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

The StreamableHTTPTransport class used hardcodes the Content-Type to application/json which prevents overriding it to include, for example, custom charsets

The faulty code is on this line

Example Code

StreamableHTTPConnectionParams(
            url="....",
            headers={"Content-Type": "application/json; charset=utf-8"},
        ),

Python & MCP Python SDK

1.26.0

I can try to provide a PR but want to confirm first if this is considered an issue by the maintainers or they want to have strict control over the Content-Type header