> ## Documentation Index
> Fetch the complete documentation index at: https://docs.ag-ui.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Events

> Documentation for the events used in the Agent User Interaction Protocol SDK

# 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:

```typescript theme={null}
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",
  TOOL_CALL_RESULT = "TOOL_CALL_RESULT",
  STATE_SNAPSHOT = "STATE_SNAPSHOT",
  STATE_DELTA = "STATE_DELTA",
  MESSAGES_SNAPSHOT = "MESSAGES_SNAPSHOT",
  ACTIVITY_SNAPSHOT = "ACTIVITY_SNAPSHOT",
  ACTIVITY_DELTA = "ACTIVITY_DELTA",
  RAW = "RAW",
  CUSTOM = "CUSTOM",
  RUN_STARTED = "RUN_STARTED",
  RUN_FINISHED = "RUN_FINISHED",
  RUN_ERROR = "RUN_ERROR",
  STEP_STARTED = "STEP_STARTED",
  STEP_FINISHED = "STEP_FINISHED",
  REASONING_START = "REASONING_START",
  REASONING_MESSAGE_START = "REASONING_MESSAGE_START",
  REASONING_MESSAGE_CONTENT = "REASONING_MESSAGE_CONTENT",
  REASONING_MESSAGE_END = "REASONING_MESSAGE_END",
  REASONING_MESSAGE_CHUNK = "REASONING_MESSAGE_CHUNK",
  REASONING_END = "REASONING_END",
  REASONING_ENCRYPTED_VALUE = "REASONING_ENCRYPTED_VALUE",
  SUBAGENT_STARTED = "SUBAGENT_STARTED",
  SUBAGENT_FINISHED = "SUBAGENT_FINISHED",
  SUBAGENT_ERROR = "SUBAGENT_ERROR",
}
```

## BaseEvent

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

```typescript theme={null}
type BaseEvent = {
  type: EventType // Discriminator field
  timestamp?: number
  rawEvent?: any
  metadata?: Record<string, 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 |
| `metadata` | `Record<string, any>` (optional) | Extra information attached to the event |

`metadata` is open by key: any JSON value is allowed under a key, including
`null`. The object may be absent, but a present one is never `null` — an
explicit `null` parses as absent. The `ag-ui` key is reserved for AG-UI's own
use. Use `mergeMetadata` from `@ag-ui/core` to fold event metadata into a
message; see [Metadata](/concepts/metadata).

## Lifecycle Events

These events represent the lifecycle of an agent run.

### RunStartedEvent

Signals the start of an agent run.

```typescript theme={null}
type RunStartedEvent = BaseEvent & {
  type: EventType.RUN_STARTED
  threadId: string
  runId: string
  parentRunId?: string
  input?: RunAgentInput
}
```

| Property | Type | Description |
| - | - | - |
| `threadId` | `string` | ID of the conversation thread |
| `runId` | `string` | ID of the agent run |
| `parentRunId` | `string` (optional) | (Optional) Lineage pointer for branching/time travel. If present, refers to a prior run within the same thread |
| `input` | `RunAgentInput` (optional) | (Optional) The exact agent input payload sent to the agent for this run. May omit messages already in history |

### TokenUsage

A reusable, numeric-only token usage summary carried on terminal run events.
It intentionally contains **only** provider/model labels and token counts — no
prompts, completions, messages, or identifiers. The counts follow the
protocol's [accounting](/spec/1.0/events/lifecycle#token-usage): the input
and output counts are totals, and the cache and reasoning counts are parts of
them, never additions.

