docs(server,examples): document extension capabilities with a runnable example (#2387)

examples/extension-capabilities/server.ts

@@ -0,0 +1,37 @@
+/**
+ * Declares one extension capability, `com.example/feature-flags`, with a small
+ * settings object. The entry is advertised to every peer — by the `initialize`
+ * result on legacy connections and by `server/discover` on modern ones.
+ *
+ * One binary, either transport — selected by `--http --port <N>` (defaults to
+ * stdio). See `examples/CONTRIBUTING.md` for the canonical shape.
+ */
+import { createServer } from 'node:http';
+
+import { parseExampleArgs } from '@mcp-examples/shared';
+import { toNodeHandler } from '@modelcontextprotocol/node';
+import { createMcpHandler, McpServer } from '@modelcontextprotocol/server';
+import { serveStdio } from '@modelcontextprotocol/server/stdio';
+
+function buildServer(): McpServer {
+    const mcp = new McpServer({ name: 'extension-capabilities-server', version: '1.0.0' });
+
+    // Declare the extension and its settings before connecting.
+    mcp.server.registerCapabilities({
+        extensions: { 'com.example/feature-flags': { flags: ['dark-mode', 'beta-search'] } }
+    });
+
+    return mcp;
+}
+
+const { transport, port } = parseExampleArgs();
+
+if (transport === 'stdio') {
+    void serveStdio(buildServer);
+    console.error('[server] serving over stdio');
+} else {
+    const handler = createMcpHandler(buildServer);
+    createServer(toNodeHandler(handler)).listen(port, () => {
+        console.error(`[server] listening on http://127.0.0.1:${port}/mcp`);
+    });
+}