When a `notFound()`, `forbidden()`, or `unauthorized()` error escapes
into the outer prerender recovery path, we were falling back to the
generic error shell flow.
In the `cacheComponents` case, that could leave us with:
- error HTML rendered from `ErrorApp`
- Flight data reused from the aborted prerender prelude
- references to Flight chunks that were never emitted
That is what caused the client-side `Connection closed` failure in
#86251.
Instead of rerendering the full Flight tree or always using the generic
error RSC payload, this change:
- finds the deepest matching HTTP fallback boundary
- rerenders the normal app router payload with that segment-scoped
fallback
- tees the replacement Flight stream so Fizz can render from one copy
while prerender buffering consumes the other
- only takes this path for recoverable HTTP access fallbacks that have a
real boundary (ie a defined not-found/unauthorized/etc)
If no matching boundary exists, we keep the existing generic error
handling.
Fixes#86251Fixes#90837Closes#87041Closes#86251
Closes NAR-711
Closes NEXT-4876
If a layout or page is marked with "use client", we do not need to pass
any runtime params from the server; the client can parse them from the
URL, or (in the case of a rewrite) from the x-next-rewritten-path and
x-next-rewritten-query headers. This means the client can treat the
segment as fully static, and no dynamic request is necessary upon
navigation.
### What?
Adds build-time validation to require explicit `default.js` files for
all parallel route slots (except the implicit "children" slot). This
validation is implemented in both Webpack and Turbopack bundlers.
### Why?
Parallel routes without `default.js` files currently cause silent 404
errors when users navigate to those routes. This creates confusion and
hard-to-debug issues because the routes appear to be configured
correctly but fail at runtime without any indication of what went wrong.
By making this validation explicit at build time, developers get
immediate feedback about missing required files with clear error
messages and documentation links, catching configuration mistakes before
deployment.
### How?
**Rust/Turbopack** (`crates/next-core/src/app_structure.rs`): Added
`MissingDefaultParallelRouteIssue` that emits a build error when a
parallel route slot is missing its `default.js` file. The validation is
skipped for the "children" slot since it's implicit and doesn't require
a default file.
**Webpack**
(`packages/next/src/build/webpack/loaders/next-app-loader/index.ts`):
Added validation that throws `MissingDefaultParallelRouteError` when
`default.js` cannot be resolved. The "children" slot falls back to the
existing `PARALLEL_ROUTE_DEFAULT_PATH` behavior for backward
compatibility.
**Error Class**
(`packages/next/src/shared/lib/errors/missing-default-parallel-route-error.ts`):
New error type with helpful messaging that includes the slot path,
explanation of the requirement, and a link to documentation.
**Migration Path**: Users who want the previous 404 behavior can
explicitly create a `default.js` that calls `notFound()`, or return
`null` for empty slots:
```tsx
import { notFound } from 'next/navigation'
export default function Default() {
notFound()
}
```
Users can also run the following Deno script to generate the default
files for them:
https://gist.github.com/wyattjoh/ba7263ecb637ef399d3e3e4db63ffbd6
**Breaking Change**: This is a breaking change timed for Next.js 16
beta. Builds will now fail if parallel route slots are missing required
`default.js` files.
## What?
Rename `experimental.dynamicIO` to `experimental.cacheComponents` across
the Next.js codebase.
## Why?
We're going to be merging the functionality of the `ppr`, `dynamicIO`
and `useCache` experimental flags into the singular `cacheComponents`
flag to reduce complexity of the codebase and simplify adoption for
users wanting to experiment with experimental features.
## How?
- Renamed the configuration option from `experimental.dynamicIO` to
`experimental.cacheComponents`
- Added deprecation handling with automatic migration for the old option
name
- Updated all documentation, tests, and internal references
- Updated Rust code in SWC transforms and Turbopack
- Maintained backward compatibility with deprecation warnings
NAR-158
Zack Tanner