```typescript theme={null}
type TokenUsage = {
  provider?: string
  model?: string
  inputTokens?: number
  outputTokens?: number
  totalTokens?: number
  reasoningTokens?: number
  cachedInputTokens?: number
  cacheWriteInputTokens?: number
}
```

| Property | Type | Description |
| - | - | - |
| `provider` | `string` (optional) | Provider that reported the usage (e.g. `openai`) |
| `model` | `string` (optional) | Model that produced the usage |
| `inputTokens` | `number` (optional) | All input tokens, cache reads and writes included |
| `outputTokens` | `number` (optional) | All output tokens, reasoning included |
| `totalTokens` | `number` (optional) | `inputTokens` plus `outputTokens` |
| `reasoningTokens` | `number` (optional) | Output tokens spent on reasoning; part of `outputTokens` |
| `cachedInputTokens` | `number` (optional) | Input tokens read from a prompt cache; part of `inputTokens` |
| `cacheWriteInputTokens` | `number` (optional) | Input tokens written to a prompt cache; part of `inputTokens` |

### RunFinishedEvent

Signals the successful completion of an agent run.

```typescript theme={null}
type RunFinishedEvent = BaseEvent & {
  type: EventType.RUN_FINISHED
  threadId: string
  runId: string
  result?: any
  usage?: TokenUsage[]
}
```

| Property | Type | Description |
| - | - | - |
| `threadId` | `string` | ID of the conversation thread |
| `runId` | `string` | ID of the agent run |
| `result` | `any` (optional) | Result data from the agent run |
| `usage` | `TokenUsage[]` (optional) | Per-(provider, model) token usage for the run, when reported |

### RunErrorEvent

Signals an error during an agent run.

```typescript theme={null}
type RunErrorEvent = BaseEvent & {
  type: EventType.RUN_ERROR
  message: string
  code?: string
  usage?: TokenUsage[]
}
```

| Property | Type | Description |
| - | - | - |
| `message` | `string` | Error message |
| `code` | `string` (optional) | Error code |
| `usage` | `TokenUsage[]` (optional) | Partial token usage from model calls that completed |

### StepStartedEvent

Signals the start of a step within an agent run.

```typescript theme={null}
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.

```typescript theme={null}
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.

```typescript theme={null}
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.

```typescript theme={null}
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.

```typescript theme={null}
type TextMessageEndEvent = BaseEvent & {
  type: EventType.TEXT_MESSAGE_END
  messageId: string
}
```

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

### TextMessageChunkEvent

Convenience event that expands to `TextMessageStart` → `TextMessageContent` →
`TextMessageEnd` automatically in the JS/TS client.

```typescript theme={null}
type TextMessageChunkEvent = BaseEvent & {
  type: EventType.TEXT_MESSAGE_CHUNK
  messageId?: string // required on the first chunk for a message
  role?: "developer" | "system" | "assistant" | "user"
  delta?: string
}
```

Behavior

* Omit start/end: The client transforms chunk sequences into the standard
  start/content/end triad, so you don’t need to emit them manually.
* First chunk requirements: The first chunk for a message must include
  `messageId`. When `role` is omitted, it defaults to `assistant`.
* Streaming: Subsequent chunks with the same `messageId` emit
  `TextMessageContent` events. `TextMessageEnd` is emitted automatically when a
  different message starts or when the stream completes.

## Tool Call Events

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

### ToolCallStartEvent

Signals the start of a tool call.

```typescript theme={null}
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.

```typescript theme={null}
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.

```typescript theme={null}
type ToolCallEndEvent = BaseEvent & {
  type: EventType.TOOL_CALL_END
  toolCallId: string
}
```

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

### ToolCallResultEvent

Provides the result of a tool call execution.

```typescript theme={null}
type ToolCallResultEvent = BaseEvent & {
  type: EventType.TOOL_CALL_RESULT
  messageId: string
  toolCallId: string
  content: string
  role?: "tool"
}
```

| Property | Type | Description |
| - | - | - |
| `messageId` | `string` | ID of the conversation message this result belongs to |
| `toolCallId` | `string` | Matches the ID from the corresponding ToolCallStartEvent |
| `content` | `string` | The actual result/output content from the tool execution |
| `role` | `"tool"` (optional) | Optional role identifier, typically "tool" for tool results |

## State Management Events

These events are used to manage agent state.

### StateSnapshotEvent

Provides a complete snapshot of an agent's state.

