google--adk-python/.agents/skills/adk-architecture/references/interfaces/event.md

Event

The Event class represents a single event in the conversation history or workflow execution in the ADK. It is the core data structure used for state reconstruction, communication, and persistence.

Purpose

• Stores conversation content between users and agents.
• Captures actions taken by agents (e.g., function calls, function responses, state updates).
• Holds metadata for workflow nodes, such as execution paths and run IDs.

Key Fields

• invocation_id: The ID of the invocation this event belongs to. Non-empty before appending to a session.
• author: 'user' or the name of the agent, indicating who created the event.
• content: The actual content of the message (text, parts, etc.), inheriting from LlmResponse.
• actions: EventActions containing function calls, responses, or state changes.
• output: Generic data output from a workflow node.
• node_info: NodeInfo containing the execution path in the workflow (e.g., "A/B").
• branch: Used for branch-aware isolation when peer sub-agents shouldn't see each other's history.
• id: Unique identifier for the event.
• timestamp: The timestamp of the event.

Methods of Interest

• get_function_calls(): Returns function calls in the event.
• get_function_responses(): Returns function responses in the event.
• is_final_response(): Returns whether the event is the final response of an agent.

State Lifecycle & Immutability

• Event Immutability: Event history is immutable. Never assume that events are mutated or cleared after they are saved to a session.
• Signal & Action Persistence: When checking if a signal or action is "pending" versus "resolved", do not rely on events being modified in place.
• Compaction Side-Effects: Be aware that storing stateful flags on events (such as requested actions or transient status) can have permanent unintended effects on background compaction when those events age but remain in history.