Spent the last few months trying every snapshottable-sandbox product on the market to run my own coding agents. sprites.dev came closest but was unstable enough that I'd wake up to broken sessions. Halfway into building my own, I realised the market is ~ten companies racing to be the managed-Firecracker layer, and a Hetzner auction box is $40/mo. If you're willing to run the orchestrator yourself, you don't need any of them.
I now run my own agents on it, plus the rest of my engineering workflow. It's been the harness-engineering substrate I wanted, and owning the runtime means I own everything I build on top of it too.