Running two or more Claude Code agents on one Xcode project is the natural next step once a single agent works well. The expectation is two tasks in roughly the time of one.

Where it goes wrong is not in the code. An iOS project shares more than files: the simulator, the installed app, and the data the app saved. I wanted to see what happens when two agents reach that shared part.

I gave two Claude Code agents separate tasks on a small SwiftUI weather app, each in its own Git worktree. One added a feature. The other added a test. Neither prompt named a simulator.

Both agents picked the iPhone 18 Pro that was already booted. Both builds succeeded, and both reported success.

Then I checked what was installed. The simulator was running the second agent's build, and the first agent's build had been replaced. Neither agent's output mentioned the other, or that its build was gone.

Worktrees keep files apart.…