Duplicate Response Rendering When Using after_agent_callback in Google ADK

[bgautham-dev]

I’m facing an issue where the agent response is rendered twice in the ADK Web UI when using an after_agent_callback to modify the agent’s final output.

Context

• I’m using Google ADK (LlmAgent) with an after_agent_callback
• The callback inspects and conditionally modifies the final agent response (for example: enriching the output, state updation)
• The callback returns a modified types.Content object

Observed Behavior

• The original agent response is rendered once
• The modified response returned by after_agent_callback is rendered again
• As a result, the ADK Web UI displays duplicate responses for a single agent turn

Expected Behavior

• Only the final, modified response produced by after_agent_callback should be rendered
• The original, unmodified agent output should be suppressed or replaced

Clarification Needed

• Is this the intended behavior of after_agent_callback?

• Should after_agent_callback:

  • mutate the agent response in place, or
  • return None unless the response must be replaced?

• Is there a recommended pattern to override vs append agent output to avoid duplicate rendering?

Any guidance on the correct lifecycle or best practices for after_agent_callback would be greatly appreciated.
I can share a minimal reproducible example if helpful.

Thanks!

[adk-bot]

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

TLDR: The after_agent_callback is designed to replace the agent's final output, not append to it. The duplicate rendering you're observing is not the intended behavior.

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Hello! Based on the ADK documentation, here’s an explanation of the expected behavior for after_agent_callback:

Intended Behavior

The primary purpose of the after_agent_callback is to give you a final opportunity to inspect or modify the agent's output before it is sent to the user. The key points regarding its behavior are:

• To Replace Output: If you return a types.Content object from this callback, the ADK framework will replace the agent's original output with the new content you returned. The original response should not be rendered. [1, 2]
• To Do Nothing: If you return None, the ADK framework proceeds with the original output produced by the agent, without any modifications. [1]

Answering Your Questions

1. Is this the intended behavior?
   No, displaying both the original and the modified response is not the intended behavior. The framework should only display the final content returned by your callback.

2. Should the callback mutate in place or return None?
   You should return a new types.Content object to replace the agent's output. You should not mutate the response in place. Return None only when you decide that no modification is necessary.

3. Is there a recommended pattern to override vs. append?

   • Override: The standard pattern to override is to simply return types.Content(...) with your new content.
   • Append: To append, you would need to construct a new types.Content object that includes both the original content and your additional content, and then return that new object. However, the core issue you're facing is the duplicate rendering itself.

Recommendation

Since the behavior you are seeing is inconsistent with the documented behavior of the ADK, it might be helpful to verify that your callback always returns either a single types.Content object or None.

If the issue persists, providing a minimal reproducible example, as you offered, would be very helpful for the development team to diagnose whether this is a bug in the ADK framework or a side effect of a specific implementation pattern.

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[1] https://google.github.io/adk-docs/callbacks/types-of-callbacks/
[2] https://github.com/google/adk-docs/blob/main/examples/python/snippets/callbacks/after_agent_callback.py