Wesley Matlock iOS Articles & Videos

Testing on-device Foundation Models features with Swift Testing when the output is non-deterministic by design.

‍Stop trying to assert equality on AI output

Here’s the reframe that unlocks all of this: a language model is not a function in the mathematical sense. The same input does not map to the same output. The moment you accept that, you stop asking “what string will it return” and start asking “what must be true about whatever it returns.”

The model can reach into your app. Mid-generation. It can call a function you wrote, get a result, and fold that result into its answer — all in a single respond call. That's the Tool protocol - on-device tool calling - and it's the difference between an AI feature that talks about your data and one that talks to it.

I handed Xcode a task three months ago and walked to the kitchen for coffee. When I came back, the build was green, two files I hadn’t named were edited, and a test I’d forgotten existed was passing again. I sat down, read the diff, and reverted half of it.

That’s the honest shape of the last three months. The agent built into Xcode 26.3 is genuinely good at a specific kind of work, genuinely bad at another, and the gap between those two is exactly where you earn your keep now. This is a field report, not a launch-day hot take — three months of real work on VinylCrate, the wins and the reverts both.

I thought App Intents was going to be a Siri feature, but It turned into an architecture audit.The weirdest part wasn’t writing the intents. It was debugging failures with almost zero visibility.Six shortcuts defined. Two appeared. No logs. No errors. Just missing tiles in Shortcuts.app.

Shortcuts launches your app headlessly. App.init never runs. If your entity queries depend on app-registered services or SwiftData setup, the system can silently drop the shortcut entirely.

I built a sample app for this post called Tour Merch. It’s an App Clip plus a parent app, both written in Swift 6 with strict concurrency on. The clip lets you buy a shirt at a Metallica show with Apple Pay; the parent app lets you see every shirt you’ve bought across the tour. The whole companion repo is at the end of this post if you want to clone and poke.

What I want to walk through is the stuff that matters once you sit down to build one of these: what App Clips are actually for, how invocation works, how the project is wired, where Apple Pay forces your hand under Swift 6, and how the clip hands off to the parent app without losing the receipt.