwiftUI feature that searches local notes and tasks using Foundation Models, then profiles it to optimize performance. Key topics include setting up a local data boundary, adding search tools, and using Instruments to trace session and tool timelines. The article emphasizes treating AI models as debuggable code rather than black boxes, with practical steps for reducing model workload and improving app efficiency.