Error with default-app-name when creating session in ADK agent deployment on Agent Engine

[wcvb13]

We deployed a demo agent to Agent Engine using ADK for testing, using VertexAiSessionService for session persistence. Then, after getting the agent instance by calling agent_engines.get("xxxx"), when we call create_session, it reports an error saying 'App name default-app-name is not valid'.

Environment details

• OS type and version: MacOS
• Python version: 3.11
• pip version: 25.0
• google-cloud-aiplatform version: 1.89.0

Steps to reproduce

1. Used ADK sample code to deploy an agent to Vertex AI Agent Engine
2. Deployment was successful and received a resource name
3. Tried to interact with the deployed agent using Vertex AI SDK
4. All API calls fail with error "App name default-app-name is not valid"

Code example

import vertexai
from vertexai import agent_engines
from vertexai.preview.reasoning_engines import AdkApp

# Agent deployment code
app = AdkApp(
    agent=root_agent,
    enable_tracing=True,
    env_vars=env_vars,
    session_service_builder=session_service_builder
)

remote_agent = agent_engines.create(
    app,
    display_name="Travel Concierge",
    requirements=[
        "google-adk (>=0.0.2)",
        "google-cloud-aiplatform[agent_engines] (>=1.88.0)",
        "google-genai (>=1.5.0,<2.0.0)",
        "pydantic (>=2.10.6,<3.0.0)",
        "absl-py (>=2.2.1,<3.0.0)",
        "pydantic (>=2.10.6,<3.0.0)",
        "requests (>=2.32.3,<3.0.0)",
    ],
    extra_packages=[
        "./travel_concierge",  # The main package
        "./eval",  # The initial state files
    ],
)
print(f"Created remote agent: {remote_agent.resource_name}")

# Attempting to use the deployed agent
vertexai.init(project=project_id, location=location, staging_bucket="gs://xxxxxxxx")
remote_agent = agent_engines.get("123124123123")
print(remote_agent.operation_schemas())

# This line fails with the error
sessions = remote_agent.list_sessions(user_id=user_id)

Stack trace

Error Details: {"detail":"Agent Engine Error: An error occurred during invocation. Exception: App name default-app-name is not valid. It should either be the full ReasoningEngine resource name, or the reasoning engine id}

[yeesian]

Hi sorry for the slow response -- this does seem like a legitimate issue, and I'm working on a fix in #5287.

Just for me to doublecheck: I think you will not run into this issue if you remove the line session_service_builder=session_service_builder (it will still instantiate a vertex ai session service for you when deployed to agent engine).

[wcvb13]

Yes, if I remove session_service_builder it will be okey