# Fork a sandbox
URL: /docs/playground/fork-a-sandbox

Do the expensive setup once, then fork the live container into independent children.



`sb.fork()` copies a running container — filesystem, installed packages, everything currently on
disk — into a new sandbox that runs independently. Pay a slow `npm ci` or model download once on
a base sandbox, then fork it N ways to parallelize the work that follows.

This workflow builds a shared baseline, forks it twice, has each child modify the same file, and
shows that neither child can see the other's changes.

<WorkflowRunner workflow="fork-a-sandbox" />

## What just happened [#what-just-happened]

* The base sandbox wrote `/work/base.txt` — stand-in for expensive setup.
* `sb.fork("child-a")` and `sb.fork("child-b")` each produced a full, independent copy of the
  base container at that moment.
* Child A appended `"A was here"`; child B appended `"B was here"`. The final step `cat`s the
  file in each: A's copy has only A's line, B's has only B's. The fork is a copy, not a shared
  mount.

<Callout type="info">
  The hosted endpoint caps total sandboxes at 3 (base + 2 forks = the whole budget). If a fork step
  reports a capacity error, another reader is using the endpoint — retry in a moment, or [run
  locally](/docs/playground).
</Callout>

See also the [Parallel Test Shards](/docs/cookbooks/parallel-test-shards) cookbook.