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The runtime forwards every session event to each live subscription (the active prompt, steers, and subscriptions held open while background-task notification turns run), so one stream event can reach the TUI several times. AssistantStream.append is not idempotent, so each redelivery rendered again: after a background task completed, notification-turn responses displayed every token once per delivery (e.g. three deliveries rendered each syllable three times) even though the persisted transcript stayed clean. Drop exact redeliveries at the onEvent boundary: an event is skipped when its eventId was already seen, or when the delivered object itself was (subscriptions forward the same object reference). Distinct events never share a key, so legitimately repeated delta content still renders. The session-subscription channel is left untouched; it does not append streamed content.
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Problem
When a background task (backgrounded exec or a background agent) completes and the
runtime starts a notification turn, the streamed assistant response can render with
every token repeated N times — users have seen both 2× and 3× in the wild:
fix
onEventnow drops an event it has already processed — matched byeventId,or by the delivered object itself — before anything is appended to the
transcript. Seen keys are kept in a bounded LRU (256). Distinct events never
share a key, so legitimate output is never suppressed.
The session-subscription channel is untouched (it does not append streamed
content). The runtime-side trigger — one notification starting two concurrent
turns — lives in the vendored runtime and is out of scope here.