refactor: make Server class framework-agnostic by moving express to separate module

docs/server.md

@@ -71,7 +71,7 @@ For more detailed patterns (stateless vs stateful, JSON response mode, CORS, DNS
 MCP servers running on localhost are vulnerable to DNS rebinding attacks. Use `createMcpExpressApp()` to create an Express app with DNS rebinding protection enabled by default:
 
 ```typescript
-import { createMcpExpressApp } from '@modelcontextprotocol/sdk/server/index.js';
+import { createMcpExpressApp } from '@modelcontextprotocol/sdk/server/express.js';
 
 // Protection auto-enabled (default host is 127.0.0.1)
 const app = createMcpExpressApp();

src/examples/server/elicitationFormExample.ts

@@ -12,7 +12,7 @@ import { type Request, type Response } from 'express';
 import { McpServer } from '../../server/mcp.js';
 import { StreamableHTTPServerTransport } from '../../server/streamableHttp.js';
 import { isInitializeRequest } from '../../types.js';
-import { createMcpExpressApp } from '../../server/index.js';
+import { createMcpExpressApp } from '../../server/express.js';
 
 // Create MCP server - it will automatically use AjvJsonSchemaValidator with sensible defaults
 // The validator supports format validation (email, date, etc.) if ajv-formats is installed

src/examples/server/elicitationUrlExample.ts

@@ -11,7 +11,7 @@ import express, { Request, Response } from 'express';
 import { randomUUID } from 'node:crypto';
 import { z } from 'zod';
 import { McpServer } from '../../server/mcp.js';
-import { createMcpExpressApp } from '../../server/index.js';
+import { createMcpExpressApp } from '../../server/express.js';
 import { StreamableHTTPServerTransport } from '../../server/streamableHttp.js';
 import { getOAuthProtectedResourceMetadataUrl, mcpAuthMetadataRouter } from '../../server/auth/router.js';
 import { requireBearerAuth } from '../../server/auth/middleware/bearerAuth.js';

src/examples/server/jsonResponseStreamableHttp.ts

@@ -4,7 +4,7 @@ import { McpServer } from '../../server/mcp.js';
 import { StreamableHTTPServerTransport } from '../../server/streamableHttp.js';
 import * as z from 'zod/v4';
 import { CallToolResult, isInitializeRequest } from '../../types.js';
-import { createMcpExpressApp } from '../../server/index.js';
+import { createMcpExpressApp } from '../../server/express.js';
 
 // Create an MCP server with implementation details
 const getServer = () => {

src/examples/server/simpleSseServer.ts

@@ -3,7 +3,7 @@ import { McpServer } from '../../server/mcp.js';
 import { SSEServerTransport } from '../../server/sse.js';
 import * as z from 'zod/v4';
 import { CallToolResult } from '../../types.js';
-import { createMcpExpressApp } from '../../server/index.js';
+import { createMcpExpressApp } from '../../server/express.js';
 
 /**
  * This example server demonstrates the deprecated HTTP+SSE transport

src/examples/server/simpleStatelessStreamableHttp.ts

@@ -3,7 +3,7 @@ import { McpServer } from '../../server/mcp.js';
 import { StreamableHTTPServerTransport } from '../../server/streamableHttp.js';
 import * as z from 'zod/v4';
 import { CallToolResult, GetPromptResult, ReadResourceResult } from '../../types.js';
-import { createMcpExpressApp } from '../../server/index.js';
+import { createMcpExpressApp } from '../../server/express.js';
 
 const getServer = () => {
     // Create an MCP server with implementation details

src/examples/server/simpleStreamableHttp.ts

@@ -5,7 +5,7 @@ import { McpServer } from '../../server/mcp.js';
 import { StreamableHTTPServerTransport } from '../../server/streamableHttp.js';
 import { getOAuthProtectedResourceMetadataUrl, mcpAuthMetadataRouter } from '../../server/auth/router.js';
 import { requireBearerAuth } from '../../server/auth/middleware/bearerAuth.js';
-import { createMcpExpressApp } from '../../server/index.js';
+import { createMcpExpressApp } from '../../server/express.js';
 import {
     CallToolResult,
     ElicitResultSchema,

src/examples/server/simpleTaskInteractive.ts

@@ -11,7 +11,8 @@
 
 import { Request, Response } from 'express';
 import { randomUUID } from 'node:crypto';
-import { createMcpExpressApp, Server } from '../../server/index.js';
+import { Server } from '../../server/index.js';
+import { createMcpExpressApp } from '../../server/express.js';
 import { StreamableHTTPServerTransport } from '../../server/streamableHttp.js';
 import {
     CallToolResult,

src/examples/server/sseAndStreamableHttpCompatibleServer.ts