```typescript theme={null}
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.

```typescript theme={null}
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.

```typescript theme={null}
type MessagesSnapshotEvent = BaseEvent & {
  type: EventType.MESSAGES_SNAPSHOT
  messages: Message[]
}
```

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

### ActivitySnapshotEvent

Delivers a complete snapshot of an activity message.

```typescript theme={null}
type ActivitySnapshotEvent = BaseEvent & {
  type: EventType.ACTIVITY_SNAPSHOT
  messageId: string
  activityType: string
  content: Record<string, any>
  replace?: boolean
}
```

| Property | Type | Description |
| - | - | - |
| `messageId` | `string` | Identifier for the target `ActivityMessage` |
| `activityType` | `string` | Activity discriminator such as `"PLAN"` or `"SEARCH"` |
| `content` | `Record<string, any>` | Structured payload describing the full activity state |
| `replace` | `boolean` (optional) | Defaults to `true`; when `false` the snapshot is ignored if a message with the same ID already exists |

### ActivityDeltaEvent

Provides incremental updates to an activity snapshot using JSON Patch.

```typescript theme={null}
type ActivityDeltaEvent = BaseEvent & {
  type: EventType.ACTIVITY_DELTA
  messageId: string
  activityType: string
  patch: any[] // RFC 6902 JSON Patch operations
}
```

| Property | Type | Description |
| - | - | - |
| `messageId` | `string` | Identifier for the target `ActivityMessage` |
| `activityType` | `string` | Activity discriminator mirroring the most recent snapshot |
| `patch` | `any[]` | JSON Patch operations applied to the structured activity payload |

## Reasoning Events

These events represent the lifecycle of reasoning/thinking processes within an
agent. Reasoning events allow agents to expose their internal thought process to
the frontend, creating `ReasoningMessage` objects that persist in the message
history with the role `"reasoning"`.

### ReasoningStartEvent

Signals the start of a reasoning phase. This is a pass-through event that
notifies subscribers but does not create messages.

```typescript theme={null}
type ReasoningStartEvent = BaseEvent & {
  type: EventType.REASONING_START
  messageId: string
}
```

| Property | Type | Description |
| - | - | - |
| `messageId` | `string` | Identifier for the reasoning phase |

### ReasoningMessageStartEvent

Signals the start of a reasoning message. Creates a new `ReasoningMessage` in
the message history.

```typescript theme={null}
type ReasoningMessageStartEvent = BaseEvent & {
  type: EventType.REASONING_MESSAGE_START
  messageId: string
  role: "reasoning"
}
```

| Property | Type | Description |
| - | - | - |
| `messageId` | `string` | Unique identifier for the message |
| `role` | `"reasoning"` | Role is always "reasoning" |

### ReasoningMessageContentEvent

Represents a chunk of content in a streaming reasoning message.

```typescript theme={null}
type ReasoningMessageContentEvent = BaseEvent & {
  type: EventType.REASONING_MESSAGE_CONTENT
  messageId: string
  delta: string
}
```

| Property | Type | Description |
| - | - | - |
| `messageId` | `string` | Matches the ID from ReasoningMessageStartEvent |
| `delta` | `string` | Reasoning content chunk |

### ReasoningMessageEndEvent

Signals the end of a reasoning message.

```typescript theme={null}
type ReasoningMessageEndEvent = BaseEvent & {
  type: EventType.REASONING_MESSAGE_END
  messageId: string
}
```

| Property | Type | Description |
| - | - | - |
| `messageId` | `string` | Matches the ID from ReasoningMessageStartEvent |

### ReasoningMessageChunkEvent

Convenience event that expands to `ReasoningMessageStart` →
`ReasoningMessageContent` → `ReasoningMessageEnd` automatically in the JS/TS
client.

