User-Agent header in sHTTP transport is not forwarded to auth flow

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

Passing a custom User-Agent header to the sHTTP transport, e.g. here does not result in auth flow requests including that user agent. There also does not appear to be any way to customize this behavior externally to the SDK itself.

In particular, this becomes a problem when working with AWS WAF, as one of its baseline rules is to require all requests to include a User-Agent header, to filter out low-grade spam.

I am currently recommending to internal teams that they disable this rule for the time being, but this is not a great solution for them, as it means removing an application guardrail to work around an SDK limitation.

Example Code

print("📡 Opening StreamableHTTP transport connection with auth...")
async with streamablehttp_client(
    url=self.server_url,
    auth=oauth_auth,
    timeout=timedelta(seconds=60),
    headers={
        "User-Agent": "mcp-python-sdk/0.1.0",
    },
) as (read_stream, write_stream, get_session_id):
    await self._run_session(read_stream, write_stream, get_session_id)

Python & MCP Python SDK

Latest SDK commit: 27279bc (1.22.0)
Python 3.13.2

examples/clients/simple-auth-client/mcp_simple_auth_client/main.py の streamablehttp_client の使用箇所、特に 214-220 行付近の headers 引数から始めて、認証フローがどのようにリクエストを構築しているかを追跡してください。カスタム User-Agent がそれらのリクエストに到達することを、AWS WAF のユースケースも含めて確認してください。完了の条件は、呼び出し元がそのルールを無効化せずに SDK 経由でヘッダーを設定できることです。