vercel/next.js · error · Error

Config options `experimental.externalProxyRewritesResolve`…

Error message

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

What it means

Error "Config options `experimental.externalProxyRewritesResolve` and `experimental.externalMiddlewareRewritesResolve` cannot be set at the same time. Please use `experimental.externalProxyRewritesResolve` instead." thrown in vercel/next.js.

Solutions

  1. Remove experimental.externalMiddlewareRewritesResolve and keep only experimental.externalProxyRewritesResolve.
Defensive patterns

Strategy: validation

When it happens

Trigger: Both externalProxyRewritesResolve and externalMiddlewareRewritesResolve are set.

Common situations: Config defines both the old middleware and new proxy external-rewrites options.

AI-assisted analysis of vercel/next.js@34433fd12e (2026-08-19). Data as JSON: /api/errors/7fda295704066e32. Report an issue: GitHub.

Appendix: source

Thrown at packages/next/src/server/config.ts:1062

    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.'
    )
  }

  // Map Proxy config to Middleware config as it is currently an alias.
  if (
    userConfig.experimental?.proxyClientMaxBodySize === undefined &&
    userConfig.experimental?.middlewareClientMaxBodySize !== undefined
  ) {
    result.experimental.proxyClientMaxBodySize =

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