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Overview

Notifications are one-way messages that do not expect a response. They are used to inform the other party about events or state changes.

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Session Cancellation

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CancelNotification

Notification to cancel ongoing operations for a session. See Cancellation.
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sessionId
String
required
The ID of the session to cancel operations for
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_meta
Map<String, dynamic>
Optional metadata

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Usage

When a client sends this notification, the agent should:
  • Stop all language model requests as soon as possible
  • Abort all tool call invocations in progress
  • Send any pending session/update notifications
  • Respond to the original session/prompt request with StopReason::Cancelled

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Request Cancellation

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CancelRequestNotification

Protocol-level notification to cancel a specific in-flight JSON-RPC request. See Request Cancellation.
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requestId
RequestId
required
The request identifier to cancel (JSON-RPC Request ID)
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_meta
Map<String, dynamic>
Optional metadata

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Usage

This is a protocol-level notification that can be sent by either party to request cancellation of a specific pending request. The receiving party should make a best effort to cancel the request but may not be able to guarantee cancellation.

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Session Updates

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SessionNotification

Notification containing a session update from the agent. Used to stream real-time progress and results during prompt processing. See Agent Reports Output.
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sessionId
String
required
The ID of the session this update pertains to
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update
SessionUpdate
required
The actual update content. Can be one of many types:
  • UserMessageChunkSessionUpdate - User message content
  • AgentMessageChunkSessionUpdate - Agent message content
  • AgentThoughtChunkSessionUpdate - Agent internal thoughts
  • ToolCallSessionUpdate - New tool call
  • ToolCallUpdateSessionUpdate - Tool call status update
  • PlanSessionUpdate - Execution plan update
  • AvailableCommandsUpdateSessionUpdate - Available commands update
  • CurrentModeUpdateSessionUpdate - Mode change
  • ConfigOptionUpdate - Configuration change
  • SessionInfoUpdate - Session metadata update
  • UsageUpdate - Token usage update
See the Session Updates page for detailed information about each update type.
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_meta
Map<String, dynamic>
Optional metadata

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Usage

Clients receive these notifications throughout the lifetime of a prompt request. They should:
  • Update the UI in real-time to reflect agent progress
  • Continue accepting tool call updates even after sending a session/cancel notification
  • Handle all update types gracefully, including unknown types for forward compatibility

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Extension Notifications

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ExtNotification

Allows custom notifications that are not part of the ACP specification.
Extension notifications use method names with a leading underscore (e.g., _custom_notification). ACP reserves the _ prefix for extensions.
The structure and fields of extension notifications are determined by the specific extension being used.

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