fix: concatenate list values in deep_merge_dicts during parallel tool call merge

[enjoykumawat]

Summary

• Bug: When multiple tool calls run in parallel and each writes to the same state_delta key containing a list value, deep_merge_dicts silently drops all but the last value because lists hit the else branch and get overwritten.
• Fix: Add a list-type check in deep_merge_dicts so list values are concatenated (d1[key] + value) instead of overwritten, preserving all entries from parallel function responses.
• Tests: Added 5 unit tests covering list concatenation, scalar overwrite, nested dict merge, mixed-type handling, and an integration test for merge_parallel_function_response_events with state_delta lists.

Reproduction

# Tool A's delta: {"state_delta": {"items": ["a"]}}# Tool B's delta: {"state_delta": {"items": ["b"]}}# Before fix: {"state_delta": {"items": ["b"]}} — item "a" is lost# After fix: {"state_delta": {"items": ["a", "b"]}} — both preserved

Test plan

• All 5 new unit tests pass (test_deep_merge_dicts_* and test_merge_parallel_function_response_events_merges_state_delta_lists)
• All 32 existing tests in test_functions_simple.py pass (0 regressions)
• Manual: create parallel tool calls that accumulate list state and verify no data loss

Fixes#5190

[wuliang229]

Please move the tests to test_functions_parallel.py

[enjoykumawat]

Done! Moved all the deep_merge_dicts and state_delta list tests to test_functions_parallel.py in commit 4b3db55. Also merged latest main to pick up any upstream changes. CI is re-running now.

[enjoykumawat]

Hi @wuliang229, just following up — the tests have been moved to test_functions_parallel.py as requested (commit 4b3db55) and CI is passing. Would you have a chance to re-review when you get a moment? Happy to make any additional changes if needed.

[adk-bot]

Thank you @enjoykumawat for your contribution! 🎉

Your changes have been successfully imported and merged via Copybara in commit 1ff84eb.

Closing this PR as the changes are now in the main branch.

[enjoykumawat]

Hi @GWeale — I see this was reopened after the Copybara merge (commit 1ff84eb is already in main). Is there a follow-up you'd like from me, or was this reopened for internal tracking? Happy to help.

[enjoykumawat]

Quick question before this gets another look — digging into the Copybara history here, I see this was actually merged internally (1ff84eb, 2026-06-22) and then reverted about 4 hours later by an internal commit (fda23474, same day), with no reason given in either commit message. The PR was then reopened.

Was there a concern found internally with this approach (the list-concatenation logic in deep_merge_dicts, or something else) that led to the revert? I've since rebased this onto current main — the bug is still present there (list values in state_delta still get silently overwritten instead of concatenated) — and all tests pass, but I'd rather understand what went wrong the first time than have the same thing happen again. Happy to adjust the approach if there's a reason this shouldn't land as-is.