[Bug]: Server does not run on python < 3.13

[njbrake]

What happened?

If you go to the helloworld example https://github.com/google/a2a-python/tree/main/examples/helloworld

And adjust it to run on python 3.12, it breaks.

Relevant log output

(.venv) *[main][~/scm/a2a-python/examples/helloworld]$ python test_client.py 
Traceback (most recent call last):
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/httpx/_transports/default.py", line 101, in map_httpcore_exceptions
    yield
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/httpx/_transports/default.py", line 394, in handle_async_request
    resp = await self._pool.handle_async_request(req)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/httpcore/_async/connection_pool.py", line 256, in handle_async_request
    raise exc from None
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/httpcore/_async/connection_pool.py", line 236, in handle_async_request
    response = await connection.handle_async_request(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/httpcore/_async/connection.py", line 103, in handle_async_request
    return await self._connection.handle_async_request(request)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/httpcore/_async/http11.py", line 136, in handle_async_request
    raise exc
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/httpcore/_async/http11.py", line 106, in handle_async_request
    ) = await self._receive_response_headers(**kwargs)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/httpcore/_async/http11.py", line 177, in _receive_response_headers
    event = await self._receive_event(timeout=timeout)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/httpcore/_async/http11.py", line 217, in _receive_event
    data = await self._network_stream.read(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/httpcore/_backends/anyio.py", line 32, in read
    with map_exceptions(exc_map):
         ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nbrake/.local/share/uv/python/cpython-3.12.8-macos-aarch64-none/lib/python3.12/contextlib.py", line 158, in __exit__
    self.gen.throw(value)
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/httpcore/_exceptions.py", line 14, in map_exceptions
    raise to_exc(exc) from exc
httpcore.ReadTimeout

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/a2a/client/client.py", line 247, in _send_request
    response = await self.httpx_client.post(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/httpx/_client.py", line 1859, in post
    return await self.request(
           ^^^^^^^^^^^^^^^^^^^
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/httpx/_client.py", line 1540, in request
    return await self.send(request, auth=auth, follow_redirects=follow_redirects)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/httpx/_client.py", line 1629, in send
    response = await self._send_handling_auth(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/httpx/_client.py", line 1657, in _send_handling_auth
    response = await self._send_handling_redirects(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/httpx/_client.py", line 1694, in _send_handling_redirects
    response = await self._send_single_request(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/httpx/_client.py", line 1730, in _send_single_request
    response = await transport.handle_async_request(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/httpx/_transports/default.py", line 393, in handle_async_request
    with map_httpcore_exceptions():
         ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nbrake/.local/share/uv/python/cpython-3.12.8-macos-aarch64-none/lib/python3.12/contextlib.py", line 158, in __exit__
    self.gen.throw(value)
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/httpx/_transports/default.py", line 118, in map_httpcore_exceptions
    raise mapped_exc(message) from exc
httpx.ReadTimeout

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/test_client.py", line 45, in <module>
    asyncio.run(main())
  File "/Users/nbrake/.local/share/uv/python/cpython-3.12.8-macos-aarch64-none/lib/python3.12/asyncio/runners.py", line 194, in run
    return runner.run(main)
           ^^^^^^^^^^^^^^^^
  File "/Users/nbrake/.local/share/uv/python/cpython-3.12.8-macos-aarch64-none/lib/python3.12/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nbrake/.local/share/uv/python/cpython-3.12.8-macos-aarch64-none/lib/python3.12/asyncio/base_events.py", line 686, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/test_client.py", line 30, in main
    response = await client.send_message(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/a2a/utils/telemetry.py", line 158, in async_wrapper
    result = await func(*args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/a2a/client/client.py", line 167, in send_message
    **await self._send_request(
      ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/a2a/utils/telemetry.py", line 158, in async_wrapper
    result = await func(*args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nbrake/scm/a2a-python/examples/helloworld/.venv/lib/python3.12/site-packages/a2a/client/client.py", line 257, in _send_request
    raise A2AClientHTTPError(
a2a.client.errors.A2AClientHTTPError: HTTP Error 503: Network communication error: 
(.venv) *[main][~/scm/a2a-python/examples/helloworld]$

Code of Conduct

• I agree to follow this project's Code of Conduct

[njbrake]

To bring this bug out, I simplified the dependencies into a requirements.txt

a2a-sdk
click>=8.1.8
dotenv>=0.9.9
httpx>=0.28.1
pydantic>=2.11.4
python-dotenv>=1.1.0
uvicorn>=0.34.2

Then I pulled up a python 3.12 interpreter, in one terminal ran python . and in another terminal ran python test_client.py and that gave the error.

[njbrake]

Fixed via #98 . Thanks!