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crust analyzes a production build of a Next.js App Router project, explains the rendering and cache story of every route, and stores a snapshot so the next build can be compared with evidence. Next DevTools explains the current page. crust explains what became worse, why, and whether the PR should merge.

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What crust answers

For a single build:
  • Is this route static, partially static, ISR, dynamic, or a route handler?
  • Which dynamic API, uncached read, or route configuration produced that mode?
  • How much of the rendered route is in the static shell?
  • Which first-party modules and packages make up its first-load JavaScript?
  • Which three findings are most worth investigating first?
Across builds:
  • Did a route become less static?
  • Did a cached read become uncached?
  • Did a component leave the static shell?
  • Did first-load JavaScript grow, and which module caused it?
  • Is the regression certain enough to fail CI?

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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 and no bundle growth:
The call site named there is three frames below the page, in a file the route never mentions.
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.

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What it does

Analyze a build

Prioritized findings, route modes, first-load JS, shell composition, and source-level reasons.

Understand regressions

What fails automatically, what needs a budget, and when crust refuses to compare.

Shell engine

Predicted versus actual static shell, and the call site that broke it.

Add CI

Zero-config regression checks, optional ceilings, and a PR comment that updates in place.

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What is enforced

With a comparable baseline, crust fails CI without a budget file when:
  • a rendering mode moves down the staticness scale
  • an uncached read appears where the baseline had none
  • a route that emitted a static shell stops emitting one
Project-specific ceilings remain explicit. Configure maximum first-load bytes, maximum percentage growth, and minimum shell ratios in .perf/budgets.json.
An added route is not a regression, unknown is never assigned a direction, and builds made with different bundlers, Next majors, or snapshot schemas are not compared. Absolute ceilings can still apply because they describe the current build rather than a delta.

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How it differs

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

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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.