401 in Streamable HTTP should be handled gracefully

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

Currently, streamable_http will call response.raise_for_status() when handling POST requests. (ref).

This means that a 401 will bubble up and cause the entire program to crash.

While a 401 on server initialize could warrant an exception, we have a use case where a MCP server will return 401 for specific unauthorized operations, and allow others.

This should be handled gracefully, similar to how _handle_unexpected_content_type works, returning a JSONRPC message outlining an unauthorized exception was thrown.

This would let an agent or other client handle it instead of crashing the program.

Python & MCP Python SDK

Python 3.12 and MCP SDK v1.13.1

從 src/mcp/client/streamable_http.py 中 response.raise_for_status() 附近的 POST 處理開始，然後將其與 _handle_unexpected_content_type 進行比較。追蹤 HTTP 錯誤如何變成客戶端結果；完成的標準是：未授權操作傳回的 401 會產生一則 JSON-RPC unauthorized 錯誤訊息，客戶端可以處理該訊息而不會崩潰。