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To change the maximum length of headers emitted by React and added to a response, use the top-level reactMaxHeadersLength option in next.config.mjs. Next.js documents a default of 6000 and says the setting applies only to the App Router. It is a cap, not a way to make every proxy accept larger headers.
Set the React response-header limit
In a project using the App Router, add reactMaxHeadersLength as a top-level property in the Next.js configuration. Do not put it under experimental according to the current configuration reference. Check the documentation for your installed Next.js release before relying on the option, especially in a project on an older version.
/** @type {import('next').NextConfig} */
const nextConfig = {
reactMaxHeadersLength: 6000,
}
module.exports = nextConfigThe documented default is 6000. Set a different value only when you have a concrete need to allow more React-emitted header content; the option governs the maximum length of headers emitted by React and added to the response. See the Next.js configuration reference for the current behavior and version context.
Check whether the application or a proxy is imposing the limit
A higher Next.js cap does not raise the maximum header size accepted by a reverse proxy or other intermediary. The Next.js documentation warns that some proxies may truncate long headers. If a request fails or headers appear incomplete, identify where the truncation occurs before increasing the application limit; lowering the React limit may be appropriate when the proxy cannot support the emitted header length.
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- Confirm the setting is being used by an App Router application and is supported by the installed Next.js release.
- Inspect the response as it leaves the application and after it passes through the proxy or hosting layer.
- If the proxy truncates headers, configure or replace that intermediary where possible, or reduce the emitted header content. Increasing the Next.js cap alone cannot fix the intermediary’s limit.
Four other size flags limit different data
These four related options are useful to distinguish from reactMaxHeadersLength. They address incoming request bodies or serialized page data, not the length of React-emitted response headers. The settings are a comparison set of related size controls; they should not be assumed to be the exact other four flags implied by the title.
| Option | What it limits | Documented default or threshold | Behavior and scope |
|---|---|---|---|
experimental.proxyClientMaxBodySize |
Request-body buffering when using proxy | 10 MB | Next.js buffers a cloned request body so it can be read in proxy and by the underlying route handler. Above the limit, it buffers only the first N bytes, logs a warning, and continues with a partial body. This is not an outgoing-header limit. Next.js documentation |
serverActions.bodySizeLimit |
Incoming Server Action request body | 1 MB | Limits request-body parsing to help control resource consumption; accepts a byte value or supported size string. It concerns request content, not response headers. Server Actions are stable and enabled by default in Next.js 14. Next.js documentation |
experimental.largePageDataBytes |
Serialized Pages Router data warning threshold | 128 kB | Controls a warning threshold. Serialized data is inlined in HTML and increases document size, which can slow initial page loads on slow networks; increasing the threshold should follow a performance assessment. It is not a transport-header limit. Next.js error guide |
experimental.middlewareClientMaxBodySize |
Middleware request-body buffering | 10 MB | The Next.js 15 documentation labels this experimental and not recommended for production. Oversize bodies are buffered only to the limit, a warning is logged, and processing continues with partial body content. Check documentation for the installed release. Next.js 15 documentation |
Choose the setting by the data and execution path
- React-emitted response headers:
reactMaxHeadersLength, App Router only. - Request body shared with proxy and route handler:
experimental.proxyClientMaxBodySize. - Server Action input:
serverActions.bodySizeLimit. - Pages Router serialized data warning:
experimental.largePageDataBytes. - Middleware request-body buffering in Next.js 15:
experimental.middlewareClientMaxBodySize, with the version-specific experimental caveat.
The documented defaults and threshold values above come from Next.js documentation, not independent performance measurements. A body-buffering limit, Server Action parsing limit, or page-data warning threshold cannot be used to enlarge an HTTP response header.
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