Why it matters

Events make agents debuggable and streamable. Understanding shapes production observability.

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The architecture

Event types: message (user/model), tool call, tool response, error, control.

Metadata: author (user/model/tool), timestamp, session ID.

ADK Java event typesMessage eventsuser + modelTool eventscall + responseControl eventsstate changesStreaming events to client enables real-time UI without polling
Event categories.
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How it works end to end

Streaming: events emit as they happen. Client subscribes.

Persistence: events stored with session. Enables replay.

Filtering: emit certain events only.

Aggregation: batch related events into higher-level.