Looking for full access to session events inside of agent callbacks.

[JnBrymn]

Hello, I'm working on a Google grant project to build a therapist training simulation. The agents that we are building for the simulation will need to have exacting control over the shape of the prompt as it goes into the model, therefore we are incorporating heavy use of the before_model_callback for our work.

In order to build the prompt that we need, we can't just rewrite the supplied LLM request, because it's missing important information that is present in the other events, so we need access to all of the events currently stored in the session. To address this we have started using callback_context._invocation_context.session to pull the events. This works quite well - and is exactly what we need as a matter of fact - but we're using an underscored variable _invocation_context which gives me concern that the APIs might change without notice in the future.

What is the prescribed way to get all of the session events from inside of the model and agent callbacks?

[adk-bot]

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

TLDR: You are on the right track! While callback_context._invocation_context.session works, the prescribed and more stable way to access the session object (which contains the full event history) from within a callback is via the callback_context.session property.

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Hello! Thank you for your detailed question and for sharing your use case for the therapist training simulation project. It's a great example of advanced agent customization.

You are correct that CallbackContext is the intended object for accessing session information within callbacks.

For accessing session state, the documented approach is to use the callback_context.state property, which allows you to read and write key-value data. [2]

For your use case, you need the full history of events, which is stored on the Session object itself. Your finding to use callback_context._invocation_context.session is functionally correct because the CallbackContext does hold the InvocationContext, which in turn holds the Session. However, your concern about using an underscored property (_invocation_context) is valid, as it's considered an internal implementation detail and could change in future versions.

The intended public accessor for the session object is callback_context.session. [1] You can use this property to get the Session object and then access its event history. This will be more resilient to future changes in the ADK framework.

So, instead of:

session = callback_context._invocation_context.session
events = session.events # Or however you access the event list

You should use:

session = callback_context.session
events = session.events

This will provide the stability you're looking for while giving you the full access you need to build your custom prompts.

[1] https://google.github.io/adk-docs/context/
[2] https://google.github.io/adk-docs/sessions/state/

[JnBrymn]

callback_context.session doesn't exist last I looked