feat: Add CORS configuration for browser-based MCP clients

- Add CORSMiddleware to streamable HTTP example servers
- Configure minimal CORS with Mcp-Session-Id exposed
- Add CORS documentation section to README

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

@@ -713,6 +713,27 @@ The streamable HTTP transport supports:
 - JSON or SSE response formats
 - Better scalability for multi-node deployments
 
+#### 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:
+
+```python
+from starlette.middleware.cors import CORSMiddleware
+
+# Add CORS middleware to your Starlette app
+app.add_middleware(
+    CORSMiddleware,
+    allow_origins=["*"],  # Configure appropriately for production
+    allow_methods=["GET", "POST", "DELETE"],  # MCP streamable HTTP methods
+    expose_headers=["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
+
 ### Mounting to an Existing ASGI Server
 
 > **Note**: SSE transport is being superseded by [Streamable HTTP transport](https://modelcontextprotocol.io/specification/2025-03-26/basic/transports#streamable-http).

examples/servers/simple-streamablehttp-stateless/mcp_simple_streamablehttp_stateless/server.py

@@ -8,6 +8,7 @@
 from mcp.server.lowlevel import Server
 from mcp.server.streamable_http_manager import StreamableHTTPSessionManager
 from starlette.applications import Starlette
+from starlette.middleware.cors import CORSMiddleware
 from starlette.routing import Mount
 from starlette.types import Receive, Scope, Send
 
@@ -131,6 +132,14 @@ async def lifespan(app: Starlette) -> AsyncIterator[None]:
         ],
         lifespan=lifespan,
     )
+    
+    # Add CORS middleware to expose Mcp-Session-Id header for browser-based clients
+    starlette_app.add_middleware(
+        CORSMiddleware,
+        allow_origins=["*"],  # Allow all origins - adjust as needed for production
+        allow_methods=["GET", "POST", "DELETE"],  # MCP streamable HTTP methods
+        expose_headers=["Mcp-Session-Id"],
+    )
 
     import uvicorn
 

examples/servers/simple-streamablehttp/mcp_simple_streamablehttp/server.py

@@ -9,6 +9,7 @@
 from mcp.server.streamable_http_manager import StreamableHTTPSessionManager
 from pydantic import AnyUrl
 from starlette.applications import Starlette
+from starlette.middleware.cors import CORSMiddleware
 from starlette.routing import Mount
 from starlette.types import Receive, Scope, Send
 
@@ -159,6 +160,14 @@ async def lifespan(app: Starlette) -> AsyncIterator[None]:
         ],
         lifespan=lifespan,
     )
+    
+    # Add CORS middleware to expose Mcp-Session-Id header for browser-based clients
+    starlette_app.add_middleware(
+        CORSMiddleware,
+        allow_origins=["*"],  # Allow all origins - adjust as needed for production
+        allow_methods=["GET", "POST", "DELETE"],  # MCP streamable HTTP methods
+        expose_headers=["Mcp-Session-Id"],
+    )
 
     import uvicorn