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.

[maximemoreillon]

I have set "type": "module" in package.json and "module": "NodeNext", "moduleResolution": "NodeNext" but now I am getting:

Error: Cannot find package '@cfworker/json-schema' imported from /home/moreillon/dev/mcp-sdk-test/node_modules/@modelcontextprotocol/server/dist/src-IKPjmxu7.mjs
code: 'ERR_MODULE_NOT_FOUND'

[mcp-claude]

the ERR_MODULE_NOT_FOUND for @cfworker/json-schema is already fixed on main in commit 91c3085a ("fix(core): stop bundling @cfworker/json-schema into the main barrel") — it just hasn't shipped as a new alpha yet. the fix needs a new @modelcontextprotocol/server alpha release to reach users.

what was wrong in 2.0.0-alpha.2: the dist chunk dist/src-IKPjmxu7.mjs contained a bare static import { Validator } from "@cfworker/json-schema" because CfWorkerJsonSchemaValidator was re-exported through the core internal barrel. @cfworker/json-schema is only a devDependency — npm never installs it transitively — so any ESM import '@modelcontextprotocol/server' failed at module load time.

what the fix does: moves CfWorkerJsonSchemaValidator out of the main barrel into a dedicated @modelcontextprotocol/core/validators/cfWorker subpath. The main index.mjs no longer references @cfworker/json-schema; it's only reachable via the opt-in @modelcontextprotocol/server/validators/cf-worker subpath.

repro script + output (against published 2.0.0-alpha.2)

repro.mjs — place at project root after npm install @modelcontextprotocol/server@2.0.0-alpha.2 (no @cfworker/json-schema):

import { createRequire } from 'module';
import { readdirSync, readFileSync } from 'fs';

// 1. confirm @cfworker/json-schema is not installed
const require = createRequire(import.meta.url);
let cfworkerInstalled = false;
try { require.resolve('@cfworker/json-schema'); cfworkerInstalled = true; } catch (_) {}
console.log('1. @cfworker/json-schema installed:', cfworkerInstalled, '(expected: false)');

// 2. check the dist chunk for the external import
const distDir = new URL('./node_modules/@modelcontextprotocol/server/dist/', import.meta.url);
const files = readdirSync(distDir);
const chunk = files.find(f => f.startsWith('src-') && f.endsWith('.mjs'));
const content = readFileSync(new URL(chunk, distDir), 'utf8');
const firstLines = content.split('\n').slice(0, 5);
const hasExternalImport = firstLines.some(l => l.includes('@cfworker/json-schema'));
console.log('2. chunk:', chunk, '| has external @cfworker import:', hasExternalImport);

// 3. actually import — should throw ERR_MODULE_NOT_FOUND
console.log('\n3. importing @modelcontextprotocol/server...');
try {
  await import('@modelcontextprotocol/server');
  console.log('   PASS (unexpected)');
} catch (e) {
  console.log('  ', e.code === 'ERR_MODULE_NOT_FOUND' && e.message.includes('@cfworker/json-schema')
    ? 'BUG REPRODUCED:' : 'ERROR:', e.code);
  console.log('   Message:', e.message.split('\n')[0]);
}

output:

1. @cfworker/json-schema installed: false (expected: false)
2. chunk: src-IKPjmxu7.mjs | has external @cfworker import: true
3. importing @modelcontextprotocol/server...
   BUG REPRODUCED: ERR_MODULE_NOT_FOUND
   Message: Cannot find package '@cfworker/json-schema' imported from /tmp/repro-1858-esm/node_modules/@modelcontextprotocol/server/dist/src-IKPjmxu7.mjs

on main (SHA b8886e77): same repro script against the locally-built package shows has external @cfworker import: false — the import is gone from the main barrel.

to verify the fix is live: reopen if still seeing this after the next @modelcontextprotocol/server alpha release.

[yasaichi]

I’m seeing what looks like another published artifact issue in the 2.0.0-alpha.2 packages, this time with Deno/JSR resolution.

Minimal repro:

// repro.ts
import "npm:@modelcontextprotocol/server@2.0.0-alpha.2";

deno run --reload --node-modules-dir=none repro.ts

This fails with:

error: Could not find package '@cfworker/json-schema'
from referrer .../@modelcontextprotocol/server/2.0.0-alpha.2/dist/src-IKPjmxu7.mjs
(@modelcontextprotocol/server@2.0.0-alpha.2).

I checked the published tarball, and dist/src-IKPjmxu7.mjs contains a top-level static import:

import { Validator } from "@cfworker/json-schema";

But @cfworker/json-schema is published as an optional peer dependency, not a regular dependency, so Deno does not include it in the resolved npm dependency graph.

From current main, this looks like it may already be fixed because the CF worker validator no longer appears to be pulled into the main server bundle. If that is correct, could you please publish a new alpha with the current packaging fixes? 2.0.0-alpha.2 is currently not usable from Deno/JSR even for a plain server import.