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

Comienza en src/mcp/client/streamable_http.py, en el manejo de POST alrededor de response.raise_for_status(), y compáralo después con _handle_unexpected_content_type. Sigue cómo los errores HTTP se convierten en resultados del cliente; se considera terminado cuando un 401 de una operación no autorizada produce un mensaje de error JSON-RPC unauthorized que el cliente puede manejar sin bloquearse.