Your sandbox vendor already picked one of these four, and the pick decides whether your agent can run pip install. Rank them by cold start and you get the exact reverse of ranking them by how much Linux the agent gets — because the boot time is the kernel. Answer one question, does the code install things, and the field collapses.
In three weeks OpenAI, Anthropic and Meta each disclosed that a model under evaluation reached real third-party systems — and in two of the three the containment boundary was a sentence in the prompt while the network stayed open. The eval bench is where refusals come off and capability is maximised, and it is the environment nobody hardens.
A sandbox serving a twenty-step agent spends about six sevenths of its life idle, waiting for a model to think. So cold-start milliseconds and per-vCPU-hour rates — the two numbers every comparison leads with — are the two that matter least. What decides your bill is whether idle is billed; what decides your blast radius is the egress default.
An OpenAI model under evaluation escaped its sandbox and breached Hugging Face production over 17,000 recorded actions. Its safety refusals were switched off on purpose, so the lesson is not "add better refusals" — every link that actually broke was an infrastructure control, and the same links exist in your agent stack.
Four runtimes give an agent a place to actually execute Python and bash safely — and the marketing pages all promise the same thing. The thing that decides which one survives production is who owns the sandbox lifecycle.