proxy/middleware does not work with ESM only app when deployed to vercel

[mburumaxwell]

Link to the code that reproduces this issue

https://github.com/mburumaxwell/dependabot-azure-devops/tree/main/apps/web

To Reproduce

1. Create next 16 app
2. Set `"type": "module"
3. Add proxy.ts with some basic logic
4. Deploy to vercel
5. Make a request that

Current vs. Expected behavior

ESM-only should be supported on proxy/middleware too. Instead, I get the error this error from the application logs:

Error [ERR_REQUIRE_ESM]: require() of ES Module /var/task/apps/web/.next/server/middleware.js from /var/task/apps/web/___next_launcher.cjs not supported.
middleware.js is treated as an ES module file as it is a .js file whose nearest parent package.json contains "type": "module" which declares all .js files in that package scope as ES modules.
Instead either rename middleware.js to end in .cjs, change the requiring code to use dynamic import() which is available in all CommonJS modules, or change "type": "module" to "type": "commonjs" in /var/task/apps/web/package.json to treat all .js files as CommonJS (using .mjs for all ES modules instead).

    at /opt/rust/nodejs.js:2:13527
    at Function.Jr (/opt/rust/nodejs.js:2:13905)
    at Ue.e.<computed>.Be._load (/opt/rust/nodejs.js:2:13497)
    at TracingChannel.traceSync (node:diagnostics_channel:322:14) {
  code: 'ERR_REQUIRE_ESM'
}
Node.js process exited with exit status: 1. The logs above can help with debugging the issue.
Error [ERR_REQUIRE_ESM]: require() of ES Module /var/task/apps/web/.next/server/middleware.js from /var/task/apps/web/___next_launcher.cjs not supported.
middleware.js is treated as an ES module file as it is a .js file whose nearest parent package.json contains "type": "module" which declares all .js files in that package scope as ES modules.
Instead either rename middleware.js to end in .cjs, change the requiring code to use dynamic import() which is available in all CommonJS modules, or change "type": "module" to "type": "commonjs" in /var/task/apps/web/package.json to treat all .js files as CommonJS (using .mjs for all ES modules instead).

    at /opt/rust/nodejs.js:2:13527
    at Function.Jr (/opt/rust/nodejs.js:2:13905)
    at Ue.e.<computed>.Be._load (/opt/rust/nodejs.js:2:13497)
    at TracingChannel.traceSync (node:diagnostics_channel:322:14) {
  code: 'ERR_REQUIRE_ESM'
}
Node.js process exited with exit status: 1. The logs above can help with debugging the issue.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.1.0: Mon Oct 20 19:32:41 PDT 2025; root:xnu-12377.41.6~2/RELEASE_ARM64_T6000
  Available memory (MB): 16384
  Available CPU cores: 8
Binaries:
  Node: 24.10.0
  npm: 11.6.0
  Yarn: N/A
  pnpm: 10.13.1
Relevant Packages:
  next: 16.0.3 // Latest available version is detected (16.0.3).
  eslint-config-next: N/A
  react: 19.2.0
  react-dom: 19.2.0
  typescript: 5.9.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Turbopack, Middleware

Which stage(s) are affected? (Select all that apply)

Vercel (Deployed)

Additional context

No response

[rosbitskyy]

Summary: ESM-only Support for Proxy/Middleware

Problem (What Was)

When deploying Next.js apps with "type": "module" in package.json to Vercel, proxy/middleware failed with:

Error [ERR_REQUIRE_ESM]: require() of ES Module /var/task/apps/web/.next/server/middleware.js from /var/task/apps/web/___next_launcher.cjs not supported.

The loadNodeMiddleware() method was using require() to load middleware.js, which fails in ESM-only projects because .js files are treated as ES modules.

Solution (What Changed)

1. Added isESMProject() method - Detects if package.json has "type": "module"
2. Updated loadNodeMiddleware() to:
   • Check if project is ESM-only
   • Use import() instead of require() for ESM projects
   • Handle Windows paths with pathToFileURL()
   • Keep require() for CJS projects (backward compatible)
3. Added test suite - test/e2e/app-dir/app-middleware-proxy-esm/ with:
   • package.json with "type": "module"
   • Proxy middleware using ESM syntax
   • Tests for loading, rewrites, and headers

Benefits (What It Gives)

• ✅ ESM-only projects now work with proxy/middleware on Vercel
• ✅ Backward compatible - CJS projects still use require()
• ✅ Windows support via pathToFileURL()
• ✅ Handles top-level await via await import()

How to Test

Manual Testing:

1. Create Next.js app with "type": "module" in package.json
2. Add proxy.js or middleware.js with ESM syntax (import/export)
3. Deploy to Vercel or run next build && next start
4. Verify middleware/proxy works without ERR_REQUIRE_ESM error

Automated Testing:

# Run the new test suite
npm test -- test/e2e/app-dir/app-middleware-proxy-esm/app-middleware-proxy-esm.test.ts

Test Results Example

Before Fix:

Error [ERR_REQUIRE_ESM]: require() of ES Module .../middleware.js not supported.
Node.js process exited with exit status: 1

After Fix:

✓ should load proxy middleware in ESM-only project (1234ms)
✓ should handle proxy rewrite in ESM-only project (567ms)
✓ should add headers from proxy in ESM-only project (890ms)

Test Suites: 1 passed, 1 total
Tests:       3 passed, 3 total

Example Test Output:

GET / 200 in 45ms (compile: 12ms, proxy.ts: 8ms, render: 25ms)
GET /test 200 in 23ms
GET /api/test 200 in 15ms
x-from-proxy: hello-from-proxy ✓

Files Changed

• packages/next/src/server/next-server.ts - Core fix
• test/e2e/app-dir/app-middleware-proxy-esm/ - Test suite (8 new files)

Total: 9 files changed, 124 insertions(+), 7 deletions(-)

Fixes #86434