So I know the "node based"/"boxes and arrows" style interfaces come up a lot around here... Is there any current consensus on good/best options for a library to support building/prototyping them? Does everyone just roll their own from scratch?

I haven't used it but this looks nice https://reactflow.dev/

This isn’t what you’re asking for... I (we) use existing diagram editors and “make do” with what they are able to draw. The editor(s) we use save diagrams as XML. We parse the XML and compile to executable code. Some day, we’ll write a proper box-and-arrow editor using a box-and-arrow programming language (properly reactive). Currently, we’re using draw.io by force of momentum. I’ve used yEd and have looked at Excalidraw (both would be suitable).

Are you trying to implement the node editor UI into some other user-facing aspect of your project, or are you trying to use the node diagram for something internal to the project?

We went home-grown for our main programming interface. We also have an embedded state machine diagramme editor which is constrained enough that we use graphviz for that and let it auto-layout.

I always roll my own 🤷 The interface is where I can do interesting work. If someone has an off the shelf solution for the semantics of a node-wire interface, I would take that :)

Are you trying to implement the node editor UI into some other user-facing aspect of your project, or are you trying to use the node diagram for something internal to the project?

I'm way more in the "prototype" than "project" phase... but my goal is to explore some ideas for tools/interfaces/experiences around building and managing these types of things, somewhat in general, though hopefully with some demos actually working on/with some real systems. So I suppose the answer is a little of both.