Events

The Agent User Interaction Protocol SDK uses a streaming event-based architecture. Events are the fundamental units of communication between agents and the frontend. This section documents the event types and their properties.

EventType Enum

The EventType enum defines all possible event types in the system:

enum EventType {
  TEXT_MESSAGE_START = "TEXT_MESSAGE_START",
  TEXT_MESSAGE_CONTENT = "TEXT_MESSAGE_CONTENT",
  TEXT_MESSAGE_END = "TEXT_MESSAGE_END",
  TOOL_CALL_START = "TOOL_CALL_START",
  TOOL_CALL_ARGS = "TOOL_CALL_ARGS",
  TOOL_CALL_END = "TOOL_CALL_END",
  STATE_SNAPSHOT = "STATE_SNAPSHOT",
  STATE_DELTA = "STATE_DELTA",
  MESSAGES_SNAPSHOT = "MESSAGES_SNAPSHOT",
  RAW = "RAW",
  CUSTOM = "CUSTOM",
  RUN_STARTED = "RUN_STARTED",
  RUN_FINISHED = "RUN_FINISHED",
  RUN_ERROR = "RUN_ERROR",
  STEP_STARTED = "STEP_STARTED",
  STEP_FINISHED = "STEP_FINISHED",
}

BaseEvent

All events inherit from the BaseEvent type, which provides common properties shared across all event types.

type BaseEvent = {
  type: EventType // Discriminator field
  timestamp?: number
  rawEvent?: any
}

Property	Type	Description
`type`	`EventType`	The type of event (discriminator field for the union)
`timestamp`	`number` (optional)	Timestamp when the event was created
`rawEvent`	`any` (optional)	Original event data if this event was transformed

Lifecycle Events

These events represent the lifecycle of an agent run.

RunStartedEvent

Signals the start of an agent run.

type RunStartedEvent = BaseEvent & {
  type: EventType.RUN_STARTED
  threadId: string
  runId: string
}

Property	Type	Description
`threadId`	`string`	ID of the conversation thread
`runId`	`string`	ID of the agent run

RunFinishedEvent

Signals the successful completion of an agent run.

type RunFinishedEvent = BaseEvent & {
  type: EventType.RUN_FINISHED
  threadId: string
  runId: string
}

Property	Type	Description
`threadId`	`string`	ID of the conversation thread
`runId`	`string`	ID of the agent run

RunErrorEvent

Signals an error during an agent run.

type RunErrorEvent = BaseEvent & {
  type: EventType.RUN_ERROR
  message: string
  code?: string
}

Property	Type	Description
`message`	`string`	Error message
`code`	`string` (optional)	Error code

StepStartedEvent

Signals the start of a step within an agent run.

type StepStartedEvent = BaseEvent & {
  type: EventType.STEP_STARTED
  stepName: string
}

Property	Type	Description
`stepName`	`string`	Name of the step

StepFinishedEvent

Signals the completion of a step within an agent run.

type StepFinishedEvent = BaseEvent & {
  type: EventType.STEP_FINISHED
  stepName: string
}

Property	Type	Description
`stepName`	`string`	Name of the step

Text Message Events

These events represent the lifecycle of text messages in a conversation.

TextMessageStartEvent

Signals the start of a text message.

type TextMessageStartEvent = BaseEvent & {
  type: EventType.TEXT_MESSAGE_START
  messageId: string
  role: "assistant"
}

Property	Type	Description
`messageId`	`string`	Unique identifier for the message
`role`	`"assistant"`	Role is always “assistant”

TextMessageContentEvent

Represents a chunk of content in a streaming text message.

type TextMessageContentEvent = BaseEvent & {
  type: EventType.TEXT_MESSAGE_CONTENT
  messageId: string
  delta: string // Non-empty string
}

Property	Type	Description
`messageId`	`string`	Matches the ID from TextMessageStartEvent
`delta`	`string`	Text content chunk (non-empty)

TextMessageEndEvent

Signals the end of a text message.

type TextMessageEndEvent = BaseEvent & {
  type: EventType.TEXT_MESSAGE_END
  messageId: string
}

Property	Type	Description
`messageId`	`string`	Matches the ID from TextMessageStartEvent

Tool Call Events

These events represent the lifecycle of tool calls made by agents.

