Events

The Agent User Interaction Protocol Python 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

from ag_ui.core import EventType The EventType enum defines all possible event types in the system:

class EventType(str, Enum):
    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"

BaseEvent

from ag_ui.core import BaseEvent All events inherit from the BaseEvent class, which provides common properties shared across all event types.

class BaseEvent(ConfiguredBaseModel):
    type: EventType
    timestamp: Optional[int] = None
    raw_event: Optional[Any] = None

Property	Type	Description
`type`	`EventType`	The type of event (discriminator field for the union)
`timestamp`	`Optional[int]`	Timestamp when the event was created
`raw_event`	`Optional[Any]`	Original event data if this event was transformed

Lifecycle Events

These events represent the lifecycle of an agent run.

RunStartedEvent

from ag_ui.core import RunStartedEvent Signals the start of an agent run.

class RunStartedEvent(BaseEvent):
    type: Literal[EventType.RUN_STARTED]
    thread_id: str
    run_id: str
    parent_run_id: Optional[str] = None
    input: Optional[RunAgentInput] = None

Property	Type	Description
`thread_id`	`str`	ID of the conversation thread
`run_id`	`str`	ID of the agent run
`parent_run_id`	`Optional[str]`	(Optional) Lineage pointer for branching/time travel. If present, refers to a prior run within the same thread
`input`	`Optional[RunAgentInput]`	(Optional) The exact agent input payload sent to the agent for this run. May omit messages already in history

RunFinishedEvent

from ag_ui.core import RunFinishedEvent Signals the successful completion of an agent run.

class RunFinishedEvent(BaseEvent):
    type: Literal[EventType.RUN_FINISHED]
    thread_id: str
    run_id: str
    result: Optional[Any] = None

Property	Type	Description
`thread_id`	`str`	ID of the conversation thread
`run_id`	`str`	ID of the agent run
`result`	`Optional[Any]`	Result data from the agent run

RunErrorEvent

from ag_ui.core import RunErrorEvent Signals an error during an agent run.

class RunErrorEvent(BaseEvent):
    type: Literal[EventType.RUN_ERROR]
    message: str
    code: Optional[str] = None

Property	Type	Description
`message`	`str`	Error message
`code`	`Optional[str]`	Error code

StepStartedEvent

from ag_ui.core import StepStartedEvent Signals the start of a step within an agent run.

class StepStartedEvent(BaseEvent):
    type: Literal[EventType.STEP_STARTED]
    step_name: str

Property	Type	Description
`step_name`	`str`	Name of the step

StepFinishedEvent

from ag_ui.core import StepFinishedEvent Signals the completion of a step within an agent run.

class StepFinishedEvent(BaseEvent):
    type: Literal[EventType.STEP_FINISHED]
    step_name: str

Property	Type	Description
`step_name`	`str`	Name of the step

Text Message Events

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

TextMessageStartEvent

from ag_ui.core import TextMessageStartEvent Signals the start of a text message.

class TextMessageStartEvent(BaseEvent):
    type: Literal[EventType.TEXT_MESSAGE_START]
    message_id: str
    role: Literal["assistant"]

Property	Type	Description
`message_id`	`str`	Unique identifier for the message
`role`	`Literal["assistant"]`	Role is always “assistant”

TextMessageContentEvent

from ag_ui.core import TextMessageContentEvent Represents a chunk of content in a streaming text message.

class TextMessageContentEvent(BaseEvent):
    type: Literal[EventType.TEXT_MESSAGE_CONTENT]
    message_id: str
    delta: str  # Non-empty string

    def model_post_init(self, __context):
        if len(self.delta) == 0:
            raise ValueError("Delta must not be an empty string")

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

TextMessageEndEvent

from ag_ui.core import TextMessageEndEvent Signals the end of a text message.

class TextMessageEndEvent(BaseEvent):
    type: Literal[EventType.TEXT_MESSAGE_END]
    message_id: str

Property	Type	Description
`message_id`	`str`	Matches the ID from TextMessageStartEvent

Tool Call Events

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

ToolCallStartEvent

from ag_ui.core import ToolCallStartEvent Signals the start of a tool call.

class ToolCallStartEvent(BaseEvent):
    type: Literal[EventType.TOOL_CALL_START]
    tool_call_id: str
    tool_call_name: str
    parent_message_id: Optional[str] = None

Property	Type	Description
`tool_call_id`	`str`	Unique identifier for the tool call
`tool_call_name`	`str`	Name of the tool being called
`parent_message_id`	`Optional[str]`	ID of the parent message

ToolCallArgsEvent

from ag_ui.core import ToolCallArgsEvent Represents a chunk of argument data for a tool call.

class ToolCallArgsEvent(BaseEvent):
    type: Literal[EventType.TOOL_CALL_ARGS]
    tool_call_id: str
    delta: str

Property	Type	Description
`tool_call_id`	`str`	Matches the ID from ToolCallStartEvent
`delta`	`str`	Argument data chunk

ToolCallEndEvent

from ag_ui.core import ToolCallEndEvent Signals the end of a tool call.

