feat: Add CORS configuration for browser-based MCP clients

- Add cors middleware to example servers with Mcp-Session-Id exposed - Add CORS documentation section to README - Configure minimal CORS settings (only expose required headers) This enables browser-based clients to connect to MCP servers by properly exposing the Mcp-Session-Id header required for session management. Reported-by: Jerome

Author: jerome3o-anthropic

README.md

@@ -584,6 +584,26 @@ app.listen(3000);
 > );
 > ```
 
+
+#### CORS Configuration for Browser-Based Clients
+
+If you'd like your server to be accessible by browser-based MCP clients, you'll need to configure CORS headers. The `Mcp-Session-Id` header must be exposed for browser clients to access it:
+
+```typescript
+import cors from 'cors';
+
+// Add CORS middleware before your MCP routes
+app.use(cors({
+ origin: '*', // Configure appropriately for production
+ exposedHeaders: ['Mcp-Session-Id']
+}));
+```
+
+This configuration is necessary because:
+- The MCP streamable HTTP transport uses the `Mcp-Session-Id` header for session management
+- Browsers restrict access to response headers unless explicitly exposed via CORS
+- Without this configuration, browser-based clients won't be able to read the session ID from initialization responses
+
 #### Without Session Management (Stateless)
 
 For simpler use cases where session management isn't needed:

src/examples/server/jsonResponseStreamableHttp.ts

@@ -4,6 +4,7 @@ import { McpServer } from '../../server/mcp.js';
 import { StreamableHTTPServerTransport } from '../../server/streamableHttp.js';
 import { z } from 'zod';
 import { CallToolResult, isInitializeRequest } from '../../types.js';
+import cors from 'cors';
 
 
 // Create an MCP server with implementation details
@@ -81,6 +82,12 @@ const getServer = () => {
 const app = express();
 app.use(express.json());
 
+// Configure CORS to expose Mcp-Session-Id header for browser-based clients
+app.use(cors({
+ origin: '*', // Allow all origins - adjust as needed for production
+ exposedHeaders: ['Mcp-Session-Id']
+}));
+
 // Map to store transports by session ID
 const transports: { [sessionId: string]: StreamableHTTPServerTransport } = {};
 

src/examples/server/simpleStatelessStreamableHttp.ts

@@ -3,6 +3,7 @@ import { McpServer } from '../../server/mcp.js';
 import { StreamableHTTPServerTransport } from '../../server/streamableHttp.js';
 import { z } from 'zod';
 import { CallToolResult, GetPromptResult, ReadResourceResult } from '../../types.js';
+import cors from 'cors';
 
 const getServer = () => {
  // Create an MCP server with implementation details
@@ -96,6 +97,12 @@ const getServer = () => {
 const app = express();
 app.use(express.json());
 
+// Configure CORS to expose Mcp-Session-Id header for browser-based clients
+app.use(cors({
+ origin: '*', // Allow all origins - adjust as needed for production
+ exposedHeaders: ['Mcp-Session-Id']
+}));
+
 app.post('/mcp', async (req: Request, res: Response) => {
  const server = getServer();
  try {

src/examples/server/sseAndStreamableHttpCompatibleServer.ts

@@ -6,6 +6,7 @@ import { SSEServerTransport } from '../../server/sse.js';
 import { z } from 'zod';
 import { CallToolResult, isInitializeRequest } from '../../types.js';
 import { InMemoryEventStore } from '../shared/inMemoryEventStore.js';
+import cors from 'cors';
 
 /**
  * This example server demonstrates backwards compatibility with both:
@@ -71,6 +72,12 @@ const getServer = () => {
 const app = express();
 app.use(express.json());
 
+// Configure CORS to expose Mcp-Session-Id header for browser-based clients
+app.use(cors({
+ origin: '*', // Allow all origins - adjust as needed for production
+ exposedHeaders: ['Mcp-Session-Id']
+}));
+
 // Store transports by session ID
 const transports: Record<string, StreamableHTTPServerTransport | SSEServerTransport> = {};