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InMemorySessionService allows duplicate events to be appended during concurrent state broadcasts #5723
Closed as not planned
Description
Description
In InMemorySessionService.append_event, there is no deduplication check before appending an Event to session.events. When an orchestrator or background task broadcasts shared state updates to multiple concurrent agent sessions, race conditions can cause the exact same event ID to be appended multiple times to a session's history.
Proposed Solution
Add a simple, universal deduplication check at the beginning of append_event:
async def append_event(self, session: Session, event: Event) -> Event:
if event.partial:
return event
if any(e.id == event.id for e in session.events):
return event
# ...
Activity
klateefa commentedon May 18, 2026
@chriskinzel A possible resolution would be to add event ID deduplication in
append_event(), and potentially add synchronization/locking to avoid concurrent race conditions during appends.Could you also confirm which ADK version you're using and whether this still reproduces on the latest release? If not please use latest version of
ADKand also share more details to help us work on this.Thanks!
fix(sessions): deduplicate events in InMemorySessionService.append_ev…
surajksharma07 commentedon May 29, 2026
@chriskinzel Thanks for the detailed report and for linking PR #5727 — looks like this caught some good community traction, with he-yufeng's PR #5743 and devteamaegis's PR #5815 (which already has a committed fix) both targeting the same append_event deduplication gap.
Since the fix is actively being tracked across those PRs and we haven't heard back in 2 weeks since @klateefa's question about your ADK version, going ahead and closing this out — the bug itself is being handled on the PR side.
Keep an eye on PR #5815 for the merge; once that lands, the deduplication check will be in the main branch.
Feel free to reopen if there's additional context you wanted to share or if the merged fix doesn't fully cover your concurrent broadcast scenario!
fix: deduplicate events in InMemorySessionService.append_event
DeanChensj commentedon Oct 8, 2026
Following up after an architectural review of ADK's session lifecycle:
ADK does not broadcast the same
Eventinstance (or duplicateevent.id) across multiple concurrentSessionreferences for the samesession_id:ParallelAgentand parallel workflow branches), all branches share oneInvocationContext.sessionobject and serializeappend_eventcalls through the runner's event queue.Sessionsnapshots are rejected withStaleSessionErroron persistent backends (DatabaseSessionService,SqliteSessionService,FirestoreSessionService), and duplicateevent.idvalues violate storage uniqueness constraints.app:oruser:scoped state keys (State.APP_PREFIX/State.USER_PREFIX), and callget_session()when a fresh session snapshot is needed.Marking this issue as
not_plannedfor clarity so future contributors are not misled by the concurrent-broadcast premise.docs(sessions): correct the rationale for InMemorySessionService even…