LlmAgent output_key is not written to session state when before_agent_callback short-circuits execution #4837
Description
Describe the Bug:
When LlmAgent uses both output_key and before_agent_callback, a types.Content returned from the callback correctly short-circuits execution and is shown to the user, but session.state[output_key] is never updated. This would silently break downstream agents, tools, or prompt templates that rely on output_key , since the UI shows the correct response while the backend state remains stale or empty.
Steps to Reproduce:
We can reproduce using this repro code
from google.adk.agents import LlmAgent
from google.adk.agents.callback_context import CallbackContext
from google.adk.runners import InMemoryRunner
from google.genai import types
def cache_callback(callback_context: CallbackContext):
return types.Content(parts=[types.Part(text="cached answer")])
agent = LlmAgent(
name="my_agent",
model="gemini-2.0-flash",
instruction="Answer questions.",
output_key="result",
before_agent_callback=cache_callback,
)
runner = InMemoryRunner(agent=agent, app_name="test")
session = await runner.session_service.create_session(app_name="test", user_id="user1")
async for _ in runner.run_async(
user_id="user1", session_id=session.id,
new_message=types.Content(parts=[types.Part(text="hello")]),
):
pass
session = await runner.session_service.get_session(app_name="test", user_id="user1", session_id=session.id)
print(session.state.get("result")) Expected Behavior:
session.state["result"] should equal "cached answer", the same text the callback returned to the user.
Observed Behavior:
session.state.get("result") returns None. No error or warning is raised.
Environment Details:
- ADK Library Version (pip show google-adk): 1.27.1
- Desktop OS: Windows
- Python Version (python -V): 3.12.12
How often has this issue occurred?:
- Always (100%)