ToolCallStartEvent

Signals the start of a tool call.

type ToolCallStartEvent = BaseEvent & {
  type: EventType.TOOL_CALL_START
  toolCallId: string
  toolCallName: string
  parentMessageId?: string
}

Property	Type	Description
`toolCallId`	`string`	Unique identifier for the tool call
`toolCallName`	`string`	Name of the tool being called
`parentMessageId`	`string` (optional)	ID of the parent message

ToolCallArgsEvent

Represents a chunk of argument data for a tool call.

type ToolCallArgsEvent = BaseEvent & {
  type: EventType.TOOL_CALL_ARGS
  toolCallId: string
  delta: string
}

Property	Type	Description
`toolCallId`	`string`	Matches the ID from ToolCallStartEvent
`delta`	`string`	Argument data chunk

ToolCallEndEvent

Signals the end of a tool call.

type ToolCallEndEvent = BaseEvent & {
  type: EventType.TOOL_CALL_END
  toolCallId: string
}

Property	Type	Description
`toolCallId`	`string`	Matches the ID from ToolCallStartEvent

State Management Events

These events are used to manage agent state.

StateSnapshotEvent

Provides a complete snapshot of an agent’s state.

type StateSnapshotEvent = BaseEvent & {
  type: EventType.STATE_SNAPSHOT
  snapshot: any // StateSchema
}

Property	Type	Description
`snapshot`	`any`	Complete state snapshot

StateDeltaEvent

Provides a partial update to an agent’s state using JSON Patch.

type StateDeltaEvent = BaseEvent & {
  type: EventType.STATE_DELTA
  delta: any[] // JSON Patch operations (RFC 6902)
}

Property	Type	Description
`delta`	`any[]`	Array of JSON Patch operations

MessagesSnapshotEvent

Provides a snapshot of all messages in a conversation.

type MessagesSnapshotEvent = BaseEvent & {
  type: EventType.MESSAGES_SNAPSHOT
  messages: Message[]
}

Property	Type	Description
`messages`	`Message[]`	Array of message objects

Special Events

RawEvent

Used to pass through events from external systems.

type RawEvent = BaseEvent & {
  type: EventType.RAW
  event: any
  source?: string
}

Property	Type	Description
`event`	`any`	Original event data
`source`	`string` (optional)	Source of the event

CustomEvent

Used for application-specific custom events.

type CustomEvent = BaseEvent & {
  type: EventType.CUSTOM
  name: string
  value: any
}

Property	Type	Description
`name`	`string`	Name of the custom event
`value`	`any`	Value associated with the event

Event Schemas

The SDK uses Zod schemas to validate events:

const EventSchemas = z.discriminatedUnion("type", [
  TextMessageStartEventSchema,
  TextMessageContentEventSchema,
  TextMessageEndEventSchema,
  ToolCallStartEventSchema,
  ToolCallArgsEventSchema,
  ToolCallEndEventSchema,
  StateSnapshotEventSchema,
  StateDeltaEventSchema,
  MessagesSnapshotEventSchema,
  RawEventSchema,
  CustomEventSchema,
  RunStartedEventSchema,
  RunFinishedEventSchema,
  RunErrorEventSchema,
  StepStartedEventSchema,
  StepFinishedEventSchema,
])

This allows for runtime validation of events and provides TypeScript type inference.

TypeScript

Python

​Events

​EventType Enum

​BaseEvent

​Lifecycle Events

​RunStartedEvent

​RunFinishedEvent

​RunErrorEvent

​StepStartedEvent

​StepFinishedEvent

​Text Message Events

​TextMessageStartEvent

​TextMessageContentEvent

​TextMessageEndEvent

​Tool Call Events

​ToolCallStartEvent

​ToolCallArgsEvent

​ToolCallEndEvent

​State Management Events

​StateSnapshotEvent

​StateDeltaEvent

​MessagesSnapshotEvent

​Special Events

​RawEvent

​CustomEvent

​Event Schemas