class ToolCallEndEvent(BaseEvent):
    type: Literal[EventType.TOOL_CALL_END]
    tool_call_id: str

Property	Type	Description
`tool_call_id`	`str`	Matches the ID from ToolCallStartEvent

ToolCallResultEvent

from ag_ui.core import ToolCallResultEvent Provides the result of a tool call execution.

class ToolCallResultEvent(BaseEvent):
    message_id: str
    type: Literal[EventType.TOOL_CALL_RESULT]
    tool_call_id: str
    content: str
    role: Optional[Literal["tool"]] = None

Property	Type	Description
`message_id`	`str`	ID of the conversation message this result belongs to
`tool_call_id`	`str`	Matches the ID from the corresponding ToolCallStartEvent
`content`	`str`	The actual result/output content from the tool execution
`role`	`Optional[Literal["tool"]]`	Optional role identifier, typically “tool” for tool results

State Management Events

These events are used to manage agent state.

StateSnapshotEvent

from ag_ui.core import StateSnapshotEvent Provides a complete snapshot of an agent’s state.

class StateSnapshotEvent(BaseEvent):
    type: Literal[EventType.STATE_SNAPSHOT]
    snapshot: State

Property	Type	Description
`snapshot`	`State`	Complete state snapshot

StateDeltaEvent

from ag_ui.core import StateDeltaEvent Provides a partial update to an agent’s state using JSON Patch.

class StateDeltaEvent(BaseEvent):
    type: Literal[EventType.STATE_DELTA]
    delta: List[Any]  # JSON Patch (RFC 6902)

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

MessagesSnapshotEvent

from ag_ui.core import MessagesSnapshotEvent Provides a snapshot of all messages in a conversation.

class MessagesSnapshotEvent(BaseEvent):
    type: Literal[EventType.MESSAGES_SNAPSHOT]
    messages: List[Message]

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

ActivitySnapshotEvent

from ag_ui.core import ActivitySnapshotEvent Delivers a complete snapshot of an activity message.

class ActivitySnapshotEvent(BaseEvent):
    type: Literal[EventType.ACTIVITY_SNAPSHOT]
    message_id: str
    activity_type: str
    content: Any
    replace: bool = True

Property	Type	Description
`message_id`	`str`	Identifier for the target `ActivityMessage`
`activity_type`	`str`	Activity discriminator such as `"PLAN"` or `"SEARCH"`
`content`	`Any`	Structured payload describing the full activity state
`replace`	`bool` (default `True`)	When `False`, the snapshot is ignored if a message with the same ID already exists

ActivityDeltaEvent

from ag_ui.core import ActivityDeltaEvent Provides incremental updates to an activity snapshot using JSON Patch.

class ActivityDeltaEvent(BaseEvent):
    type: Literal[EventType.ACTIVITY_DELTA]
    message_id: str
    activity_type: str
    patch: List[Any]

Property	Type	Description
`message_id`	`str`	Identifier for the target `ActivityMessage`
`activity_type`	`str`	Activity discriminator mirroring the most recent snapshot
`patch`	`List[Any]`	JSON Patch operations applied to the structured activity content

Special Events

RawEvent

from ag_ui.core import RawEvent Used to pass through events from external systems.

class RawEvent(BaseEvent):
    type: Literal[EventType.RAW]
    event: Any
    source: Optional[str] = None

Property	Type	Description
`event`	`Any`	Original event data
`source`	`Optional[str]`	Source of the event

CustomEvent

from ag_ui.core import CustomEvent Used for application-specific custom events.

class CustomEvent(BaseEvent):
    type: Literal[EventType.CUSTOM]
    name: str
    value: Any

Property	Type	Description
`name`	`str`	Name of the custom event
`value`	`Any`	Value associated with the event

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

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

class ReasoningStartEvent(BaseEvent):
    type: Literal[EventType.REASONING_START]
    message_id: str

Property	Type	Description
`message_id`	`str`	Identifier for the reasoning phase

ReasoningMessageStartEvent

from ag_ui.core import ReasoningMessageStartEvent Signals the start of a reasoning message. Creates a new ReasoningMessage in the message history.

class ReasoningMessageStartEvent(BaseEvent):
    type: Literal[EventType.REASONING_MESSAGE_START]
    message_id: str
    role: Literal["assistant"]

Property	Type	Description
`message_id`	`str`	Unique identifier for the message
`role`	`Literal["assistant"]`	Role is always “assistant”

ReasoningMessageContentEvent

from ag_ui.core import ReasoningMessageContentEvent Represents a chunk of content in a streaming reasoning message.

class ReasoningMessageContentEvent(BaseEvent):
    type: Literal[EventType.REASONING_MESSAGE_CONTENT]
    message_id: str
    delta: str

Property	Type	Description
`message_id`	`str`	Matches the ID from ReasoningMessageStartEvent
`delta`	`str`	Reasoning content chunk

ReasoningMessageEndEvent

from ag_ui.core import ReasoningMessageEndEvent Signals the end of a reasoning message.

class ReasoningMessageEndEvent(BaseEvent):
    type: Literal[EventType.REASONING_MESSAGE_END]
    message_id: str

