Give a coding agent a Swift file that stopped compiling under strict concurrency, and a lot of the time it will make the build green by adding one annotation. The error goes away. The data race it was warning about does not.
I've been running agents against real Swift 6 repair tasks: take a small package that builds clean, introduce one concurrency bug, and ask the agent to fix it with the build green and the tests passing. The setup matters. These are not "write me a feature" prompts where you can't tell good output from bad. There is a right answer and a wrong answer, and the compiler under -strict-concurrency=complete is standing right there to tell them apart.
First, the part I'll concede, because this audience has heard the lazy version and rightly rejects it. Frontier models write good Swift concurrency code. Ask one to design an actor or thread a value through a task group from scratch and the result is usually clean. Writing the code was never the bottleneck. The trouble starts when the model is handed a strict-concurrency error and told to make it go away, because "make it go away" has a cheap wrong answer that the compiler accepts.
Here's a concrete one. A value type that crosses into concurrent code, declared Sendable:
