fix: preserve function-call IDs across partial and final SSE streaming events

[stakeswky]

Summary

Fixes #4609

When SSE streaming is active (the default since ADK 1.22+), _finalize_model_response_event is called once per LlmResponse chunk — once with partial=True for each streaming chunk, and once with partial=False for the final response. Each call creates a fresh Event via model_validate, which means the function calls in the final event have empty IDs. When populate_client_function_call_id runs on the final event it generates a brand-new adk-{uuid}, different from the one assigned to the partial event.

Root Cause

# _finalize_model_response_event (before fix)
finalized_event = Event.model_validate({
    **model_response_event.model_dump(exclude_none=True),
    **llm_response.model_dump(exclude_none=True),  # overwrites content, losing IDs
})
functions.populate_client_function_call_id(finalized_event)  # assigns NEW uuid!

This breaks HITL workflows:

1. Partial event yields function call with ID-A → consumer captures ID-A
2. Final event yields same function call with ID-B → ADK persists ID-B in session
3. Consumer submits FunctionResponse with ID-A → ADK can't find it → hard error

Fix

After populate_client_function_call_id assigns IDs to a partial event, write the content (including IDs) back to model_response_event. On the next call (final event), extract those IDs before the merge overwrites content, then restore them by position before populate_client_function_call_id runs. This ensures partial and final events for the same function call share the same adk-* ID.

# Extract prior IDs before merge overwrites content
prior_fc_ids = [fc.id for fc in (model_response_event.get_function_calls() or [])]

finalized_event = Event.model_validate({...})  # merge

# Restore previously-assigned IDs by position
if prior_fc_ids:
    for idx, fc in enumerate(function_calls):
        if idx < len(prior_fc_ids) and prior_fc_ids[idx]:
            fc.id = prior_fc_ids[idx]

populate_client_function_call_id(finalized_event)  # only assigns for still-empty IDs

# Persist IDs back for next call
model_response_event.content = finalized_event.content

Testing

Added two unit tests:

• test_finalize_model_response_event_consistent_fc_id_across_partial_and_final: verifies partial and final events share the same ID
• test_finalize_model_response_event_preserves_llm_assigned_id: verifies LLM-assigned IDs are not overwritten

All 350 existing tests/unittests/flows/llm_flows/ tests pass.

[gemini-code-assist]

Warning

You have reached your daily quota limit. Please wait up to 24 hours and I will start processing your requests again!

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[adk-bot]

Hello @stakeswky, thank you for your contribution!

Before we can merge this PR, we need you to sign the Contributor License Agreement (CLA). You can find more information and sign the CLA at https://cla.developers.google.com/.

Response from ADK Triaging Agent

[ryanaiagent]

Hi @stakeswky , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Please fix formatting errors by running autoformat.sh

[stakeswky]

Thanks! I ran and pushed the formatting fix in commit , and re-ran the targeted unit tests ().\n\nI still need to complete CLA on my side before merge can proceed.

[stakeswky]

Correction to my previous comment (shell escaped text got mangled): I ran ./autoformat.sh via uv, pushed formatting updates in commit fe469b4, and re-ran the targeted unit test file (20 passed). CLA is still pending on my side before merge can proceed.

[stakeswky]

Updated this branch to fix CLA/email issues:\n- removed merge commit authored with non-matching email\n- replaced autoformat commit authored as user@example.com\n- all commits now use stakeswky@gmail.com\n\nPlease re-run CLA checks.