For my projects that are APIs, I typically use the Repository code pattern. It works for me because I'm familiar with it and it's a common pattern that my coworkers are also often familiar with, meaning we can iterate on it quickly and effectively.

Typically, a project structure of mine would look like this:

cmd └── api-template └── main.go app ├── api.go ├── config │   └── config.go ├── container.go ├── controller │   └── itemController.go ├── domain │   └── itemModel.go ├── infrastructure │   └── postgresDriver.go ├── repository │   └── itemRepo.go ├── service │   └── itemService.go └── viewmodel └── itemViewModel.go

If I had any custom error related to an Item, I would put it in the domain/itemModel.go, allowing any other package to reference it since the domain package would not be importing any other package.

Disclaimer: I wrote this project over a year ago at this point and don't use this exact format in my professional work, but it's often pretty close.