fix: add root src/index.ts so the advertised root export resolves (#2273)

.changeset/fix-root-export-missing-index.md

@@ -0,0 +1,7 @@
+---
+'@modelcontextprotocol/sdk': patch
+---
+
+Fix the advertised root package export failing with `ERR_MODULE_NOT_FOUND`. `package.json` maps the `.` export to `dist/{esm,cjs}/index.{js,d.ts}`, but there was no `src/index.ts`, so `tsc` never emitted those files and they were absent from the published tarball — both
+`import '@modelcontextprotocol/sdk'` and `require('@modelcontextprotocol/sdk')` threw before consumer code ran. Adds a side-effect-free `src/index.ts` re-exporting the shared protocol surface (`./types.js`) and the in-memory transport (`./inMemory.js`); `Client` and `Server`
+remain on their `./client` / `./server` subpath exports to avoid ambiguous re-export collisions. Subpath imports are unchanged.

src/index.ts

@@ -0,0 +1,16 @@
+/**
+ * Root entry point for `@modelcontextprotocol/sdk`.
+ *
+ * `package.json` maps the root `.` export to `dist/{esm,cjs}/index.{js,d.ts}`,
+ * but there was no `src/index.ts`, so `tsc` never emitted those files and they
+ * were absent from the published tarball. Importing the package root therefore
+ * failed with `ERR_MODULE_NOT_FOUND` before any consumer code ran (#2273).
+ *
+ * Re-export the shared protocol surface (types, schemas, constants) and the
+ * in-memory transport. `Client` and `Server` intentionally stay on their
+ * `./client` / `./server` subpath exports to avoid TS2308 ambiguous re-export
+ * collisions at the root.
+ */
+
+export * from './types.js';
+export * from './inMemory.js';

test/issues/test_2273_root_export.test.ts

@@ -0,0 +1,35 @@
+/**
+ * Regression test for https://github.com/modelcontextprotocol/typescript-sdk/issues/2273
+ *
+ * `package.json` advertises a root `.` export that maps to
+ * `dist/{esm,cjs}/index.{js,d.ts}`, but v1.x had no `src/index.ts`, so `tsc`
+ * emitted nothing for the root and importing `@modelcontextprotocol/sdk` failed
+ * with `ERR_MODULE_NOT_FOUND` before any consumer code ran.
+ *
+ * This exercises the root barrel directly (the same module `tsc` compiles into
+ * the advertised dist files): it must resolve and re-export the shared protocol
+ * surface and the in-memory transport.
+ */
+
+import { describe, it, expect } from 'vitest';
+import * as root from '../../src/index.js';
+import { LATEST_PROTOCOL_VERSION, SUPPORTED_PROTOCOL_VERSIONS, JSONRPC_VERSION, InMemoryTransport } from '../../src/index.js';
+
+describe('root package export (#2273)', () => {
+    it('re-exports the shared protocol constants', () => {
+        expect(LATEST_PROTOCOL_VERSION).toBe('2025-11-25');
+        expect(SUPPORTED_PROTOCOL_VERSIONS).toContain(LATEST_PROTOCOL_VERSION);
+        expect(JSONRPC_VERSION).toBe('2.0');
+    });
+
+    it('re-exports the in-memory transport as a working class', () => {
+        expect(typeof InMemoryTransport).toBe('function');
+        const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+        expect(clientTransport).toBeInstanceOf(InMemoryTransport);
+        expect(serverTransport).toBeInstanceOf(InMemoryTransport);
+    });
+
+    it('re-exports a non-empty protocol surface', () => {
+        expect(Object.keys(root).length).toBeGreaterThan(0);
+    });
+});