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The Vercel adapter provides comprehensive support for Incremental Static Regeneration (ISR), enabling pages to be updated after deployment without a full rebuild.

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Prerender output handling

ISR pages are processed through the prerender outputs system: 
await handlePrerenderOutputs(outputs.prerenders, {
  config,
  hasAppEntries: outputs.appPages.length > 0,
  varyHeader: routing.rsc.varyHeader,
  vercelOutputDir,
  nodeOutputsParentMap,
  rscContentType: routing.rsc.contentTypeHeader,
});
Location: index.ts:198-206

​
Prerender configuration

Each ISR page generates a .prerender-config.json file with the following structure:
1

Configuration generation

const prerenderConfigPath = path.join(
  functionsDir,
  `${normalizeIndexPathname(
    output.pathname,
    config
  )}.prerender-config.json`
);
Location: outputs.ts:464-469
2

Revalidation settings

{
  group: output.groupId,
  expiration:
    typeof output.fallback?.initialRevalidate !== 'undefined'
      ? output.fallback?.initialRevalidate
      : 1,
  staleExpiration: output.fallback?.initialExpiration,
  sourcePath: parentNodeOutput?.pathname,
  passQuery: true,
  allowQuery: output.config.allowQuery,
  allowHeader: output.config.allowHeader,
  bypassToken: output.config.bypassToken,
  experimentalBypassFor: output.config.bypassFor,
  initialHeaders,
  initialStatus: output.fallback?.initialStatus,
  fallback: prerenderFallbackPath ? path.posix.relative(...) : null,
}
Location: outputs.ts:586-627

​
Fallback modes

The adapter handles different ISR fallback modes:

​
Fallback: true

Pages with fallback enabled generate a fallback file: 
const prerenderFallbackPath = fallbackHasFilePath(output.fallback)
  ? path.join(
      functionsDir,
      `${normalizeIndexPathname(
        output.pathname,
        config
      )}.prerender-fallback${path.extname(output.fallback.filePath)}`
    )
  : undefined;
Location: outputs.ts:471-478
The fallback file is served while the page is being generated in the background.

​
Fallback: false

Pages with fallback: false use a special mapping to prevent 404s: 
const prerenderFallbackFalseMap: Record<string, string[]> = {};

for (const prerender of outputs.prerenders) {
  if (
    prerender.parentFallbackMode === false &&
    !prerender.pathname.includes('_next/data') &&
    !prerender.pathname.endsWith('.rsc')
  ) {
    const parentOutput = nodeOutputsParentMap.get(prerender.parentOutputId);
    const parentPage = parentOutput.pathname.substring(
      config.basePath.length
    );
    let currentMap = prerenderFallbackFalseMap[parentPage];

    if (!currentMap) {
      currentMap = prerenderFallbackFalseMap[parentPage] = [];
    }
    currentMap.push(prerender.pathname.substring(config.basePath.length));
  }
}
Location: index.ts:142-169
With fallback: false, only pre-generated paths will be accessible. Paths not included during build will return 404.

​
Partial Prerendering (PPR)

The adapter supports Next.js Partial Prerendering with postponed state: 
if (
  output.fallback?.postponedState &&
  fallbackHasFilePath(output.fallback) &&
  prerenderFallbackPath
) {
  const fallbackContent = await fs.readFile(
    output.fallback.filePath,
    'utf8'
  );
  await writeIfNotExists(
    prerenderFallbackPath,
    `${output.fallback.postponedState}${fallbackContent}`
  );
  const originContentType = isRscOutput
    ? rscContentType
    : 'text/html; charset=utf-8';

  initialHeaders['content-type'] =
    `application/x-nextjs-pre-render; state-length=${
      output.fallback.postponedState.length
    }; origin=${JSON.stringify(originContentType)}`;
}
Location: outputs.ts:544-564

​
PPR chain configuration

PPR outputs include chain configuration: 
chain: output.pprChain
  ? {
      ...output.pprChain,
      outputPath: path.posix.join(
        './',
        `${normalizeIndexPathname(output.pathname, config)}`
      ),
    }
  : undefined,
Location: outputs.ts:617-625 ISR pages share the same function code through symlinks: 
if (output.pathname !== parentNodeOutput.pathname) {
  await fs.mkdir(path.dirname(prerenderFunctionDir), {
    recursive: true,
  });
  await fs
    .symlink(
      path.relative(
        path.dirname(prerenderFunctionDir),
        parentFunctionDir
      ),
      prerenderFunctionDir
    )
    .catch((err) => {
      if (!(typeof err === 'object' && err && err.code === 'EEXIST')) {
        throw err;
      }
    });
}
Location: outputs.ts:517-535
Symlinks reduce deployment size by sharing function code between the dynamic route and its prerendered variants.

​
Revalidation configuration

​
Time-based revalidation

The expiration field controls how long before the page is revalidated: 
expiration:
  typeof output.fallback?.initialRevalidate !== 'undefined'
    ? output.fallback?.initialRevalidate
    : 1,

​
Stale-while-revalidate

The staleExpiration field enables serving stale content: 
staleExpiration: output.fallback?.initialExpiration,

​
On-demand revalidation

Bypass tokens enable on-demand revalidation: 
bypassToken: output.config.bypassToken,
experimentalBypassFor: output.config.bypassFor,
Location: outputs.ts:604-605

​
Query and header allowlists

ISR pages can specify which query parameters and headers affect caching: 
passQuery: true,
allowQuery: output.config.allowQuery,
allowHeader: output.config.allowHeader,
Location: outputs.ts:599-602
  • passQuery: true sends query parameters to the function
  • allowQuery specifies which query params create cache variants
  • allowHeader specifies which headers create cache variants

​
Initial response configuration

Prerendered pages can specify initial response metadata: 
initialHeaders,
initialStatus: output.fallback?.initialStatus,
Location: outputs.ts:607-608

​
RSC prerendering

App Router RSC routes receive special handling: 
const isRscOutput = path.extname(output.pathname) === '.rsc';

initialHeaders['content-type'] =
  `application/x-nextjs-pre-render; state-length=${
    output.fallback.postponedState.length
  }; origin=${JSON.stringify(rscContentType)}`;
Location: outputs.ts:542-579

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