Agreed! I hate the start of a project with an LLM. The code it writes is always expletive, and wrangling it with rules, hooks, plans, and what have you is just a series of half measures. I'm basically in a fistfight with a cheerful drunk keyboard.

Once there's enough code, tests, and docs to appease the prefill gods and give it an understanding of what I expect in terms of code quality, style, and TDD etiquette, I can expect more consistent performance.

That said, I've never been comfortable enough with it to let it write more than a few of functions at a time without a diff/checking-in.

I've seen folks with 500K LOC branches written by an LLM and my heart just sinks...

It's easy in greenfield projects because they are smaller and less things to break.

They work well on existing projects if the codebase already has a good architecture and has good consistent (and prevalent) patterns.

If you start with architecture docs that outline the high level goals and what the core abstractions should be for a greenfield project and refine it as you fill in the actual code, you can get pretty good results.

That gives the first few agents the "north star" and then subsequent agents have the actual code, the runtime behavior, and the user experience expectations to work against after that, in addition to the architecture docs, that provide hard boundaries to vibe off of.

My experience with greenfield is that when you're just starting out, you have to be really strict and careful about overall architecture, establish a framework, provide structure, separate concerns well.

If the project is not throwaway, this first set of iterations is critical, without guidance LLMs will just bolt on things and produce special-cased spaghetti, even the very-frontier models.

Once the framework is established, it's smooth sailing.

But I haven't gotten to "very large" size yet, I expect it produces another kind of coherence problems.

I’ve had a good experience with a greenfield project.

The single thing that seems to have helped is that we all agreed to use OpenSpec early on, and to commit the specs alongside the code.

I have no affiliation with OpenSpec and I don’t suspect it’s doing anything unique here, but having the intent develop alongside the code in the repository seems to have ensured that agents have a more holistic view of the project.

It’s a night/day difference when I use an agent against this codebase that integrates its changes using OpenSpec and those that ignore it.

Depends on what the existing project looks like ;-)

One repo at my work is DDD-esque, clean architecture, modular monolith.

It suuuucks to work on. Even without AI, editing any feature requires navigating through a bunch of app, infra, domain layers instead of living in a vertical feature slice. Indirection from cargo cult design patterns we don't need, like Mediator. With AI, degradation in quality and accuracy is plainly obvious. Everything needs hand-holding. AI is dumber than our average human dev so the creaky architecture made things 10x worse

The greenfield one we started with AI a few months ago is kicking along nicely. Warts exist, there are some really large files and questionable bits, but it's way easier to work on because we didn't make the architecture some convoluted dwemer mine thingy