`Session not found` errors when using `adk web` with an App object

[carlosafonso]

Describe the bug
When using adk web to test an agent, if the agent uses an App object then a Session not found error is raised at the very first input event.

To Reproduce

1. Create a new agent with adk create
2. Use a very basic agent, as follows

from google.adk.agents.llm_agent import Agent
from google.adk.apps import App

root_agent = Agent(
  model='gemini-2.5-flash',
  name='greeter_agent',
  description='An agent that provides a friendly greeting.',
  instruction='Reply with Hello, World!',
)

app = App(
  name="agents",
  root_agent=root_agent,
)

3. Run the UI with adk web
4. Send any input to the agent
5. You will see the following error in the console

2025-11-12 17:57:26,521 - ERROR - adk_web_server.py:1431 - Error in event_generator: Session not found: 294d0036-c513-4c60-b1b7-ee4a34e4ef09
Traceback (most recent call last):
  File "/home/user/test-agent/.venv/lib/python3.12/site-packages/google/adk/cli/adk_web_server.py", line 1421, in event_generator
    async for event in agen:
  File "/home/user/test-agent/.venv/lib/python3.12/site-packages/google/adk/runners.py", line 443, in run_async
    async for event in agen:
  File "/home/user/test-agent/.venv/lib/python3.12/site-packages/google/adk/runners.py", line 380, in _run_with_trace
    raise ValueError(message)
ValueError: Session not found: 294d0036-c513-4c60-b1b7-ee4a34e4ef09

6. Comment out the app declaration (i.e., leave only the root_agent declaration), then try again
7. The agent works as expected

Expected behavior
The agent should return a response in both cases.

Screenshots
N/A

Desktop (please complete the following information):

• OS: Ubuntu Linux
• Python version(python -V): 3.12.3
• ADK version(pip show google-adk): 1.18.0

Model Information:

• Are you using LiteLLM: No
• Which model is being used: gemini-2.5-flash

Additional context
N/A

[zmy1116]

I know this! it's because the app name name="agents" mismatch with with the name of the agent adk web uses

so in your http://localhost:8000/dev-ui/?app=xxx, you need to make sure that xxx has the same name as the one you defined in

app = App(
  name="agents",
  root_agent=root_agent,
)

[carlosafonso]

Hey @zmy1116 , thanks for the suggestion. If I set app in the query string to agents, the UI redirects to the old name, which seems to be the name of the top-level folder.

If I change the name attribute of App to the name of the folder, then it seems to work. However I get this warning in the console:

(In my case, multimodal_agent is the name of the folder which holds the agent code.)

2025-11-13 08:00:12,974 - WARNING - runners.py:266 - App name mismatch detected. The runner is configured with app name "multimodal_agent", but the root agent was loaded from "/home/user/test-agent/.venv/lib/python3.12/site-packages/google/adk/agents", which implies app name "agents".

I think we can close this for now, but I'm unsure what the above means.

[EdmondDantes21]

This is not super intuitive, perhaps it should be added to the dedicated section in the documentation? https://google.github.io/adk-docs/apps/

[jamesarosen]

We experienced the same thing. The error message was a bit confusing because we were setting app_name, but it didn't match the directory name.

We had this:

# /path/to/multimodal_agent/agent.py
app = App(
  app_name = "multimodal"
  # ...
)

And got the error message

ERROR - adk_web_server.py:1431 - Error in event_generator: Session not
found: d4a0a06f-ea91-44ba-9130-96443c94eede. The runner is configured
with app name "multimodal", but the root agent was loaded from
"/app/.venv/lib/python3.12/site-packages/google/adk/agents", which
implies app name "agents". Ensure the runner app_name matches that
directory or pass app_name explicitly when constructing the runner.
The mismatch prevents the runner from locating the session.

I would have expected to see something like

The runner is configured with app name "multimodal", but
the root agent was loaded from "/path/to/multimodal_agent", which
implies app name "multimodal_agent".