Per-workspace agent sandboxes: on-demand, internet-enabled, no-escape isolation on top of ComputePlane #54

Summary

Cozystack needs a per-workspace sandbox layer for agentic-coding and untrusted-code workloads: one fully isolated Linux environment per workspace, spun up on demand from an image, with public internet access, snapshot/resume, and no practical escape path.

This is the layer above ComputePlane. #17 (merged) gives us the no-escape boundary between untrusted workloads and the management plane. It does not give us the per-workspace unit that lives inside that boundary — that is what this issue tracks.

Motivation

The request comes from mattia-eleuteri, whose end customer runs an agentic-coding product and currently depends on a hosted Firecracker-based sandbox service. The commercial driver is sovereignty: the managed micro-VM offerings that fit the requirement cleanest are all outside the EU/CH, which rules them out for the customer. A self-hosted equivalent on Cozystack is the reason this is worth building rather than buying.

Requirements

The sandbox must let an agent read and write files, run shell commands, install dependencies from the public internet (npm, pnpm, pip, uv), run builds and tests, and use ordinary dev tooling (git, Node, Python, ripgrep). The file API and shell execution must share one workspace root.

Hard isolation requirements:

• the sandbox may reach the public internet;
• it must not reach the host, the orchestrator, the container-runtime socket, the metadata service, the private network, the platform database, application secrets, or model/API keys;
• sandbox A must not reach sandbox B's files or its localhost.

Accepted risk: a sandbox can exfiltrate its own workspace data, because internet access is a requirement. Not accepted: escape to the host, the platform, or another tenant.

Prior art and substrate options already evaluated

• bwrap — shared host kernel; an escape compromises the worker and potentially every workspace on it. Usable as a protocol, not as the final isolation boundary.
• gVisor — a workable self-hosted fallback (user-space kernel boundary, per-workspace netns, egress firewall, cgroup limits), but operationally heavy: runsc/containerd lifecycle, cleanup, image patching, and DNS/firewall correctness all land on the operator.
• Kata Containers / micro-VM runtimes — the cleanest boundary, but they need hardware virtualization on the host, which ordinary cloud VMs without nested virtualization do not expose. On bare metal this constraint does not apply, which is the common Cozystack deployment.
• kubernetes-sigs/agent-sandbox — surfaced after the original discussion and covers a meaningful part of the design. Whether we adopt it as the substrate or keep our own abstraction is the main open question below.

Relationship to existing proposals

• #17 — ComputePlane, merged. The isolation boundary this work sits inside.
• #28 — VMSession, still open and untouched since 8 July: on-demand per-workspace VM cloned from a master image, no-escape networking, auto-teardown. It was written directly from this request and reviewed by Mattia Valzelli (@mattia).

Open questions

1. Do we implement VMSession as proposed in docs(design-proposals): add ephemeral VM sessions (VMSession) proposal #28, adopt kubernetes-sigs/agent-sandbox as the substrate, or express docs(design-proposals): add ephemeral VM sessions (VMSession) proposal #28's contract on top of it? This decision blocks the rest and should be settled before implementation starts.
2. Snapshot/resume: Mattia proposed Kata plus LINSTOR snapshots. With Blockstor replacing LINSTOR, what does the snapshot path look like?
3. Does a sandbox get its own ComputePlane, or do many sandboxes share one? This drives both the security story and the cost per workspace.
4. What is the intended tenant-facing API — a new kind, or a preset on an existing one? #26 pushed ComputePlane toward composable presets rather than a new kind; the same argument likely applies here.

Next step

Settle question 1, then either revive #28 or supersede it. Hidora offered to prototype on their lab.