`TypeError: Plain typing.TypeAlias is not valid as type argument` in vertexai/_genai/_agent_engines_utils.py on Python 3.10

Start in vertexai/_genai/_agent_engines_utils.py at line 289 and review the runtime ADKAgent annotation path. Reproduce the import on Python 3.10 with the affected dependency versions, then verify that importing the module no longer raises the TypeError while the optional google.adk import behavior remains intact.

Environment details

• Python: 3.10.1
• google-cloud-aiplatform: 1.140.0
• google-adk: 1.26.0
• OS: Linux (Kubernetes)
• Buggy versions: 1.136.0, 1.137.0, 1.138.0, 1.139.0, 1.140.0
• Python versions affected: 3.10.x
• Last working version: 1.135.0

Steps to reproduce

Not sure this was encountered while using google adk which has this as a dependency.
Full traceback of error after making a POST request to create a session which triggered this error:

Traceback (most recent call last):
  File "/root/.pyenv/versions/3.10/lib/python3.10/site-packages/google/adk/cli/adk_web_server.py", line 660, in _create_session
    session = await self.session_service.create_session(
  File "/root/.pyenv/versions/3.10/lib/python3.10/site-packages/google/adk/sessions/vertex_ai_session_service.py", line 115, in create_session
    api_response = await api_client.agent_engines.sessions.create(
  File "/root/.pyenv/versions/3.10/lib/python3.10/site-packages/vertexai/_genai/client.py", line 111, in agent_engines
    self._agent_engines = importlib.import_module(
  File "/root/.pyenv/versions/3.10.1/lib/python3.10/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1050, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1027, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1006, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 688, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 883, in exec_module
  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
  File "/root/.pyenv/versions/3.10/lib/python3.10/site-packages/vertexai/_genai/agent_engines.py", line 34, in <module>
    from . import _agent_engines_utils
  File "/root/.pyenv/versions/3.10/lib/python3.10/site-packages/vertexai/_genai/_agent_engines_utils.py", line 289, in <module>
    ADKAgent: Optional[TypeAlias] = BaseAgent
  File "/root/.pyenv/versions/3.10.1/lib/python3.10/typing.py", line 311, in inner
    return func(*args, **kwds)
  File "/root/.pyenv/versions/3.10.1/lib/python3.10/typing.py", line 402, in __getitem__
    return self._getitem(self, parameters)
  File "/root/.pyenv/versions/3.10.1/lib/python3.10/typing.py", line 528, in Optional
    arg = _type_check(parameters, f"{self} requires a single type.")
  File "/root/.pyenv/versions/3.10.1/lib/python3.10/typing.py", line 171, in _type_check
    raise TypeError(f"Plain {arg} is not valid as type argument")
TypeError: Plain typing.TypeAlias is not valid as type argument

Root Cause

File: vertexai/_genai/_agent_engines_utils.py
Line: 289

if typing.TYPE_CHECKING:
    from google.adk.agents import BaseAgent

    ADKAgent: TypeAlias = BaseAgent
else:
    try:
        from google.adk.agents import BaseAgent

        ADKAgent: Optional[TypeAlias] = BaseAgent  # this is the line causing issues
    except (ImportError, AttributeError):
        ADKAgent = None  # type: ignore[no-redef]

Not sure the syntax Optional[TypeAlias] valid 3.10? Confusing though since I think the syntax is valid unclear why it would cause this runtime error.

Introduced in: https://github.com/googleapis/python-aiplatform/commit/d8cc50dd67

Workaround

Pin to version 1.135.0 or earlier:

google-cloud-aiplatform = "1.135.0"