test_streamable_http.py::test_streamablehttp_client_resumption hangs intermittently

[davemssavage]

Describe the bug
When running pytest on codespaces tests/shared/test_streamable_http.py::test_streamablehttp_client_resumption occaisionally hangs

To Reproduce
Steps to reproduce the behavior:

1. Go to main branch of code
2. Open branch in fresh codespace
3. In codespace terminal install uv - e.g. curl -LsSf https://astral.sh/uv/install.sh | sh
4. In codespace terminal install dev dependencies e.g. uv sync --frozen --all-extras --dev
5. In codespace terminal run uv run pytest -vv
6. In codespace terminal check tests completed
7. Repeat 5 until test hangs (usually within 1-5 repeats but I have seen up to 10 successes prior to a hang)

Expected behavior
Tests run to completion and do not hang

Screenshots

Desktop (please complete the following information):

• OS: Github codeworkspace
• Browser Firefox
• Version main

Additional context
As can be seen from the screen shot their appears to be a port open, I suspect a race condition meaning the test some times gets stuck, I've not yet had a chance to figure out what the test is doing

[davemssavage]

doing some checks with print statements indicates the hang is here:

https://github.com/modelcontextprotocol/python-sdk/blob/main/tests/shared/test_streamable_http.py#L1126

then

https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/shared/session.py#L280

On the receiving side it seems to get to here:

https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/client/streamable_http.py#L232

Which just blocks waiting for event_source.aiter_sse() to yield an sse event but nothing arrives

This may be a simple deadlock where each side is waiting for the other to send a message

[davemssavage]

Noting also that this might be a known issue on the httpx-sse library:

florimondmanca/httpx-sse#4

Further digging and encode/httpx#2186 (comment) seems to suggest a hacky fix