nextjs - 💡(How to fix) Fix unstable_getResponseFromNextConfig does not support async function configs [1 comments, 2 participants]

Description

unstable_getResponseFromNextConfig from next/experimental/testing/server does not support Next.js configs that are exported as async functions (the (phase, ctx) => Promise<NextConfig> pattern). It passes the config value directly to loadCustomRoutes, which accesses properties like config.rewrites and config.redirects immediately — without first checking if the config is a function and resolving it.

This means any next.config.mjs that uses a plugin returning an async function wrapper (e.g. withWorkflow, or any custom (phase, ctx) => config pattern) will silently produce null for all rewrite/redirect/header matches in tests.

Reproduction

// next.config.mjs
import { withSomePlugin } from 'some-plugin';

const config = {
  async rewrites() {
    return [{ source: '/old', destination: '/new' }];
  },
};

// withSomePlugin returns async (phase, ctx) => resolvedConfig
export default withSomePlugin(config);

// test.ts
import { unstable_getResponseFromNextConfig, getRewrittenUrl } from 'next/experimental/testing/server';

const nextConfig = (await import('./next.config.mjs')).default;
// nextConfig is an async function, not a config object

const response = await unstable_getResponseFromNextConfig({
  url: '/old',
  nextConfig, // ← function, not object
});

getRewrittenUrl(response); // → null (expected: '/new')

Root cause

In next/dist/experimental/testing/server/config-testing-utils.js:

async function unstable_getResponseFromNextConfig({ url, nextConfig, ... }) {
  // ...
  const routes = await loadCustomRoutes(nextConfig);
  //                                    ^^^^^^^^^
  // nextConfig may be an async (phase, ctx) => config function
  // but loadCustomRoutes expects a plain config object

loadCustomRoutes then accesses config.rewrites, config.assetPrefix, etc. directly. Since a function doesn't have these properties, everything resolves to undefined and no routes match.

Expected behavior

unstable_getResponseFromNextConfig should resolve function configs the same way Next.js does at build time — by calling the function with the appropriate phase and context before passing the result to loadCustomRoutes. Something like:

let resolvedConfig = nextConfig;
if (typeof resolvedConfig === 'function') {
  resolvedConfig = await resolvedConfig('phase-production-build', { defaultConfig: {} });
}
const routes = await loadCustomRoutes(resolvedConfig);

Version

[email protected] (but likely affects all versions since the testing utility was introduced)