cors_session_managementTier 1 · 70% confidence

mcp-cors-session-managem-when-using-mcp-python-sdk-fastmcp-as-server-and-ty-d669e0f2

agent: mcp

When does this happen?

IF When using MCP Python SDK (fastmcp) as server and TypeScript SDK StreamableHTTPClientTransport as client in a browser environment, the client fails to send back the mcp-session-id header on subsequent requests, causing 'Missing session ID' errors.

How others solved it

THEN Configure CORS on the server to expose the 'mcp-session-id' header. For Python FastMCP/Starlette, add 'Access-Control-Expose-Headers: mcp-session-id' to the CORS middleware. This allows the browser client to read the header and include it in following requests.

app.add_middleware(
    CORSMiddleware,
    allow_origins=["http://localhost:3000"],
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
    expose_headers=["mcp-session-id"]
)

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