fix: incorrect loading of ESM based configurations (#32515)

* add regression test

* fix: make sure esm configs are indeed loaded as ESM

Author: AtofStryker

cli/CHANGELOG.md

@@ -6,6 +6,7 @@ _Released 9/23/2025 (PENDING)_
 **Bugfixes:**
 
 - In development mode, Electron `stderr` is piped directly to Cypress' `stderr` to make it clear why Electron failed to start, if it fails to start. Fixes [#32358](https://github.com/cypress-io/cypress/issues/32358). Addressed in [32468](https://github.com/cypress-io/cypress/pull/32468).
+- Fixed an issue where ESM Cypress configurations were not being interpreted correctly. Fixes [#32493](https://github.com/cypress-io/cypress/issues/32493). Fixed in [#32515](https://github.com/cypress-io/cypress/pull/32515).
 
 **Misc:**
 

packages/server/lib/plugins/child/run_require_async_child.js

@@ -83,29 +83,37 @@ function run (ipc, file, projectRoot) {
   // Config file loading of modules is tested within
   // system-tests/projects/config-cjs-and-esm/*
   const loadFile = async (file) => {
+    let errorLoadingCJSConfig = null
+
     try {
       debug('Loading file %s', file)
 
       return require(file)
     } catch (err) {
       if (!err.stack.includes('[ERR_REQUIRE_ESM]') && !err.stack.includes('SyntaxError: Cannot use import statement outside a module')) {
-        throw err
+        errorLoadingCJSConfig = err
       }
     }
 
-    debug('User is loading an ESM config file')
+    debug('User may be trying to load an ESM config file')
 
     try {
       // We cannot replace the initial `require` with `await import` because
       // Certain modules cannot be dynamically imported.
       // pathToFileURL for windows interop: https://github.com/nodejs/node/issues/31710
       const fileURL = pathToFileURL(file).href
 
-      debug(`importing esm file %s`, fileURL)
+      debug(`importing config as esm file %s`, fileURL)
+      const config = await import(fileURL)
 
-      return await import(fileURL)
+      return config
     } catch (err) {
       debug('error loading file via native Node.js module loader %s', err.message)
+      if (errorLoadingCJSConfig) {
+        debug('CJS loading initially failed. Rethrowing %s', errorLoadingCJSConfig.message)
+        throw errorLoadingCJSConfig
+      }
+
       throw err
     }
   }

system-tests/projects/config-cjs-and-esm/config-with-mjs/cypress.config.mjs

@@ -1,4 +1,10 @@
 import { defineConfig } from 'cypress'
+import { resolve } from 'node:path'
+
+const root = import.meta.dirname
+
+// should fail if not loading ESM correctly
+resolve(root, 'dist')
 
 export default defineConfig({
   e2e: { supportFile: false },

system-tests/test/issue_32493_spec.ts

@@ -0,0 +1,12 @@
+import systemTests from '../lib/system-tests'
+
+describe('e2e issue 32493: ESM config loading should not fail', () => {
+  systemTests.setup()
+
+  systemTests.it('loads the config file', {
+    spec: 'app.cy.js',
+    browser: 'chrome',
+    project: 'config-cjs-and-esm/config-with-mjs',
+    expectedExitCode: 0,
+  })
+})