Harcoded Content-Type in the client cannot be overriden

[lvalladares]

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

python-sdk/src/mcp/client/streamable_http.py

Line 93 in fb2276b

"content-type": "application/json",

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

[Kludex]

I don't think this is an issue.

utf-8 should be inferred from application/json by clients. It's in the RFC for that content type.

[lvalladares]

You're right that the RFC of JSON includes UTF-8 as the default charset, but in Java, some old servers like RestEasy follow the Servlet spec that pins the charset to ISO-8859-1 (https://jakarta.ee/specifications/servlet/6.0/jakarta-servlet-spec-6.0.html#request-data-encoding), so adding this would simplify the compatibility with legacy systems