What do you think about extending the WP_REST_Controller so we can make use of its built-in functionality including links so we can do things like embedding models with a controller response?

The WP_REST_Controller implementation is honestly not great for implementing general controllers. It is extremely opinionated in terms of CRUD endpoints, i.e. it only works well for a set of endpoints that allow CRUD operations for a single resource (which neither of the endpoints in the WP AI Client fit into). For any other kind of endpoint, extending WP_REST_Controller means implementing a ton of methods with just some stub "This is not implemented."

I'm not sure why we would need links here. Can you clarify what you mean by "embedding models with a controller response"? If we want to do this, maybe something we can do in a follow up issue/PR?

I get what you mean. I can't say I've ever found it particularly painful for things like this, but it's fine. I only thought of it because it makes HAL linking easy.

I mistyped when I wrote "embedding models with a controller response". What I meant was embedding models within a provider response. So I could do /providers/google?_embed=models and have the models for that provider included in the response so I can one-shot it.

Ah, that makes sense.

Adding those links in order to embed sub-results is pretty straightforward even without the WP_REST_Controller base class. How about we open an issue for this? I'm happy to take this on, but would love to unblock this main piece here.

Sure! It's certainly not a blocker. 👍

I opened #39 for that 👍