Yes, you can build feature in OP with SDK have done this. Works well...but this is something completely different to agents.

Subagents and the auth implementation are linked because Anthropic's initial strategy was to have a prompt-based interaction which, because of the progress in model performance, has ended up being limiting as users want to run things without prompting. This is why they developed Claude Code Web (this product is more similiar to what this feature will do than subagents, subagents are similar if you have a very shallow understanding...the purpose of this change is to abstract away human interaction, i assume that will use subagents but the context/prompt management is quite different).

Oh really? I was looking at the Agent SDK for an idea and the docs seemed to imply that wasn't the case.

    Unless previously approved, we do not allow third party developers to offer Claude.ai login or rate limits for their products, including agents built on the Claude Agent SDK. Please use the API key authentication methods described in this document instead.

I didn't dig deeper, but I'd pick it back up for a little personal project if I could just use my current subscription. Does it just use your local CC session out of the box?

You can’t resell - that’s the third party language. You can build and use for your own purposes. And yes it just picks up your local sessions out of the box.