vercel/next.js · error

Config options `experimental.externalProxyRewritesResolve` a

Error message

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

What it means

`experimental.externalMiddlewareRewritesResolve` was renamed to `experimental.externalProxyRewritesResolve`. The check at config.ts:988-995 rejects setting both keys because the alias-mapping step needs exactly one source value. Use the proxy-prefixed key going forward.

Source

Thrown at packages/next/src/server/config.ts:992

    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 =

View on GitHub (pinned to 0ae8c72462)

Solutions

1. Remove `experimental.externalMiddlewareRewritesResolve` and keep `experimental.externalProxyRewritesResolve`.
2. Confirm the external resolver module path or function is correct.
3. Audit other `externalMiddleware*` keys for the same rename pattern.

Example fix

// before
module.exports = { experimental: { externalProxyRewritesResolve: './resolver', externalMiddlewareRewritesResolve: './resolver' } }
// after
module.exports = { experimental: { externalProxyRewritesResolve: './resolver' } }

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

When it happens

Trigger: Setting both `experimental.externalProxyRewritesResolve` and `experimental.externalMiddlewareRewritesResolve` in the config.

Common situations: Config migration during upgrade where the old experimental key was left in place. Inheriting a shared base config and overriding with the new name.

Related errors

• Config options `experimental.proxyClientMaxBodySize` and `ex
• Config options `experimental.proxyPrefetch` and `experimenta
• 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/7fda295704066e32. Report an issue: GitHub.