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The shell engine answers a question the build output cannot: why did a component stop being prerendered. It works in layers, each validating the one before it.

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Layer 1 — predict, from source

crust walks the route’s server component tree, finds <Suspense> boundaries, marks components that touch a dynamic API or an uncached read, and propagates that up to the nearest enclosing boundary. This is the only layer that can name a cause:
Two inverted rule sets share this code path, detected from the build’s own resolved config rather than by parsing next.config:
A <Suspense> boundary is not automatically a hole. If everything under it is cached, the build prerenders straight through the boundary. Treating every boundary as a hole would report a regression on every route that uses Suspense correctly.

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Layer 2 — verify, against the build output

The prediction is checked against the shell HTML the build actually wrote to .next/server/app/**. React’s markers make this plain-text work:
<!--$?--> marks a boundary still showing its fallback, and the fallback sits exactly where its hole is. Boundary ids are assigned in document order.
postponedState is never parsed. The Next docs are explicit that reading or altering it produces incorrect output — and it turns out to be unnecessary, because the shell HTML is already readable.
A route built with generateStaticParams emits both products/[slug].html — the unparameterized fallback, which is zero bytes — and products/alpha.html, the real shell. crust diffs against a parameterized shell; diffing the empty one would report that the whole shell vanished on every build.

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Layer 3 — observe, at runtime

The runtime collector records when each hole actually filled, by watching for React’s inline completion scripts:
Those B:0 ids are the same ones layer 2 read out of the prerendered shell, so the join needs no source access at all.
These completions are not in the __next_f flight payload — that carries the RSC data. A collector that watches only __next_f finds the chunks and never the swaps.
Boundaries that resolved before the collector mounted are reported with no timestamp rather than a fabricated one.

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Shell ratio

The figure crust reports is the share of the rendered route that is not inside a pending boundary, measured on visible text rather than bytes. Inline scripts and the flight payload dwarf the markup, so a byte-based ratio would track the payload instead of what a user sees.