feat: Add CORS configuration for browser-based MCP clients (#1059)

Co-authored-by: Marcelo Trylesinski <marcelotryle@gmail.com>
Co-authored-by: Felix Weinberger <fweinberger@anthropic.com>

README.md

@@ -1165,6 +1165,32 @@ 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.applications import Starlette
+from starlette.middleware.cors import CORSMiddleware
+
+# Create your Starlette app first
+starlette_app = Starlette(routes=[...])
+
+# Then wrap it with CORS middleware
+starlette_app = CORSMiddleware(
+    starlette_app,
+    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
 
 By default, SSE servers are mounted at `/sse` and Streamable HTTP servers are mounted at `/mcp`. You can customize these paths using the methods described below.

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

@@ -9,6 +9,7 @@ import mcp.types as types
 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
 
@@ -123,6 +124,15 @@ def main(
         lifespan=lifespan,
     )
 
+    # Wrap ASGI application with CORS middleware to expose Mcp-Session-Id header
+    # for browser-based clients (ensures 500 errors get proper CORS headers)
+    starlette_app = CORSMiddleware(
+        starlette_app,
+        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
 
     uvicorn.run(starlette_app, host="127.0.0.1", port=port)

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

@@ -10,6 +10,7 @@ from mcp.server.lowlevel import Server
 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
 
@@ -148,6 +149,15 @@ def main(
         lifespan=lifespan,
     )
 
+    # Wrap ASGI application with CORS middleware to expose Mcp-Session-Id header
+    # for browser-based clients (ensures 500 errors get proper CORS headers)
+    starlette_app = CORSMiddleware(
+        starlette_app,
+        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
 
     uvicorn.run(starlette_app, host="127.0.0.1", port=port)