Could you clarify what this actually is?

Would I think of this as an EC2 instance which automatically and quickly scales to zero, with pricing only for resources consumed? (CPU and RAM when up, and disk all the time?)

Yeah that's about right.

It's a fast starting and fast pausing persistent VM, with a ton of built in developer tools (including a preconfigured Claude Code) and an extra JSON API for executing commands within it so you can treat it as a sandbox.

You may find my writeup here useful: https://simonwillison.net/2026/Jan/9/sprites-dev/