Hook events

A hook callback subscribes to a specific event type and runs when that event fires during agent execution. This page is the reference for every event: when each one fires across the lifecycle, the full catalogue of event types, and which properties you can modify to change agent behavior. To write and register a callback, see Hooks.

Hook Event Lifecycle

Single-Agent Lifecycle

The following diagram shows when hook events are emitted during a typical agent invocation where tools are invoked:

Python

TypeScript

Multi-Agent Lifecycle

The following diagram shows when multi-agent hook events are emitted during orchestrator execution:

Python

TypeScript

Available Events

Python

Event	Description
AgentInitializedEvent	Triggered when an agent has been constructed and finished initialization at the end of the agent constructor.
BeforeInvocationEvent	Triggered at the beginning of a new agent invocation request
AfterInvocationEvent	Triggered at the end of an agent request, regardless of success or failure. Uses reverse callback ordering
MessageAddedEvent	Triggered when a message is added to the agent’s conversation history
BeforeModelCallEvent	Triggered before the model is invoked for inference
AfterModelCallEvent	Triggered after model invocation completes. Uses reverse callback ordering
BeforeToolsEvent	Triggered before tools are executed in a batch
BeforeToolCallEvent	Triggered before a tool is invoked
AfterToolCallEvent	Triggered after tool invocation completes. Uses reverse callback ordering
AfterToolsEvent	Triggered after tools are executed in a batch. Uses reverse callback ordering
MultiAgentInitializedEvent	Triggered when multi-agent orchestrator is initialized
BeforeMultiAgentInvocationEvent	Triggered before orchestrator execution starts
AfterMultiAgentInvocationEvent	Triggered after orchestrator execution completes. Uses reverse callback ordering
BeforeNodeCallEvent	Triggered before individual node execution starts
AfterNodeCallEvent	Triggered after individual node execution completes. Uses reverse callback ordering

TypeScript

All events extend HookableEvent, making them both streamable via agent.stream() and subscribable via hook callbacks.

Event	Description
AgentInitializedEvent	Triggered when an agent has been constructed and finished initialization at the end of the agent constructor.
BeforeInvocationEvent	Triggered at the beginning of a new agent invocation request
AfterInvocationEvent	Triggered at the end of an agent request, regardless of success or failure. Uses reverse callback ordering
MessageAddedEvent	Triggered when a message is added to the agent’s conversation history
BeforeModelCallEvent	Triggered before the model is invoked for inference
AfterModelCallEvent	Triggered after model invocation completes. Uses reverse callback ordering
ModelStreamUpdateEvent	Wraps each transient streaming delta from the model during inference. Access via .event
ContentBlockEvent	Wraps a fully assembled content block (TextBlock, ToolUseBlock, ReasoningBlock). Access via .contentBlock
ModelMessageEvent	Wraps the complete model message after all blocks are assembled. Access via .message
BeforeToolCallEvent	Triggered before a tool is invoked
AfterToolCallEvent	Triggered after tool invocation completes. Uses reverse callback ordering
BeforeToolsEvent	Triggered before tools are executed in a batch
AfterToolsEvent	Triggered after tools are executed in a batch. Uses reverse callback ordering
ToolStreamUpdateEvent	Wraps streaming progress events from tool execution. Access via .event
ToolResultEvent	Wraps a completed tool result. Access via .result
AgentResultEvent	Wraps the final agent result at the end of the invocation. Access via .result
InterruptEvent	Fires once per unanswered interrupt when the agent halts to wait for responses. Access via .interrupt
MultiAgentInitializedEvent	Triggered when a multi-agent orchestrator has finished initialization
BeforeMultiAgentInvocationEvent	Triggered before orchestrator execution starts
AfterMultiAgentInvocationEvent	Triggered after orchestrator execution completes. Uses reverse callback ordering
BeforeNodeCallEvent	Triggered before individual node execution starts
NodeStreamUpdateEvent	Wraps an inner streaming event from a node with the node’s identity. Access via .event
NodeCancelEvent	Triggered when a node is cancelled via BeforeNodeCallEvent.cancel
AfterNodeCallEvent	Triggered after individual node execution completes. Uses reverse callback ordering
NodeResultEvent	Wraps a completed node result. Access via .result
MultiAgentHandoffEvent	Triggered when execution transitions between nodes
MultiAgentResultEvent	Wraps the final multi-agent result at the end of orchestration. Access via .result

Event Properties

Most event properties are read-only to prevent unintended modifications. However, certain properties can be modified to influence agent behavior:

Python

• AfterModelCallEvent

  • retry - Request a retry of the model invocation. See Model Call Retry.

• BeforeToolsEvent

  • cancel - Cancel all tool calls in the batch with a message. See Limit Tool Counts.

• BeforeToolCallEvent

  • cancel_tool - Cancel tool execution with a message. See Limit Tool Counts.
  • selected_tool - Replace the tool to be executed. See Tool Interception.
  • tool_use - Modify tool parameters before execution. See Fixed Tool Arguments.

• AfterToolCallEvent

  • result - Modify the tool result. See Result Modification.
  • retry - Request a retry of the tool invocation. See Tool Call Retry.
  • exception (read-only) - The original exception if the tool raised one, otherwise None. See Exception Handling.

• AfterToolsEvent

  • end_turn - Halt the agent loop after the tool batch without calling the model again. Set True for a default final assistant message, a string to use as the final assistant message, or a list of content blocks to use as the final assistant message content directly. The returned result has stop_reason="end_turn".

• AfterInvocationEvent

  • resume - Trigger a follow-up agent invocation with new input. See Invocation resume.

TypeScript

• BeforeInvocationEvent

  • cancel - Cancel the agent invocation with a message.

• BeforeModelCallEvent

  • cancel - Cancel the model call with a message.

• BeforeToolsEvent

  • cancel - Cancel all tool calls in a batch with a message. See Limit Tool Counts.

• BeforeToolCallEvent

  • cancel - Cancel tool execution with a message. See Limit Tool Counts.
  • selectedTool - Replace the tool to be executed with a different Tool instance. See Tool Interception.
  • toolUse - Mutable. Rewrite name, toolUseId, or input before execution. Renaming name re-resolves the tool from the registry when selectedTool is not set. See Fixed Tool Arguments.

• AfterModelCallEvent

  • retry - Request a retry of the model invocation.

• AfterToolCallEvent

  • retry - Request a retry of the tool invocation.
  • result - Mutable. Rewrite the ToolResultBlock before it propagates to the model. See Result Modification.

• AfterToolsEvent

  • endTurn - Halt the agent loop after the tool batch without calling the model again. Set true for a default final assistant message, a string to use as the final assistant message, or a list of content blocks to use as the final assistant message content directly. The returned result has stopReason="endTurn".

• AfterInvocationEvent

  • resume - Trigger a follow-up agent invocation with new input. Setting it re-enters the agent loop under the same invocation lock. See Invocation resume.

Next Steps

• Write and register a callback against any of these events in Hooks.
• Bundle several callbacks together with Plugins.
• See the Agent API Reference for complete method documentation: Python | TypeScript

Tags

hooks11event-loop12tool-execution14