Fix flaky test test_response in test_streamable_http.py

[maxisbey]

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 test is flaky. See: https://github.com/modelcontextprotocol/python-sdk/actions/runs/19020904095/job/54315849330?pr=1565

Output:

basic_server = None, basic_server_url = 'http://127.0.0.1:50695/'

 def test_response(basic_server: None, basic_server_url: str):
 """Test response handling for a valid request."""
 mcp_url = f"{basic_server_url}/mcp"
 response = requests.post(
 mcp_url,
 headers={
 "Accept": "application/json, text/event-stream",
 "Content-Type": "application/json",
 },
 json=INIT_REQUEST,
 )
> assert response.status_code == 200
E assert 404 == 200
E + where 404 = <Response [404]>.status_code

tests/shared/test_streamable_http.py:624: AssertionError

Example Code

Python & MCP Python SDK

1.19.0

[maxisbey]

The 404 here was the bind-then-close port race: under pytest-xdist, two workers could pick the same ephemeral port, the readiness poll succeeded against the other worker's server, and the request got that server's 404. As of #2767 (with #2764/#2765), test_response and the rest of these files run in process with no sockets or ports at all, so this failure mode no longer exists. Closing.

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