# What is a sandbox fork?

A sandbox fork is a new sandbox copied from a running one, with its files, memory and running processes exactly as they were.

**On Runtime one call makes 1 to 10 running copies, and the snapshot the fork
takes for itself is free.** Copies are billed as new sandboxes of the same
size, so a 2 vCPU, 4 GiB copy costs $0.03125 an hour while it waits and $0.08
with both CPUs busy ([pricing](/docs/pricing)).

## Why AI agents fork

An agent often reaches a point worth branching from: the repository is cloned,
the dependencies are installed, the tests have run once, a server is up. From
there it may want to try three fixes, sample several model answers, or run
many evaluation episodes. A fork gives each attempt its own copy of that
moment, memory and processes included, so none repeats the setup and none can
disturb another.

A fork differs from a fresh sandbox built from an image. An image holds files.
A fork also holds what was in memory: a loaded model, a warmed cache, a shell
halfway through a session, a running dev server.

## Runtime's forks in facts

| Fact                | Value                                                                            |
| ------------------- | -------------------------------------------------------------------------------- |
| What is copied      | Files, memory and running processes                                              |
| Copies per call     | 1 to 10, all running when the call answers                                       |
| Size                | Each copy gets the source's vCPUs, memory, disk and CPU setting                  |
| Price               | Each copy is billed as a create of that size would be                            |
| The fork's snapshot | Deleted when the fork ends and not billed, unless you keep it                    |
| Source sandbox      | Paused for the moment the fork takes (about a second when fresh), then woken     |
| Where copies run    | On the source's server                                                           |
| Funding             | `"trial"` or `"paid"`, as a create; without it, copies keep the source's funding |

A sandbox created with `pausable: false` cannot be forked, because a fork
pauses its source for a moment. If one copy fails, the error's
`details.startedSandboxIds` names the copies that did start
([snapshots and forks](/docs/javascript#snapshots-and-forks)).

## Fork a prepared sandbox

```ts check
import { Sandbox } from "withruntime";

await using base = await Sandbox.create();
await base.exec("pip install --quiet requests");
const [a, b] = await base.fork({ count: 2 }); // both running, answered together
await Promise.all([a!.stop(), b!.stop()]);
```

```python check
from withruntime import Runtime

runtime = Runtime()
with runtime.sandboxes.create() as base:
    base.exec("pip install --quiet requests")
    for fork in base.fork(count=2):
        fork.stop()
```

The CLI does the same with `runtime sandbox fork <id> --count 3`. Pass
`keepSnapshot: true` to keep the fork's snapshot and start more copies later;
it is then billed as [snapshot](/glossary/sandbox-snapshot) storage, $0.08 per
decimal GB per 30-day month (rate in force since 23 September 2026). An agent
connected through MCP forks with the `runtime_sandbox_fork` tool.

## Related

- [What is a sandbox snapshot?](/glossary/sandbox-snapshot)
- [RL environments](/use-cases/rl-environments)
- [Agent evals and SWE-bench](/use-cases/agent-evals-and-swe-bench)
- [A sandbox for a coding agent](/use-cases/coding-agent-sandbox)
- [Pricing](/docs/pricing#snapshots-images-and-volumes)

Facts on this page were checked on 25 September 2026.