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The Next.js option for changing the maximum length of response headers emitted by React is reactMaxHeadersLength. It is a top-level setting in next.config.mjs, defaults to 6000 in the current Next.js documentation, and applies only to the App Router. It is a cap—not a way to make a proxy accept headers it cannot handle.
How to change the React response-header limit
For an App Router project, set reactMaxHeadersLength at the top level of the Next.js configuration object, not inside experimental:
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/** @type {import('next').NextConfig} */
const nextConfig = {
reactMaxHeadersLength: 8000,
}
export default nextConfigThe example raises the configured cap to 8,000; choose a value based on your application and the limits of every intermediary in its request path. The current Next.js configuration reference documents a default of 6000 and says the option is App Router-only. Check the documentation for the Next.js version installed in your project before relying on a setting that may differ by release.
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This setting concerns the length of headers emitted by React and added to the response. If that emitted header content exceeds the configured maximum, it may be limited. The option does not enlarge request bodies, page data, or a reverse proxy’s own header capacity.
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Why raising the cap may not fix the problem
A response passes through the application and potentially through a reverse proxy or other intermediary before it reaches a client. Each component can have its own limits. Next.js warns that some reverse proxies cannot handle long headers and may truncate them; increasing the application-side cap can therefore make an intermediary compatibility issue worse. If a header is missing or shortened, identify where it changes rather than assuming the Next.js cap is the only constraint.
- Confirm the affected route uses the App Router.
- Check the installed Next.js version and the relevant configuration reference.
- Inspect the response at the application and after any proxy to see whether truncation happens upstream.
- Set a larger cap only if the response genuinely needs it and the infrastructure on its path supports it.
Four other size settings that limit different things
These are related Next.js size controls, but they are not four other ways to increase response-header length. Their scope and behavior differ:
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| Setting | What it controls | Documented default or threshold | What happens at the limit |
|---|---|---|---|
reactMaxHeadersLength |
Headers emitted by React and added to a response; App Router only. | 6000, according to the current Next.js configuration reference. | Caps emitted header length; Next.js warns that a proxy may truncate long headers. |
experimental.proxyClientMaxBodySize |
Request-body buffering so a body can be read in proxy and by the underlying route handler. | 10 MB, according to the current Next.js configuration reference. | Only the first configured number of bytes is buffered; a warning is logged and processing continues with a partial body. |
serverActions.bodySizeLimit |
Incoming Server Action request bodies. | 1 MB, according to the current Next.js configuration reference. | Limits request-body parsing to help control resource use; it does not set a response-header limit. |
experimental.largePageDataBytes |
A Pages Router serialized page-data warning threshold. | 128 kB, according to the Next.js error guide. | Controls when the warning is shown. Serialized data is inlined in HTML, so increasing the threshold without assessing performance can increase document size and slow initial loading on slow networks. |
experimental.middlewareClientMaxBodySize |
Request-body buffering in middleware, documented in the Next.js 15 reference. | 10 MB, in the versioned Next.js 15 documentation. | Only the configured number of bytes is buffered; a warning is logged and the request continues with partial body content. |
The figures in this table are documentation defaults or thresholds, not independent performance measurements. The middleware setting is marked experimental and not recommended for production in the Next.js 15 configuration reference; consult the documentation for your installed release.
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- A response header is too long or truncated: investigate
reactMaxHeadersLengthand the limits of proxies or other intermediaries. - A request body is incomplete in proxy or a route handler: check
experimental.proxyClientMaxBodySize; it governs buffered request bytes. - A Server Action request body is rejected or constrained: examine
serverActions.bodySizeLimit. - A Pages Router page-data warning appears: review the serialized payload and, if justified,
experimental.largePageDataBytes. - Middleware receives only part of a request body: the Next.js 15 documentation describes
experimental.middlewareClientMaxBodySize; verify its availability and status for your actual version.
For Server Actions, the Next.js configuration reference says they became stable in Next.js 14 and are enabled by default there. Its bodySizeLimit accepts bytes or a supported size string, but remains an incoming-body setting. The serialized page-data threshold is covered in the Next.js large page data error guide.
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