vercel/next.js · error · Error

Config options `experimental.proxyPrefetch` and…

Error message

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

What it means

Config validation in assignDefaultsAndValidate: both experimental.proxyClientMaxBodySize and its renamed legacy alias experimental.middlewareClientMaxBodySize are set at once. Because one option was renamed to the other, setting both is ambiguous; the guard throws and instructs the user to keep only proxyClientMaxBodySize.

Solutions

1. Remove experimental.middlewarePrefetch and keep only experimental.proxyPrefetch.

When it happens

Trigger: Both proxyPrefetch and middlewarePrefetch are set.

Common situations: Config defines both the deprecated middlewarePrefetch and the new proxyPrefetch.

AI-assisted analysis of vercel/next.js@34433fd12e (2026-08-19). Data as JSON: /api/errors/fb480ec5b1e2efe9. Report an issue: GitHub.

Appendix: source

Thrown at packages/next/src/server/config.ts:1054

        '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.'
    )
  }
  if (
    userConfig.skipProxyUrlNormalize !== undefined &&
    userConfig.skipMiddlewareUrlNormalize !== undefined
  ) {
    throw new Error(
      'Config options `skipProxyUrlNormalize` and `skipMiddlewareUrlNormalize` cannot be set at the same time. Please use `skipProxyUrlNormalize` instead.'
    )

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