Response from ADK Answering Agent (experimental, answer may be inaccurate)

TLDR: This error is a safeguard in ADK's session management. It happens when you try to update a session using an outdated session object, which can occur in concurrent environments like a web server. To fix this, you should reload the session from the session service right before you need to make an update, instead of reusing an older session object.

Hello! This ValueError is an important data integrity feature within the ADK's persistent session management. Here’s an explanation of what it means and how you can resolve it.

What Causes the "Stale Session" Error?

The ADK SessionService is designed to store the history and state of conversations. [7, 3] When using a persistent backend like a database (e.g., SQLite), it needs to prevent race conditions where multiple processes or requests might try to update the same session at the same time.

The error you're seeing occurs because of a sequence like this:

1. Your application code loads a session from the database. This session object is now in memory with a specific last_update_time. [12, 20]
2. Meanwhile, another request, thread, or process also interacts with the same session, adds a new event (like a user message or a tool result), and saves it back to the database. The update_time for that session in the database is now newer.
3. Your original code, which is still holding the now "stale" (outdated) session object, tries to append another event.
4. Before committing the change, the SessionService compares the last_update_time of your in-memory session object with the update_time in the database. [11] It sees that the database version is newer and raises the ValueError to stop you from accidentally overwriting the changes made in step 2.

This is a crucial safeguard to prevent data loss and corruption in your conversation history.

How to Fix It

The solution is to ensure your code is always working with the most recent version of the session.

Guideline: Reload the session from the SessionService before performing an update.

Instead of holding onto a Session object for a long time or across different user interactions, fetch it from the service right when you need it.

Example Pattern:

Incorrect (may lead to error):

# 1. Get session at the start of a long process
session = await session_service.get_session(app_name="app", user_id="user", session_id="123")

# ... some other process might update the session in the DB here ...

# 2. Try to append an event using the old session object
await session_service.append_event(session, new_event) # <-- This can raise the ValueError

Correct:

# In each independent operation, get the latest session first
async def handle_new_message(message_text):
    # 1. Get the LATEST session from storage
    session = await session_service.get_session(app_name="app", user_id="user", session_id="123")

    # 2. Create and append the new event
    new_event = Event(...)
    await session_service.append_event(session, new_event) # <-- This is safe

Since you are using the adk_web_server [4], it's very likely that concurrent requests are the cause. Review your agent and server logic to ensure you're fetching a fresh session for each request or operation that modifies it.