It isn't. Anthropic tried building a fairly simple piece of software (a C compiler) with a full spec, thousands of human-written tests, and a reference implementation - all of which were made available to the agent and the model trained on. It's hard to imagine a better tested, better-specified project, and we're talking about 20KLOC. Their agents worked for two weeks and produced a 100KLOC codebase that was unsalvageable - any fix to one thing broke another [1]. Again, their attempt was to write software that's smaller, better tested, and better specified than virtually any piece of real software and the agents still failed.

Today's agents are simply not capable enough to write evolvable software without close supervision to save them from the catastrophic mistakes they make on their own with alarming frequency.

Specifically, if you look at agent-generated code, it is typically highly defensive, even against bugs in its own code. It establishes an invariant and then writes a contingency in case the invariant doesn't hold. I once asked it to maintain some data structure so that it could avoid a costly loop. It did, but in the same round it added a contingency (that uses the expensive loop) in the code that consumes the data structure in case it maintained it incorrectly.

This makes it very hard for both humans and the agent to find later bugs and know what the invariants are. How do you test for that? You may think you can spec against that, but you can't, because these are code-level invariants, not behavioural invariants. The best you can do is ask the agent to document every code-level invariant it establishes and rely on it. That can work for a while, but after some time there's just too much, and the agent starts ignoring the instructions.

I think that people who believe that agents produce fine-but-messy code without close supervision either don't carefully review the code or abandon the project before it collapses. There's no way people who use agents a lot and supervise them closely believe they can just work on their own.

[1]: https://www.anthropic.com/engineering/building-c-compiler