> ## Documentation Index
> Fetch the complete documentation index at: https://docs.run.cloud/llms.txt
> Use this file to discover all available pages before exploring further.

# Linux sandboxes

> Run untrusted code in Firecracker microVMs, billed per second.

A sandbox is a Firecracker microVM with its own kernel — not a container. Spawn
one in well under a second, run whatever you like inside it, snapshot the
filesystem, and destroy it. You are billed per second of runtime against the
resources you reserved, plus any CPU you consume above the reservation.

Use them to:

* run untrusted or agent-generated code with a real isolation boundary;
* give a coding agent a disposable machine it can break;
* run builds and tests in parallel without contending for one CI box;
* snapshot an expensive setup step once and restore it many times.

Start with [Run a sandbox](/sandboxes/run-sandbox). For reusing a prepared
filesystem, see [Snapshots](/sandboxes/snapshots).

## Defaults and sizing

A sandbox with no resources specified reserves **0.125 vCPU and 128 MiB** —
enough for a shell, a small script, or an idle agent waiting for work.

CPU is a floor, not a ceiling: every sandbox auto-scales above its reservation,
so short compute spikes are fast without reserving headroom you rarely use —
see [Auto-scaling CPU](#auto-scaling-cpu) for how that bills. **Memory is a hard
limit** — a VM cannot exceed the RAM it booted with, so anything memory-hungry
must ask for more up front.

Ask for the exact resources you need with `cpu` in fractional cores and
`memory` in MiB. The maximum is **32 vCPUs and 112 GiB RAM**.

## Auto-scaling CPU

Your reserved vCPU is the billing floor and is always charged while the sandbox
runs. CPU actually consumed above the floor bills as a separate
**CPU (burst)** invoice line at the same rate as reserved CPU — auto-scaling is
not a premium, and there is nothing to turn on. Consumption is measured per
second from the VM's cgroup. Memory is unaffected: it bills on the reservation,
which is also the VM's hard ceiling.

For example, a default 0.125 vCPU sandbox that compiles at a full 1 vCPU for
10 minutes costs about $0.0005 for the floor (75 vCPU-seconds) plus about
$0.0034 for the burst (525 vCPU-seconds) — roughly \$0.004 total. The monthly
free grant applies to burst like any other usage.

## Limits

Each organization may hold **100 concurrent sandboxes** until your account is
upgraded. Reach out to [support@run.cloud](mailto:support@run.cloud) and we'll
assist you promptly. The count is of sandboxes you have not destroyed, whatever
state they are in — a paused sandbox still counts. Destroy the ones you are
finished with to free capacity.

Creating a sandbox is refused with `402` when the organization is over its
credit limit or past its payment grace period. See
[Access Control and Metering](/billing/access-control).