Last week I showed an agent making a Swift 6 data race disappear with one word. It marked a struct @unchecked Sendable, the build went green, every test still passed, and the race was exactly as present as before. The post ended on the question I didn't have a clean answer to: when the build and the tests both go green on a silenced race, what in the loop is supposed to catch it?
A red build is a signal you can trust. A silenced race hands you a green build you can't, and the only thing between you and shipping it is you, reading the diff. I spent this week building tests that put the red back on the board. Here is the one that worked, and the honest edges of it.
The setup from last time: a value type that crosses into concurrent code, carrying a mutable class the agent no longer wanted the compiler to complain about.
public final class AuditPen {
public var ink: Int
