Eval next.config export functions in the try catch

packages/next/src/server/config.ts

@@ -1769,6 +1769,7 @@ export default async function loadConfig(
     configFileName = basename(path)
 
     let userConfigModule: any
+    let loadedConfig: NextConfig
     try {
       const envBefore = Object.assign({}, process.env)
 
@@ -1809,6 +1810,18 @@ export default async function loadConfig(
 
         return userConfigModule
       }
+
+      // `normalizeConfig` invokes the user's exported config function (or
+      // awaits its returned promise) if it is one. Errors thrown from that
+      // call belong to the same "failed to load config" category as parse
+      // errors from `import()` above, so we keep them inside this try/catch
+      // to attach the same framing message.
+      loadedConfig = Object.freeze(
+        (await normalizeConfig(
+          phase,
+          interopDefault(userConfigModule)
+        )) as NextConfig
+      )
     } catch (err) {
       // Capture the error for MCP tool reporting
       NextInstanceErrorState.nextConfig.push(err)
@@ -1820,13 +1833,6 @@ export default async function loadConfig(
       throw err
     }
 
-    const loadedConfig = Object.freeze(
-      (await normalizeConfig(
-        phase,
-        interopDefault(userConfigModule)
-      )) as NextConfig
-    )
-
     if (loadedConfig.experimental) {
       for (const name of Object.keys(
         loadedConfig.experimental

test/production/config-evaluation-error/config-evaluation-error.test.ts

@@ -0,0 +1,68 @@
+import { nextTestSetup } from 'e2e-utils'
+
+describe('next.config evaluation error', () => {
+  describe('production mode', () => {
+    const { next, skipped } = nextTestSetup({
+      files: __dirname,
+      skipStart: true,
+      skipDeployment: true,
+    })
+    if (skipped) return
+
+    async function buildAndGetOutput(): Promise<string> {
+      const start = next.cliOutput.length
+      await next.build()
+      return next.cliOutput.slice(start)
+    }
+
+    it('should report a helpful error when the config function throws synchronously', async () => {
+      await next.patchFile(
+        'next.config.js',
+        `
+        module.exports = () => {
+          return { foo: new Uint8Array(5_000_000_000) }
+        }
+      `
+      )
+      const output = await buildAndGetOutput()
+
+      expect(output).toContain('Invalid typed array length')
+      expect(output).toContain(
+        'Failed to load next.config.js, see more info here https://nextjs.org/docs/messages/next-config-error'
+      )
+    })
+
+    it('should report a helpful error when the config module throws at the top level', async () => {
+      await next.patchFile(
+        'next.config.js',
+        `
+        const buf = new Uint8Array(5_000_000_000)
+        module.exports = { foo: buf }
+      `
+      )
+      const output = await buildAndGetOutput()
+
+      expect(output).toContain('Invalid typed array length')
+      expect(output).toContain(
+        'Failed to load next.config.js, see more info here https://nextjs.org/docs/messages/next-config-error'
+      )
+    })
+
+    it('should report a helpful error when the config function rejects', async () => {
+      await next.patchFile(
+        'next.config.js',
+        `
+        module.exports = async () => {
+          throw new Error('boom from async config plugin')
+        }
+      `
+      )
+      const output = await buildAndGetOutput()
+
+      expect(output).toContain('boom from async config plugin')
+      expect(output).toContain(
+        'Failed to load next.config.js, see more info here https://nextjs.org/docs/messages/next-config-error'
+      )
+    })
+  })
+})

test/production/config-evaluation-error/pages/index.js

@@ -0,0 +1,3 @@
+export default function Page(props) {
+  return <p>index page</p>
+}