It's talking to a model over an API. Currently using Claude. Certainly would not be reasonable to do from scratch. The basic starting point to make a coding assistant is basically reading text from the user, feeding it to the model over the API, and giving it a couple of basic tools. Really the models can handle starting with just the ability to execute shell commands (with confirmation, unless you're braver than me), and from that you can bootstrap by asking it to suggest and write additional tools for itself.
This is a really cool idea and your comment has inspired me to get some friends together to do something similar. If you're interested we should collaborate so that we can compare results.
