vercel/next.js · error

Config options `experimental.proxyClientMaxBodySize` and `ex

Error message

Config options `experimental.proxyClientMaxBodySize` and `experimental.middlewareClientMaxBodySize` cannot be set at the same time. Please use `experimental.proxyClientMaxBodySize` instead.

What it means

This version of Next.js aliases the older `experimental.middlewareClientMaxBodySize` to the newer `experimental.proxyClientMaxBodySize` (proxy is the renamed middleware layer). The check at config.ts:971-979 forbids setting both at once because the aliasing logic would otherwise conflict over which value wins. Only one source of truth is allowed.

Source

Thrown at packages/next/src/server/config.ts:976

    if (typeof bodySizeLimit === 'number') {
      value = bodySizeLimit
    } else {
      value = bytes.parse(bodySizeLimit)
    }

    if (value === null || isNaN(value) || value < 1) {
      throw new Error(
        'Server Actions Size Limit must be a valid number or filesize format larger than 1MB: https://nextjs.org/docs/app/api-reference/next-config-js/serverActions#bodysizelimit'
      )
    }
  }

  // Throw if both Middleware and Proxy config are set.
  if (
    userConfig.experimental?.proxyClientMaxBodySize !== undefined &&
    userConfig.experimental?.middlewareClientMaxBodySize !== undefined
  ) {
    throw new Error(
      'Config options `experimental.proxyClientMaxBodySize` and `experimental.middlewareClientMaxBodySize` cannot be set at the same time. Please use `experimental.proxyClientMaxBodySize` instead.'
    )
  }
  if (
    userConfig.experimental?.proxyPrefetch !== undefined &&
    userConfig.experimental?.middlewarePrefetch !== undefined
  ) {
    throw new Error(
      'Config options `experimental.proxyPrefetch` and `experimental.middlewarePrefetch` cannot be set at the same time. Please use `experimental.proxyPrefetch` instead.'
    )
  }
  if (
    userConfig.experimental?.externalProxyRewritesResolve !== undefined &&
    userConfig.experimental?.externalMiddlewareRewritesResolve !== undefined
  ) {
    throw new Error(
      'Config options `experimental.externalProxyRewritesResolve` and `experimental.externalMiddlewareRewritesResolve` cannot be set at the same time. Please use `experimental.externalProxyRewritesResolve` instead.'
    )

View on GitHub (pinned to 0ae8c72462)

Solutions

1. Remove `experimental.middlewareClientMaxBodySize` and keep only `experimental.proxyClientMaxBodySize`.
2. Confirm the value is correct for your deployment (it caps the body size of proxied client requests).
3. Re-run the build to ensure no other alias conflicts remain.

Example fix

// before
module.exports = { experimental: { proxyClientMaxBodySize: '5mb', middlewareClientMaxBodySize: '5mb' } }
// after
module.exports = { experimental: { proxyClientMaxBodySize: '5mb' } }

const e = config.experimental ?? {};
if (e.proxyClientMaxBodySize !== undefined && e.middlewareClientMaxBodySize !== undefined) {
  throw new Error('set only experimental.proxyClientMaxBodySize');
}

When it happens

Trigger: Setting both `experimental.proxyClientMaxBodySize` and `experimental.middlewareClientMaxBodySize` to any value (including undefined check is `!== undefined`, so explicitly setting either triggers it).

Common situations: Upgrading Next.js and partially migrating config (added the new key without removing the old one). Copying a config snippet that uses the old name into a project that already uses the new name.

Related errors

• Config options `experimental.proxyPrefetch` and `experimenta
• Config options `experimental.externalProxyRewritesResolve` a
• Config options `skipProxyUrlNormalize` and `skipMiddlewareUr
• Client Max Body Size must be a valid number (bytes) or files
• Client Max Body Size must be larger than 0 bytes

AI-assisted analysis of vercel/next.js@0ae8c72462 (2026-08-06). Data as JSON: /api/errors/82ddd87321d2d7b6. Report an issue: GitHub.