fix(server): propagate negotiated protocol version to transport

Author: rechedev9

.changeset/fix-server-protocol-version.md

@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/server': patch
+---
+
+fix(server): propagate negotiated protocol version to transport in _oninitialize

packages/server/src/server/server.ts

@@ -438,6 +438,8 @@ export class Server extends Protocol<ServerContext> {
             ? requestedVersion
             : (this._supportedProtocolVersions[0] ?? LATEST_PROTOCOL_VERSION);
 
+        this.transport?.setProtocolVersion?.(protocolVersion);
+
         return {
             protocolVersion,
             capabilities: this.getCapabilities(),

packages/server/test/server/server.test.ts

@@ -0,0 +1,25 @@
+import { Client } from '@modelcontextprotocol/client';
+import { InMemoryTransport, LATEST_PROTOCOL_VERSION } from '@modelcontextprotocol/core';
+import { Server } from '../../src/server/server.js';
+
+describe('Server', () => {
+    describe('_oninitialize', () => {
+        it('should propagate negotiated protocol version to transport', async () => {
+            const server = new Server({ name: 'test', version: '1.0.0' }, { capabilities: {} });
+            const client = new Client({ name: 'test-client', version: '1.0.0' });
+
+            const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+
+            const setProtocolVersion = vi.fn();
+            (serverTransport as { setProtocolVersion?: (version: string) => void }).setProtocolVersion = setProtocolVersion;
+
+            await server.connect(serverTransport);
+            await client.connect(clientTransport);
+
+            expect(setProtocolVersion).toHaveBeenCalledWith(LATEST_PROTOCOL_VERSION);
+
+            await client.close();
+            await server.close();
+        });
+    });
+});