```typescript theme={null}
type ReasoningMessageChunkEvent = BaseEvent & {
  type: EventType.REASONING_MESSAGE_CHUNK
  messageId?: string // required on the first chunk for a message
  delta?: string
}
```

Behavior

* Omit start/end: The client transforms chunk sequences into the standard
  start/content/end triad.
* First chunk requirements: The first chunk for a message must include
  `messageId`.
* Streaming: Subsequent chunks with the same `messageId` emit
  `ReasoningMessageContent` events. `ReasoningMessageEnd` is emitted
  automatically when a different message starts or when the stream completes.

### ReasoningEndEvent

Signals the end of a reasoning phase. This is a pass-through event that notifies
subscribers but does not modify messages.

```typescript theme={null}
type ReasoningEndEvent = BaseEvent & {
  type: EventType.REASONING_END
  messageId: string
}
```

| Property | Type | Description |
| - | - | - |
| `messageId` | `string` | Identifier for the reasoning phase |

### ReasoningEncryptedValueEvent

Attaches an encrypted value to a message or tool call. When this event is
emitted, it finds the referenced entity by `entityId` and sets its
`encryptedValue` field.

```typescript theme={null}
type ReasoningEncryptedValueEvent = BaseEvent & {
  type: EventType.REASONING_ENCRYPTED_VALUE
  subtype: "tool-call" | "message"
  entityId: string
  encryptedValue: string
}
```

| Property | Type | Description |
| - | - | - |
| `subtype` | `"tool-call" \| "message"` | The type of entity this value belongs to |
| `entityId` | `string` | ID of the tool call or message to attach the value |
| `encryptedValue` | `string` | The encrypted value to attach to the entity |

## Subagent Events

These events report that the agent delegated work to a child agent, so a frontend
can attribute output to the subagent that produced it. Attribution itself travels
as an optional `subagentRunId` on most other event types.

`subagentRunId` identifies **one invocation**, not a reusable subagent
definition — the same subagent run twice yields two different values. See
[Subagents](/concepts/subagents) for the full model.

### SubagentStartedEvent

Announces a new subagent invocation and names it for display.

```typescript theme={null}
type SubagentStartedEvent = BaseEvent & {
  type: EventType.SUBAGENT_STARTED
  subagentRunId: string
  name: string
  description?: string
  parentSubagentRunId?: string
  parentToolCallId?: string
  parentMessageId?: string
}
```

| Property | Type | Description |
| - | - | - |
| `subagentRunId` | `string` | Opaque identifier for this invocation |
| `name` | `string` | Declared subagent name or type, for display |
| `description` | `string` | Optional description |
| `parentSubagentRunId` | `string` | Optional enclosing subagent, when nesting |
| `parentToolCallId` | `string` | Optional tool call that spawned this subagent |
| `parentMessageId` | `string` | Optional message holding that tool call |

### SubagentFinishedEvent

Marks a subagent invocation as complete.

```typescript theme={null}
type SubagentFinishedEvent = BaseEvent & {
  type: EventType.SUBAGENT_FINISHED
  subagentRunId: string
  result?: any
  outcome?: { type: "success" } | { type: "suspended"; interruptIds?: string[] }
}
```

| Property | Type | Description |
| - | - | - |
| `subagentRunId` | `string` | Matches the id from `SubagentStartedEvent` |
| `result` | `any` | Optional payload, mirroring `RunFinishedEvent.result` |
| `outcome` | `SubagentFinishedOutcome` | Optional discriminated union; omitted means success (the legacy reading). `{ type: "suspended", interruptIds?: string[] }` says the subagent is checkpointed awaiting outside input; `interruptIds` names the run-level interrupts whose answers resume it. |

### SubagentErrorEvent

Marks a subagent invocation as failed.

