CommonJS export paths broken - missing index.js file in dist/cjs

Describe the bug

The CommonJS exports in @modelcontextprotocol/sdk package are broken due to missing dist/cjs/index.js file. The package.json exports configuration points to non-existent files, causing MODULE_NOT_FOUND errors when importing from the main package entry point in CommonJS/Node.js environments.

To Reproduce

Steps to reproduce the behavior:

1. Create a Node.js project with CommonJS module resolution (or TypeScript compiling to CommonJS)
2. Install @modelcontextprotocol/sdk@^1.17.3
3. Try to import from the main export: import { McpError } from "@modelcontextprotocol/sdk"
4. Run the application with Node.js
5. Observe the MODULE_NOT_FOUND error

Minimal reproduction:

// test.js
const { McpError } = require('@modelcontextprotocol/sdk');
console.log(McpError);

node test.js
# Error: Cannot find module '/path/to/node_modules/@modelcontextprotocol/sdk/dist/cjs/index.js'

Expected behavior

The main package import should work without errors:

import { McpError } from "@modelcontextprotocol/sdk";
// Should successfully import McpError class

Logs

Error: Cannot find module '/Users/adam/src/maxcare-ai/monorepo/node_modules/@modelcontextprotocol/sdk/dist/cjs/index.js'
    at createEsmNotFoundErr (node:internal/modules/cjs/loader:1447:15)
    at finalizeEsmResolution (node:internal/modules/cjs/loader:1436:9)
    at resolveExports (node:internal/modules/cjs/loader:671:14)
    at Module._findPath (node:internal/modules/cjs/loader:738:31)
    at Module._resolveFilename (node:internal/modules/cjs/loader:1396:27)
    at defaultResolveImpl (node:internal/modules/cjs/loader:1051:19)
    at resolveForCJSWithHooks (node:internal/modules/cjs/loader:1056:22)
    at Module._load (node:internal/modules/cjs/loader:1219:37)
    at TracingChannel.traceSync (node:diagnostics_channel:322:14)
    at wrapModuleLoad (node:internal/modules/cjs/loader:238:24) {
  code: 'MODULE_NOT_FOUND',
  path: '/Users/adam/src/maxcare-ai/monorepo/node_modules/@modelcontextprotocol/sdk'
}

Additional context

• Package version: 1.17.3 (also affects 1.17.4)
• Node.js version: 24.7.0
• Environment: TypeScript project compiled to CommonJS

Root cause analysis:
The package.json exports configuration references:

"exports": {
  ".": {
    "import": "./dist/esm/index.js",
    "require": "./dist/cjs/index.js"  // ❌ This file doesn't exist
  }
}

Actual file structure:

dist/cjs/
├── types.js        ✅ (contains McpError)
├── server/index.js ✅
├── client/index.js ✅
├── cli.js          ✅
└── index.js        ❌ missing

Current workaround:
Using the full path import works: import { McpError } from "@modelcontextprotocol/sdk/types.js"

Suggested fix:
Either create the missing dist/cjs/index.js file that re-exports the main classes/functions, or update the exports configuration to point to existing files like types.js for the main export.

Start by checking package.json exports alongside the dist/cjs file structure, then run the provided test.js reproduction with require('@modelcontextprotocol/sdk'). Done means the main CommonJS package import resolves successfully and exposes McpError without a MODULE_NOT_FOUND error.