ERR_PACKAGE_PATH_NOT_EXPORTED with @modelcontextprotocol/express

[maximemoreillon]

Describe the bug

I am getting a ERR_PACKAGE_PATH_NOT_EXPORTED with message No "exports" main defined in C:\Users\myusersmyproject\node_modules\@modelcontextprotocol\express\package.json when running the @modelcontextprotocol/express example as per the usage guide here

import { createMcpExpressApp } from '@modelcontextprotocol/express';
const app = createMcpExpressApp();

This looks similar to #726 , #427

To Reproduce

Node version 24.14.1 LTS on Windows 11.

Project initialized with

npm i -D typescript
npx tsc --init

verbatimModuleSyntax commented out in tsconfig

Expected behavior

Having no error

Additional context

Dependencies:

{
  // ...
  "dependencies": {
    "@modelcontextprotocol/express": "^2.0.0-alpha.2",
    "@modelcontextprotocol/node": "^2.0.0-alpha.2",
    "@modelcontextprotocol/server": "^2.0.0-alpha.2",
    "express": "^5.2.1"
  },
  "devDependencies": {
    "@types/express": "^5.0.6",
    "typescript": "^6.0.2"
  }
}

[mcp-claude]

confirmed: ERR_PACKAGE_PATH_NOT_EXPORTED when requiring @modelcontextprotocol/express from a CommonJS project. The package (and its peer dep @modelcontextprotocol/server) only exports an "import" (ESM) condition — no "require" — so require() from a CJS module throws this error. TypeScript's default tsc --init produces CJS output, which is the common path that hits this.

workaround: add "type": "module" to your package.json and set "module": "NodeNext" + "moduleResolution": "NodeNext" in tsconfig.json. this makes TypeScript output ESM, which the package's "import" condition satisfies.

repro script + output

repro.mjs (place at project root with @modelcontextprotocol/express installed):

import { createRequire } from 'module';
const require = createRequire(import.meta.url);

// Test 1: ESM import (should succeed)
console.log('Test 1: ESM import...');
try {
  const mod = await import('@modelcontextprotocol/express');
  console.log('  PASS: createMcpExpressApp:', typeof mod.createMcpExpressApp);
} catch (e) {
  console.log('  FAIL:', e.message);
}

// Test 2: CJS require — reproduces the bug
console.log('\nTest 2: CJS require()...');
try {
  const mod = require('@modelcontextprotocol/express');
  console.log('  PASS (unexpected):', typeof mod.createMcpExpressApp);
} catch (e) {
  console.log('  BUG REPRODUCED:', e.code);
  console.log('  Message:', e.message);
}

command + output:

$ node repro.mjs
Test 1: ESM import...
  PASS: createMcpExpressApp: function

Test 2: CJS require()...
  BUG REPRODUCED: ERR_PACKAGE_PATH_NOT_EXPORTED
  Message: No "exports" main defined in /tmp/repro-1858/node_modules/@modelcontextprotocol/express/package.json

code path:

• packages/middleware/express/package.json exports field only has "import" condition — no "require":

  "exports": {
    ".": {
      "types": "./dist/index.d.mts",
      "import": "./dist/index.mjs"
    }
  }

• packages/middleware/express/tsdown.config.ts line 6: format: ['esm'] — CJS output is never built
• same pattern in packages/middleware/fastify, hono, node, and packages/server
• Node.js CJS loader: resolveExports finds no "require" condition → throws ERR_PACKAGE_PATH_NOT_EXPORTED

Adding CJS output to express alone would not be sufficient because @modelcontextprotocol/server (peer dep, also ESM-only) would then fail the same way from within the express CJS bundle. A full fix requires adding CJS support across all v2 packages, which is a significant build system change.