Agent Substrate was built to provide a few (important) things over Agent Sandbox:

* More efficient usage of compute by timeslicing agents (Substrate Actors), which requires fast suspend and resume (using gVisor or cloud-hypervisor snapshots), as well as keeping the K8s control plane out of the critical path (so agents can't be stored as resources in the K8s database).

* Deep inspection of outgoing requests using an egress gateway

* Minimizing the exposure of credentials to unpredictable agent control (so they can't upload access tokens to pastebin).

Achieving those goals ultimately required a significantly different design from Agent Sandbox.