```typescript theme={null}
type SubagentErrorEvent = BaseEvent & {
  type: EventType.SUBAGENT_ERROR
  subagentRunId: string
  message: string
  code?: string
}
```

| Property | Type | Description |
| - | - | - |
| `subagentRunId` | `string` | Matches the id from `SubagentStartedEvent` |
| `message` | `string` | Human-readable error message |
| `code` | `string` | Optional error code |

### Attribution on other events

Most event types accept an optional `subagentRunId`. An event without it belongs
to the parent agent, so a stream that never sets the field behaves exactly as it
did before subagents existed.

`RunStartedEvent`, `RunFinishedEvent` and `RunErrorEvent` are not attributable —
they describe the run as a whole. `MessagesSnapshotEvent` carries attribution
per-message instead, since one snapshot mixes messages from several producers.

`StateSnapshotEvent` and `StateDeltaEvent` are attributable, but attribution on
them is provenance rather than ownership — it records which subagent produced the
update. State stays run-scoped, so an attributed snapshot or delta is applied to
the run's one state document just as an unattributed one is. There is no
per-subagent state.

Subscribers can react to the lifecycle directly via `onSubagentStartedEvent`,
`onSubagentFinishedEvent` and `onSubagentErrorEvent`.

## Special Events

### RawEvent

Used to pass through events from external systems.

```typescript theme={null}
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.

```typescript theme={null}
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 |

## Deprecated Events

<Warning>
  The `THINKING_*` events are deprecated and will be removed in version 1.0.0.
  New implementations should use `REASONING_*` events instead.
</Warning>

### Thinking Events (Deprecated)

The following event types are deprecated:

| Deprecated Event | Replacement |
| - | - |
| `THINKING_START` | `REASONING_START` |
| `THINKING_END` | `REASONING_END` |
| `THINKING_TEXT_MESSAGE_START` | `REASONING_MESSAGE_START` |
| `THINKING_TEXT_MESSAGE_CONTENT` | `REASONING_MESSAGE_CONTENT` |
| `THINKING_TEXT_MESSAGE_END` | `REASONING_MESSAGE_END` |

See [Reasoning Migration](/concepts/reasoning#migration-from-thinking-events)
for detailed migration guidance.

## Event Schemas

The SDK uses Zod schemas to validate events:

```typescript theme={null}
const EventSchemas = z.discriminatedUnion("type", [
  TextMessageStartEventSchema,
  TextMessageContentEventSchema,
  TextMessageEndEventSchema,
  ToolCallStartEventSchema,
  ToolCallArgsEventSchema,
  ToolCallEndEventSchema,
  ToolCallResultEventSchema,
  StateSnapshotEventSchema,
  StateDeltaEventSchema,
  MessagesSnapshotEventSchema,
  ActivitySnapshotEventSchema,
  ActivityDeltaEventSchema,
  RawEventSchema,
  CustomEventSchema,
  RunStartedEventSchema,
  RunFinishedEventSchema,
  RunErrorEventSchema,
  StepStartedEventSchema,
  StepFinishedEventSchema,
  ReasoningStartEventSchema,
  ReasoningMessageStartEventSchema,
  ReasoningMessageContentEventSchema,
  ReasoningMessageEndEventSchema,
  ReasoningMessageChunkEventSchema,
  ReasoningEndEventSchema,
  ReasoningEncryptedValueEventSchema,
])
```

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

### ToolCallChunkEvent

Convenience event that expands to `ToolCallStart` → `ToolCallArgs` →
`ToolCallEnd` automatically in the JS/TS client.

```typescript theme={null}
type ToolCallChunkEvent = BaseEvent & {
  type: EventType.TOOL_CALL_CHUNK
  toolCallId?: string // required on the first chunk for a tool call
  toolCallName?: string // required on the first chunk for a tool call
  parentMessageId?: string
  delta?: string
}
```

Behavior

* Omit start/end: The client transforms chunk sequences into the standard
  start/args/end triad.
* First chunk requirements: The first chunk must include both `toolCallId` and
  `toolCallName`; `parentMessageId` is propagated to `ToolCallStart` if given.
* Streaming: Subsequent chunks with the same `toolCallId` emit `ToolCallArgs`.
  `ToolCallEnd` is emitted automatically when the tool call changes or when the
  stream completes.


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