Deploy to AgentEngine is not working

[v360-eduardo-marques]

I built a simple agent and I'm trying to deploy to AgentEngine. When I run adk deploy agent_engine its seems to work. But when I try to connect to the agent, I get the following error:

google.genai.errors.ClientError: 400 FAILED_PRECONDITION. {'error': {'code': 400, 'message': 'Reasoning Engine Execution failed.\nPlease refer to our documentation (https://cloud.google.com/vertex-ai/generative-ai/docs/agent-engine/troubleshooting/use) for checking logs and other troubleshooting tips.\nError Details: {"detail":"Agent Engine Error: (\\"FastAPI could not JSON-encode the response from invocation method %s. Error: %s. Invocation method\'s original response: %r\\", \'async_create_session\', ValueError([TypeError(\\"\'coroutine\' object is not iterable\\"), TypeError(\'vars() argument must have __dict__ attribute\')]), <coroutine object AdkApp.async_create_session at 0x7f70a598a320>)"}', 'status': 'FAILED_PRECONDITION'}}

To Reproduce

Steps to reproduce the behavior:

1. Install adk pip install google-adk==1.15.1
2. Create simple agent

from google.adk.agents import Agent

system_instruction = (
    "Just say hi and be nice"
)

root_agent = Agent(
    name="my_test_agent",
    model='gemini-2.5-flash',
    instruction=system_instruction,
    description="Test agent",
)

3. Deploy the agent

adk deploy agent_engine \
    --project=<project_id> \
    --region=us-central1 \
    --staging_bucket=gs://<bucket> \
    --display_name=my_agent \
    my_agent

4. Try to connect to remote agent and create a session

import asyncio
import vertexai

async def main():
    # Create a local session to maintain conversation history
    client = vertexai.Client(project="<project_id>", location="us-central1")
    remote_app = client.agent_engines.get(name='projects/<project_id>/locations/us-central1/reasoningEngines/<agent_id>')
    remote_session = await remote_app.async_create_session(user_id="u_456")
    print(remote_session)

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

5. Stacktrace of the error

Traceback (most recent call last):
  File "/Users/ezmarques/git/plataforma/agent-demo/main.py", line 24, in <module>
    asyncio.run(main())
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/asyncio/runners.py", line 190, in run
    return runner.run(main)
           ^^^^^^^^^^^^^^^^
  File "/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 "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/asyncio/base_events.py", line 654, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/Users/ezmarques/git/plataforma/agent-demo/main.py", line 8, in main
    remote_session = await remote_app.async_create_session(user_id="u_456")
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ezmarques/git/plataforma/agent-demo/.venv/lib/python3.11/site-packages/vertexai/_genai/_agent_engines_utils.py", line 1481, in _method
    response = await self.api_async_client._query(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ezmarques/git/plataforma/agent-demo/.venv/lib/python3.11/site-packages/vertexai/_genai/agent_engines.py", line 2159, in _query
    response = await self._api_client.async_request(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ezmarques/git/plataforma/agent-demo/.venv/lib/python3.11/site-packages/google/genai/_api_client.py", line 1325, in async_request
    result = await self._async_request(
             ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ezmarques/git/plataforma/agent-demo/.venv/lib/python3.11/site-packages/google/genai/_api_client.py", line 1270, in _async_request
    return await self._async_retry(  # type: ignore[no-any-return]
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ezmarques/git/plataforma/agent-demo/.venv/lib/python3.11/site-packages/tenacity/asyncio/__init__.py", line 111, in __call__
    do = await self.iter(retry_state=retry_state)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ezmarques/git/plataforma/agent-demo/.venv/lib/python3.11/site-packages/tenacity/asyncio/__init__.py", line 153, in iter
    result = await action(retry_state)
             ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ezmarques/git/plataforma/agent-demo/.venv/lib/python3.11/site-packages/tenacity/_utils.py", line 99, in inner
    return call(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ezmarques/git/plataforma/agent-demo/.venv/lib/python3.11/site-packages/tenacity/__init__.py", line 418, in exc_check
    raise retry_exc.reraise()
          ^^^^^^^^^^^^^^^^^^^
  File "/Users/ezmarques/git/plataforma/agent-demo/.venv/lib/python3.11/site-packages/tenacity/__init__.py", line 185, in reraise
    raise self.last_attempt.result()
          ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/concurrent/futures/_base.py", line 449, in result
    return self.__get_result()
           ^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/concurrent/futures/_base.py", line 401, in __get_result
    raise self._exception
  File "/Users/ezmarques/git/plataforma/agent-demo/.venv/lib/python3.11/site-packages/tenacity/asyncio/__init__.py", line 114, in __call__
    result = await fn(*args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ezmarques/git/plataforma/agent-demo/.venv/lib/python3.11/site-packages/google/genai/_api_client.py", line 1250, in _async_request_once
    await errors.APIError.raise_for_async_response(client_response)
  File "/Users/ezmarques/git/plataforma/agent-demo/.venv/lib/python3.11/site-packages/google/genai/errors.py", line 159, in raise_for_async_response
    raise ClientError(status_code, response_json, response)
google.genai.errors.ClientError: 400 FAILED_PRECONDITION. {'error': {'code': 400, 'message': 'Reasoning Engine Execution failed.\nPlease refer to our documentation (https://cloud.google.com/vertex-ai/generative-ai/docs/agent-engine/troubleshooting/use) for checking logs and other troubleshooting tips.\nError Details: {"detail":"Agent Engine Error: (\\"FastAPI could not JSON-encode the response from invocation method %s. Error: %s. Invocation method\'s original response: %r\\", \'async_create_session\', ValueError([TypeError(\\"\'coroutine\' object is not iterable\\"), TypeError(\'vars() argument must have __dict__ attribute\')]), <coroutine object AdkApp.async_create_session at 0x7f4757473520>)"}', 'status': 'FAILED_PRECONDITION'}}

Desktop (please complete the following information):

• OS: macOS
• Python version(python -V): 3.11.9
• ADK version(pip show google-adk): 1.15.1

Screenshots
project structure

[SaraCastellanosSF]

+1

[mertbozkir]

I'm having the literally same issue!

[svillegasz]

Same error using

google-adk==1.17.0
google-cloud-aiplatform==1.122.0

[edpark]

Same

[yanbin-pan]

Same issue with the following versions.

google-adk==1.17.0
google-genai==1.46.0

I'm trying to deploy on AE the same Custom Agent code as shown in the docs.
https://google.github.io/adk-docs/agents/custom-agents/