InMemorySessionService allows duplicate events to be appended during concurrent state broadcasts

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
    # ...

@chriskinzel

@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 ADK and also share more details to help us work on this.
Thanks!

fix(sessions): deduplicate events in InMemorySessionService.append_ev…

@chriskinzel

@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

Following up after an architectural review of ADK's session lifecycle:

ADK does not broadcast the same Event instance (or duplicate event.id) across multiple concurrent Session references for the same session_id:

Within a single invocation (including ParallelAgent and parallel workflow branches), all branches share one InvocationContext.session object and serialize append_event calls through the runner's event queue.
Across separate invocations, concurrent writes from stale Session snapshots are rejected with StaleSessionError on persistent backends (DatabaseSessionService, SqliteSessionService, FirestoreSessionService), and duplicate event.id values violate storage uniqueness constraints.
To share state across concurrent sessions, use app: or user: scoped state keys (State.APP_PREFIX / State.USER_PREFIX), and call get_session() when a fresh session snapshot is needed.

Marking this issue as not_planned for clarity so future contributors are not misled by the concurrent-broadcast premise.

docs(sessions): correct the rationale for InMemorySessionService even…

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InMemorySessionService allows duplicate events to be appended during concurrent state broadcasts #5723

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InMemorySessionService allows duplicate events to be appended during concurrent state broadcasts #5723

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klateefa commented on May 18, 2026

surajksharma07 commented on May 29, 2026

DeanChensj commented on Oct 8, 2026

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