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

# Introduction

> What crust measures, and the one thing nothing else catches.

crust builds a static map of a Next.js App Router project from source, joins it to the build
output, and stores a snapshot per build — so a regression traces back to the commit and the import
that caused it.

## The failure it exists to catch

Under Cache Components, removing a cache directive deep in a utility silently shrinks the static
shell. No build error. No bundle growth. Nothing surfaces it.

```text theme={null}
crust diff  cfdcf500 → 4a802397

/products/[slug]  543.2 kB  +0.0 kB
    shell 100% → 45%
    ✂ <ProductGallery> left the shell — uncached fetch at lib/http.ts:3
```

The call site named there is three frames below the page, in a file the route never mentions.

<Note>
  Under the older `experimental_ppr` model a dynamic API outside a Suspense boundary threw a build
  error, so you found out immediately. Cache Components inverts the default: absence of caching is
  what postpones, and it fails quietly.
</Note>

## What it does

<CardGroup cols={2}>
  <Card title="Analyze" icon="chart-simple" href="/reference/cli#analyze">
    Route table, first-load JS, server/client boundaries, and why each route is dynamic.
  </Card>

  <Card title="Diff" icon="code-compare" href="/reference/cli#diff">
    Per-route deltas naming the module responsible, not just the route that grew.
  </Card>

  <Card title="Shell engine" icon="layer-group" href="/concepts/shell-engine">
    Predicted versus actual static shell, and the call site that broke it.
  </Card>

  <Card title="CI" icon="circle-check" href="/guides/ci">
    Budgets on bundle size and shell ratio, with a PR comment that updates in place.
  </Card>
</CardGroup>

## Why not the tools you already have

|                         | Route bundles   | History and blame     | Shell composition |
| ----------------------- | --------------- | --------------------- | ----------------- |
| `@next/bundle-analyzer` | Yes             | No                    | No                |
| Speed Insights          | No              | No source attribution | No                |
| size-limit in CI        | Not route-aware | Budgets only          | No                |
| **crust**               | Yes             | Yes                   | Yes               |

Route-level regression blame and shell composition, together, is the gap.

## Support

Next.js 15 and 16, webpack and Turbopack. Outside that range crust refuses to run rather than emit
numbers it cannot stand behind. See [support and non-goals](/support).