# Parallel Test Shards
URL: /docs/cookbooks/parallel-test-shards

Install dependencies once, then fork into parallel sandboxes to shard work across them.



<CookbookMeta difficulty="Intermediate" time="~5 min" primitives="[&#x22;fork&#x22;, &#x22;Promise.all&#x22;]" />

Install dependencies once, then fork the sandbox into N independent copies that each run a shard
of the test suite in parallel — cutting wall-clock time roughly by the number of shards, without
repeating the install in every shard.

## Setup [#setup]

<Steps>
  <Step>
    Start OpenSandbox in Docker (one-time setup):

    ```bash
    bunx alineo-cli init
    ```
  </Step>
</Steps>

## Run it [#run-it]

<CookbookPlayground
  cwd="cookbooks/parallel-test-shards"
  repoHref="https://github.com/DrejT/alineo/tree/main/cookbooks/parallel-test-shards"
  commands="[
  { command: &#x22;bun install&#x22; },
  { command: &#x22;bun start&#x22; },
]"
  output="`Base sandbox: installing pytest, writing 3 test files...
Forking into 3 shards...

shard-1 exit 0 2 passed
shard-2 exit 0 2 passed
shard-3 exit 1 1 passed, 1 failed

Overall: FAILED (2/3 shards green)`"
/>

## What it does [#what-it-does]

<Steps>
  <Step>
    Creates one base sandbox, installs `pytest`, and writes three small test files to it.
  </Step>

  <Step>
    Calls `base.fork()` three times — each fork branches off the base sandbox's state, so none of
    them repeat the `pip install`.
  </Step>

  <Step>
    Runs a different test file in each fork with `Promise.all`, in parallel.
  </Step>

  <Step>
    Aggregates each shard's exit code and a one-line summary into an overall pass/fail report.
  </Step>
</Steps>

<Callout title="Generalizing to a real repo">
  Install dependencies and discover shard boundaries once in the base sandbox, then fork once per
  shard (or per CPU core) instead of paying setup cost N times. See [Forking
  Sandboxes](/docs/core/concepts/forking) for the underlying `sb.fork()` primitive.
</Callout>

<Callout type="warn" title="Server proxy">
  All examples default to `useServerProxy: true` — traffic routes through the OpenSandbox server so
  Docker bridge IPs don't need to be reachable directly. Set `USE_SERVER_PROXY=false` to disable
  (e.g. when using `uvx opensandbox-server` on the host).
</Callout>