@@ -6,7 +6,7 @@ import { SSEServerTransport } from '../../server/sse.js';
 import * as z from 'zod/v4';
 import { CallToolResult, isInitializeRequest } from '../../types.js';
 import { InMemoryEventStore } from '../shared/inMemoryEventStore.js';
-import { createMcpExpressApp } from '../../server/index.js';
+import { createMcpExpressApp } from '../../server/express.js';
 
 /**
  * This example server demonstrates backwards compatibility with both:

src/examples/server/ssePollingExample.ts

@@ -15,7 +15,7 @@
 import { Request, Response } from 'express';
 import { randomUUID } from 'node:crypto';
 import { McpServer } from '../../server/mcp.js';
-import { createMcpExpressApp } from '../../server/index.js';
+import { createMcpExpressApp } from '../../server/express.js';
 import { StreamableHTTPServerTransport } from '../../server/streamableHttp.js';
 import { CallToolResult } from '../../types.js';
 import { InMemoryEventStore } from '../shared/inMemoryEventStore.js';

src/examples/server/standaloneSseWithGetStreamableHttp.ts

@@ -3,7 +3,7 @@ import { randomUUID } from 'node:crypto';
 import { McpServer } from '../../server/mcp.js';
 import { StreamableHTTPServerTransport } from '../../server/streamableHttp.js';
 import { isInitializeRequest, ReadResourceResult } from '../../types.js';
-import { createMcpExpressApp } from '../../server/index.js';
+import { createMcpExpressApp } from '../../server/express.js';
 
 // Create an MCP server with implementation details
 const server = new McpServer({

src/server/express.ts

@@ -0,0 +1,74 @@
+import express, { Express } from 'express';
+import { hostHeaderValidation, localhostHostValidation } from './middleware/hostHeaderValidation.js';
+
+/**
+ * Options for creating an MCP Express application.
+ */
+export interface CreateMcpExpressAppOptions {
+    /**
+     * The hostname to bind to. Defaults to '127.0.0.1'.
+     * When set to '127.0.0.1', 'localhost', or '::1', DNS rebinding protection is automatically enabled.
+     */
+    host?: string;
+
+    /**
+     * List of allowed hostnames for DNS rebinding protection.
+     * If provided, host header validation will be applied using this list.
+     * For IPv6, provide addresses with brackets (e.g., '[::1]').
+     *
+     * This is useful when binding to '0.0.0.0' or '::' but still wanting
+     * to restrict which hostnames are allowed.
+     */
+    allowedHosts?: string[];
+}
+
+/**
+ * Creates an Express application pre-configured for MCP servers.
+ *
+ * When the host is '127.0.0.1', 'localhost', or '::1' (the default is '127.0.0.1'),
+ * DNS rebinding protection middleware is automatically applied to protect against
+ * DNS rebinding attacks on localhost servers.
+ *
+ * @param options - Configuration options
+ * @returns A configured Express application
+ *
+ * @example
+ * ```typescript
+ * // Basic usage - defaults to 127.0.0.1 with DNS rebinding protection
+ * const app = createMcpExpressApp();
+ *
+ * // Custom host - DNS rebinding protection only applied for localhost hosts
+ * const app = createMcpExpressApp({ host: '0.0.0.0' }); // No automatic DNS rebinding protection
+ * const app = createMcpExpressApp({ host: 'localhost' }); // DNS rebinding protection enabled
+ *
+ * // Custom allowed hosts for non-localhost binding
+ * const app = createMcpExpressApp({ host: '0.0.0.0', allowedHosts: ['myapp.local', 'localhost'] });
+ * ```
+ */
+export function createMcpExpressApp(options: CreateMcpExpressAppOptions = {}): Express {
+    const { host = '127.0.0.1', allowedHosts } = options;
+
+    const app = express();
+    app.use(express.json());
+
+    // If allowedHosts is explicitly provided, use that for validation
+    if (allowedHosts) {
+        app.use(hostHeaderValidation(allowedHosts));
+    } else {
+        // Apply DNS rebinding protection automatically for localhost hosts
+        const localhostHosts = ['127.0.0.1', 'localhost', '::1'];
+        if (localhostHosts.includes(host)) {
+            app.use(localhostHostValidation());
+        } else if (host === '0.0.0.0' || host === '::') {
+            // Warn when binding to all interfaces without DNS rebinding protection
+            // eslint-disable-next-line no-console
+            console.warn(
+                `Warning: Server is binding to ${host} without DNS rebinding protection. ` +
+                    'Consider using the allowedHosts option to restrict allowed hosts, ' +
+                    'or use authentication to protect your server.'
