Here is a minimal reproducer:

import asyncio


async def supervisor():
    async with asyncio.TaskGroup() as tg:
        for _ in range(20):
            tg.create_task(send())


async def send():
    _, writer = await asyncio.open_connection("localhost", 8888)
    writer.write(b"test")
    await writer.drain()


if __name__ == "__main__":
    asyncio.run(supervisor())

This results in:

  + Exception Group Traceback (most recent call last):
  |   File "/tmp/repro.py", line 17, in <module>
  |     asyncio.run(supervisor())
  |   File "/opt/local/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/asyncio/runners.py", line 190, in run
  |     return runner.run(main)
  |            ^^^^^^^^^^^^^^^^
  |   File "/opt/local/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/asyncio/runners.py", line 118, in run
  |     return self._loop.run_until_complete(task)
  |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |   File "/opt/local/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/asyncio/base_events.py", line 654, in run_until_complete
  |     return future.result()
  |            ^^^^^^^^^^^^^^^
  |   File "/tmp/repro.py", line 5, in supervisor
  |     async with asyncio.TaskGroup() as tg:
  |   File "/opt/local/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/asyncio/taskgroups.py", line 145, in __aexit__
  |     raise me from None
  | ExceptionGroup: unhandled errors in a TaskGroup (1 sub-exception)
  +-+---------------- 1 ----------------
    | Traceback (most recent call last):
    |   File "/tmp/repro.py", line 11, in send
    |     _, writer = await asyncio.open_connection("localhost", 8888)
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |   File "/opt/local/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/asyncio/streams.py", line 48, in open_connection
    |     transport, _ = await loop.create_connection(
    |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |   File "/opt/local/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/asyncio/base_events.py", line 1113, in create_connection
    |     transport, protocol = await self._create_connection_transport(
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |   File "/opt/local/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/asyncio/base_events.py", line 1143, in _create_connection_transport
    |     transport = self._make_socket_transport(sock, protocol, waiter)
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |   File "/opt/local/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/asyncio/selector_events.py", line 61, in _make_socket_transport
    |     return _SelectorSocketTransport(self, sock, protocol, waiter,
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |   File "/opt/local/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/asyncio/selector_events.py", line 936, in __init__
    |     base_events._set_nodelay(self._sock)
    |   File "/opt/local/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/asyncio/base_events.py", line 196, in _set_nodelay
    |     sock.setsockopt(socket.IPPROTO_TCP, socket.TCP_NODELAY, 1)
    | OSError: [Errno 22] Invalid argument
    +------------------------------------

on both Linux and Mac. If I only create a single task, then it succeeds every time. Am I misunderstanding what the StreamWriter object is? My intention here is to create multiple TCP client connections using asyncio.