Constant DEFAULT_WRITE_TIMEOUT 

pub const DEFAULT_WRITE_TIMEOUT: Duration;

Default deadline for handing one event to the background persistence processor (5 seconds).

This bounds the persistence send, not the broadcast one. Broadcast sends really are non-blocking — a lagging SSE consumer is dropped events, never a stalled writer — and reasoning only about that channel is why this constant was plumbed through four layers and applied by nothing until 2026-08-19. The persistence channel is a bounded mpsc, and send on a full bounded mpsc waits for a free slot with no deadline of its own. With the background processor stalled (a webhook absorbing up to HandlerLimits::push_delivery_timeout per config, or a slow store), EventQueueWriter::write blocked forever once the channel filled — measured at 1,024 events, still blocked eight seconds later.

Exceeding this deadline is reported to the executor as an error rather than dropped: an event that cannot reach the persistence processor is task state that will not be stored, and failing the task says so where silently discarding it would leave the store disagreeing with what the agent did.