+            );
+        }
+    }
+
+    return app;
+}

src/server/index.test.ts

@@ -3,7 +3,7 @@ import supertest from 'supertest';
 import { Client } from '../client/index.js';
 import { InMemoryTransport } from '../inMemory.js';
 import type { Transport } from '../shared/transport.js';
-import { createMcpExpressApp } from './index.js';
+import { createMcpExpressApp } from './express.js';
 import {
     CreateMessageRequestSchema,
     CreateMessageResultSchema,

src/server/index.ts

@@ -1,6 +1,4 @@
-import express, { Express } from 'express';
 import { mergeCapabilities, Protocol, type NotificationOptions, type ProtocolOptions, type RequestOptions } from '../shared/protocol.js';
-import { hostHeaderValidation, localhostHostValidation } from './middleware/hostHeaderValidation.js';
 import {
     type ClientCapabilities,
     type CreateMessageRequest,
@@ -669,75 +667,3 @@ export class Server<
         return this.notification({ method: 'notifications/prompts/list_changed' });
     }
 }
-
-/**
- * Options for creating an MCP Express application.
- */
-export interface CreateMcpExpressAppOptions {
-    /**
-     * The hostname to bind to. Defaults to '127.0.0.1'.
-     * When set to '127.0.0.1', 'localhost', or '::1', DNS rebinding protection is automatically enabled.
-     */
-    host?: string;
-
-    /**
-     * List of allowed hostnames for DNS rebinding protection.
-     * If provided, host header validation will be applied using this list.
-     * For IPv6, provide addresses with brackets (e.g., '[::1]').
-     *
-     * This is useful when binding to '0.0.0.0' or '::' but still wanting
-     * to restrict which hostnames are allowed.
-     */
-    allowedHosts?: string[];
-}
-
-/**
- * Creates an Express application pre-configured for MCP servers.
- *
- * When the host is '127.0.0.1', 'localhost', or '::1' (the default is '127.0.0.1'),
- * DNS rebinding protection middleware is automatically applied to protect against
- * DNS rebinding attacks on localhost servers.
- *
- * @param options - Configuration options
- * @returns A configured Express application
- *
- * @example
- * ```typescript
- * // Basic usage - defaults to 127.0.0.1 with DNS rebinding protection
- * const app = createMcpExpressApp();
- *
- * // Custom host - DNS rebinding protection only applied for localhost hosts
- * const app = createMcpExpressApp({ host: '0.0.0.0' }); // No automatic DNS rebinding protection
- * const app = createMcpExpressApp({ host: 'localhost' }); // DNS rebinding protection enabled
- *
- * // Custom allowed hosts for non-localhost binding
- * const app = createMcpExpressApp({ host: '0.0.0.0', allowedHosts: ['myapp.local', 'localhost'] });
- * ```
- */
-export function createMcpExpressApp(options: CreateMcpExpressAppOptions = {}): Express {
-    const { host = '127.0.0.1', allowedHosts } = options;
-
-    const app = express();
-    app.use(express.json());
-
-    // If allowedHosts is explicitly provided, use that for validation
-    if (allowedHosts) {
-        app.use(hostHeaderValidation(allowedHosts));
-    } else {
-        // Apply DNS rebinding protection automatically for localhost hosts
-        const localhostHosts = ['127.0.0.1', 'localhost', '::1'];
-        if (localhostHosts.includes(host)) {
-            app.use(localhostHostValidation());
-        } else if (host === '0.0.0.0' || host === '::') {
-            // Warn when binding to all interfaces without DNS rebinding protection
-            // eslint-disable-next-line no-console
-            console.warn(
-                `Warning: Server is binding to ${host} without DNS rebinding protection. ` +
-                    'Consider using the allowedHosts option to restrict allowed hosts, ' +
-                    'or use authentication to protect your server.'
