Regression in 1.18.0: Function response cannot find matching function call event when sent via separate SSE request

[srochiramani]

Describe the bug

When sending a function response via a separate /run_sse request (e.g., from an async job completion callback), ADK's event generator raises ValueError: No function call event found for function responses ids even though the function call event exists in the session history. This occurs in ADK 1.18.0 but works correctly in ADK 1.17.0.

The issue is in _rearrange_events_for_latest_function_response function which only searches through events in the current request context, not the full session history. When a function response is sent in a separate SSE request, ADK cannot find the matching function call event because it's not in the current request's event list.

To Reproduce

1. Install google-adk==1.18.0
2. Create a LongRunningFunctionTool that returns immediately with a job ID and completes asynchronously
3. Agent calls the tool in one SSE request (function call event is created)
4. Later, when the async job completes, send the function response via a separate /run_sse request:

from google.genai import types

# In update_job_status endpoint or similar async callback
function_response = types.FunctionResponse(
    id=function_call_id,  # ID from the original function call
    name='phone_call_tool',
    response=job_metadata
)
content = types.Content(role='model', parts=[types.Part(function_response=function_response)])

# Send via separate SSE request
await send_sse_message(
    user_id, 
    session_id, 
    {'newMessage': content.model_dump()}, 
    state_delta=updated_state
)

5. Error occurs:

ValueError: No function call event found for function responses ids: {'toolu_01KVJbyjbb9iRvLGq4XysjUA'}

Stack trace:

File "/usr/local/lib/python3.11/site-packages/google/adk/cli/adk_web_server.py", line 1421, in event_generator
    async for event in agen:
File "/usr/local/lib/python3.11/site-packages/google/adk/runners.py", line 443, in run_async
    async for event in agen:
...
File "/usr/local/lib/python3.11/site-packages/google/adk/flows/llm_flows/contents.py", line 402, in _get_contents
    result_events = _rearrange_events_for_latest_function_response(
File "/usr/local/lib/python3.11/site-packages/google/adk/flows/llm_flows/contents.py", line 186, in _rearrange_events_for_latest_function_response
    raise ValueError(
ValueError: No function call event found for function responses ids: {'toolu_01KVJbyjbb9iRvLGq4XysjUA'}

Expected behavior

ADK should be able to find the function call event in the session history when processing a function response sent via a separate request. The _rearrange_events_for_latest_function_response function should search the full session event history, not just the events in the current request context.

Workaround

In ADK 1.17.0, this works correctly. The function response can be sent via a separate SSE request and ADK successfully finds the matching function call in session history.

Desktop (please complete the following information):

• OS: Linux (Google Cloud Run), macOS (local development)
• Python version: Python 3.11
• ADK version:
  • Broken: google-adk==1.18.0
  • Working: google-adk==1.17.0

Model Information:

• Are you using LiteLLM: Yes
• Which model is being used: Sonnet 4.5

Additional context

• Using FunctionTool for kicking off an async operations
• Production environment uses Vertex AI Agent Engine for session storage (--session_service_uri with agentengine://)
• Local development uses in-memory session service
• The function call is made in one SSE request, and the function response is delivered later via a separate /run_sse request when the async job completes
• The function call event exists in the session history (verified by fetching session data), but ADK's event generator cannot find it when processing the function response in a separate request
• This is a regression from ADK 1.17.0 to 1.18.0

Code reference:

The issue is in google/adk/flows/llm_flows/contents.py in the _rearrange_events_for_latest_function_response function. It searches backwards through the events list parameter, but when a function response is sent via a separate request, this list only contains the function response event, not the full session history.

The function should either:

1. Load the full session history before searching for the function call event, or
2. Have access to the session's event history through the invocation context

[miguelabdonsh]

I have created PR #3553 to fix this issue.

[xuanyang15]

Thank you @srochiramani for reporting this issue! The related code change from 1.17.0 to 1.18.0 seems to be b8a2b6c, where we fixed the branch matching logic in _is_event_belongs_to_branch. Could you double check if the issue was caused by function_call and function_response having mismatched branch?

[srochiramani]

@xuanyang15 I'm not familiar with event branches but we only have one agent in the app (so only one branch?). We were basically seeing this happen when a function_response happened outside of the original function_call invocation.

[xuanyang15]

@srochiramani Thanks for the information! Do you mind providing a minimal runnable example code to reproduce this issue? It will help me locate the root cause faster.