feat: allow overriding protocol versions in Client, Server, and Transport (#1448)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

examples/server/src/customProtocolVersion.ts

@@ -0,0 +1,66 @@
+/**
+ * Example: Custom Protocol Version Support
+ *
+ * This demonstrates how to support protocol versions not yet in the SDK.
+ * First version in the list is used as fallback when client requests
+ * an unsupported version.
+ *
+ * Run with: pnpm tsx src/customProtocolVersion.ts
+ */
+
+import { randomUUID } from 'node:crypto';
+import { createServer } from 'node:http';
+
+import { NodeStreamableHTTPServerTransport } from '@modelcontextprotocol/node';
+import type { CallToolResult } from '@modelcontextprotocol/server';
+import { McpServer, SUPPORTED_PROTOCOL_VERSIONS } from '@modelcontextprotocol/server';
+
+// Add support for a newer protocol version (first in list is fallback)
+const CUSTOM_VERSIONS = ['2026-01-01', ...SUPPORTED_PROTOCOL_VERSIONS];
+
+const server = new McpServer(
+    { name: 'custom-protocol-server', version: '1.0.0' },
+    {
+        supportedProtocolVersions: CUSTOM_VERSIONS,
+        capabilities: { tools: {} }
+    }
+);
+
+// Register a tool that shows the protocol configuration
+server.registerTool(
+    'get-protocol-info',
+    {
+        title: 'Protocol Info',
+        description: 'Returns protocol version configuration',
+        inputSchema: {}
+    },
+    async (): Promise<CallToolResult> => ({
+        content: [
+            {
+                type: 'text',
+                text: JSON.stringify({ supportedVersions: CUSTOM_VERSIONS }, null, 2)
+            }
+        ]
+    })
+);
+
+// Create transport - server passes versions automatically during connect()
+const transport = new NodeStreamableHTTPServerTransport({
+    sessionIdGenerator: () => randomUUID()
+});
+
+await server.connect(transport);
+
+// Simple HTTP server
+const PORT = process.env.MCP_PORT ? Number.parseInt(process.env.MCP_PORT, 10) : 3000;
+
+createServer(async (req, res) => {
+    if (req.url === '/mcp') {
+        await transport.handleRequest(req, res);
+    } else {
+        res.writeHead(404).end('Not Found');
+    }
+}).listen(PORT, () => {
+    console.log(`MCP server with custom protocol versions on port ${PORT}`);
+    console.log(`Supported versions: ${CUSTOM_VERSIONS.join(', ')}`);
+});