-            );
-        }
-    }
-
-    return app;
-}

src/server/sse.ts

@@ -17,23 +17,23 @@ export interface SSEServerTransportOptions {
      * List of allowed host header values for DNS rebinding protection.
      * If not specified, host validation is disabled.
      * @deprecated Use the `hostHeaderValidation` middleware from `@modelcontextprotocol/sdk/server/middleware/hostHeaderValidation.js` instead,
-     * or use `createMcpExpressApp` from `@modelcontextprotocol/sdk/server/index.js` which includes localhost protection by default.
+     * or use `createMcpExpressApp` from `@modelcontextprotocol/sdk/server/express.js` which includes localhost protection by default.
      */
     allowedHosts?: string[];
 
     /**
      * List of allowed origin header values for DNS rebinding protection.
      * If not specified, origin validation is disabled.
      * @deprecated Use the `hostHeaderValidation` middleware from `@modelcontextprotocol/sdk/server/middleware/hostHeaderValidation.js` instead,
-     * or use `createMcpExpressApp` from `@modelcontextprotocol/sdk/server/index.js` which includes localhost protection by default.
+     * or use `createMcpExpressApp` from `@modelcontextprotocol/sdk/server/express.js` which includes localhost protection by default.
      */
     allowedOrigins?: string[];
 
     /**
      * Enable DNS rebinding protection (requires allowedHosts and/or allowedOrigins to be configured).
      * Default is false for backwards compatibility.
      * @deprecated Use the `hostHeaderValidation` middleware from `@modelcontextprotocol/sdk/server/middleware/hostHeaderValidation.js` instead,
-     * or use `createMcpExpressApp` from `@modelcontextprotocol/sdk/server/index.js` which includes localhost protection by default.
+     * or use `createMcpExpressApp` from `@modelcontextprotocol/sdk/server/express.js` which includes localhost protection by default.
      */
     enableDnsRebindingProtection?: boolean;
 }

src/server/streamableHttp.ts

@@ -105,23 +105,23 @@ export interface StreamableHTTPServerTransportOptions {
      * List of allowed host header values for DNS rebinding protection.
      * If not specified, host validation is disabled.
      * @deprecated Use the `hostHeaderValidation` middleware from `@modelcontextprotocol/sdk/server/middleware/hostHeaderValidation.js` instead,
-     * or use `createMcpExpressApp` from `@modelcontextprotocol/sdk/server/index.js` which includes localhost protection by default.
+     * or use `createMcpExpressApp` from `@modelcontextprotocol/sdk/server/express.js` which includes localhost protection by default.
      */
     allowedHosts?: string[];
 
     /**
      * List of allowed origin header values for DNS rebinding protection.
      * If not specified, origin validation is disabled.
      * @deprecated Use the `hostHeaderValidation` middleware from `@modelcontextprotocol/sdk/server/middleware/hostHeaderValidation.js` instead,
-     * or use `createMcpExpressApp` from `@modelcontextprotocol/sdk/server/index.js` which includes localhost protection by default.
+     * or use `createMcpExpressApp` from `@modelcontextprotocol/sdk/server/express.js` which includes localhost protection by default.
      */
     allowedOrigins?: string[];
 
     /**
      * Enable DNS rebinding protection (requires allowedHosts and/or allowedOrigins to be configured).
      * Default is false for backwards compatibility.
      * @deprecated Use the `hostHeaderValidation` middleware from `@modelcontextprotocol/sdk/server/middleware/hostHeaderValidation.js` instead,
-     * or use `createMcpExpressApp` from `@modelcontextprotocol/sdk/server/index.js` which includes localhost protection by default.
+     * or use `createMcpExpressApp` from `@modelcontextprotocol/sdk/server/express.js` which includes localhost protection by default.
      */
     enableDnsRebindingProtection?: boolean;