feat: Add CORS configuration for browser-based MCP clients #1059
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Kludex
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Jul 4, 2025
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| # 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"], | ||
| ) |
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You need to wrap the whole application instead of using add_middleware, otherwise 500s don't get in this.
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| # 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"], | ||
| ) |
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Suggested change
| # 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"], | |
| ) | |
| # Add CORS middleware to your Starlette app | |
| app = CORSMiddleware( | |
| app, | |
| allow_origins=["*"], # Configure appropriately for production | |
| allow_methods=["GET", "POST", "DELETE"], # MCP streamable HTTP methods | |
| expose_headers=["Mcp-Session-Id"], | |
| ) |
- 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
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Summary
Mcp-Session-IdheaderProblem
Browser-based MCP clients cannot access the
Mcp-Session-Idheader from initialization responses due to CORS restrictions. Without this header, they cannot establish sessions with MCP servers.Solution
This PR adds Starlette's
CORSMiddlewareto the example servers and configures it to expose theMcp-Session-Idheader viaexpose_headers. The configuration is minimal, only allowing the HTTP methods required by the MCP protocol (GET, POST, DELETE).Changes
CORSMiddlewareimport and configuration to:examples/servers/simple-streamablehttp/mcp_simple_streamablehttp/server.pyexamples/servers/simple-streamablehttp-stateless/mcp_simple_streamablehttp_stateless/server.pyTest plan
Mcp-Session-Idheader from responsesReported-by: Jerome