Property	Type	Description
`message_id`	`str`	Matches the ID from ReasoningMessageStartEvent

ReasoningMessageChunkEvent

from ag_ui.core import ReasoningMessageChunkEvent Convenience event for complete reasoning messages without manually emitting ReasoningMessageStart/ReasoningMessageEnd.

class ReasoningMessageChunkEvent(BaseEvent):
    type: Literal[EventType.REASONING_MESSAGE_CHUNK]
    message_id: Optional[str] = None  # required on first chunk for a message
    delta: Optional[str] = None

Behavior

Convenience: Some consumers (e.g., the JS/TS client) expand chunk events into the standard start/content/end sequence automatically.
First chunk requirements: The first chunk for a given message must include message_id.
Streaming: Subsequent chunks with the same message_id correspond to content pieces; completion triggers an implied end in clients that perform expansion.

ReasoningEndEvent

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

class ReasoningEndEvent(BaseEvent):
    type: Literal[EventType.REASONING_END]
    message_id: str

Property	Type	Description
`message_id`	`str`	Identifier for the reasoning phase

ReasoningEncryptedValueEvent

from ag_ui.core import ReasoningEncryptedValueEvent Attaches an encrypted value to a message or tool call. When this event is emitted, it finds the referenced entity by entity_id and sets its encrypted_value field.

ReasoningEncryptedValueSubtype = Literal["tool-call", "message"]

class ReasoningEncryptedValueEvent(BaseEvent):
    type: Literal[EventType.REASONING_ENCRYPTED_VALUE]
    subtype: ReasoningEncryptedValueSubtype
    entity_id: str
    encrypted_value: str

Property	Type	Description
`subtype`	`ReasoningEncryptedValueSubtype`	The type of entity this value belongs to
`entity_id`	`str`	ID of the tool call or message to attach the value
`encrypted_value`	`str`	The encrypted value to attach to the entity

Deprecated Events

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

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 for detailed migration guidance.

Event Discrimination

from ag_ui.core import Event The SDK uses Pydantic’s discriminated unions for event validation:

Event = Annotated[
    Union[
        TextMessageStartEvent,
        TextMessageContentEvent,
        TextMessageEndEvent,
        ToolCallStartEvent,
        ToolCallArgsEvent,
        ToolCallEndEvent,
        ToolCallResultEvent,
        StateSnapshotEvent,
        StateDeltaEvent,
        MessagesSnapshotEvent,
        ActivitySnapshotEvent,
        ActivityDeltaEvent,
        RawEvent,
        CustomEvent,
        RunStartedEvent,
        RunFinishedEvent,
        RunErrorEvent,
        StepStartedEvent,
        StepFinishedEvent,
        ReasoningStartEvent,
        ReasoningMessageStartEvent,
        ReasoningMessageContentEvent,
        ReasoningMessageEndEvent,
        ReasoningMessageChunkEvent,
        ReasoningEndEvent,
        ReasoningEncryptedValueEvent,
    ],
    Field(discriminator="type")
]

This allows for runtime validation of events and type checking at development time.

TextMessageChunkEvent

Convenience event for complete text messages without manually emitting TextMessageStart/TextMessageEnd.

from ag_ui.core import TextMessageChunkEvent

class TextMessageChunkEvent(BaseEvent):
    type: Literal[EventType.TEXT_MESSAGE_CHUNK]
    message_id: Optional[str] = None  # required on first chunk for a message
    role: Optional[TextMessageRole] = None  # defaults to "assistant" in JS client
    delta: Optional[str] = None

Behavior

Convenience: Some consumers (e.g., the JS/TS client) expand chunk events into the standard start/content/end sequence automatically, allowing producers to omit explicit start/end events when using chunks.
First chunk requirements: The first chunk for a given message must include message_id.
Streaming: Subsequent chunks with the same message_id correspond to content pieces; completion triggers an implied end in clients that perform expansion.

ToolCallChunkEvent

Convenience event for tool calls without manually emitting ToolCallStart/ToolCallEnd.

from ag_ui.core import ToolCallChunkEvent

class ToolCallChunkEvent(BaseEvent):
    type: Literal[EventType.TOOL_CALL_CHUNK]
    tool_call_id: Optional[str] = None      # required on first chunk
    tool_call_name: Optional[str] = None    # required on first chunk
    parent_message_id: Optional[str] = None
    delta: Optional[str] = None

Behavior

Convenience: Consumers may expand chunk sequences into the standard start/args/end triad (the JS/TS client does this automatically).
First chunk requirements: Include both tool_call_id and tool_call_name on the first chunk.
Streaming: Subsequent chunks with the same tool_call_id correspond to args pieces; completion triggers an implied end in clients that perform expansion.

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

​ToolCallResultEvent

​State Management Events

​StateSnapshotEvent

​StateDeltaEvent

​MessagesSnapshotEvent

​ActivitySnapshotEvent

​ActivityDeltaEvent

​Special Events

​RawEvent

​CustomEvent

​Reasoning Events

​ReasoningStartEvent

​ReasoningMessageStartEvent

​ReasoningMessageContentEvent

​ReasoningMessageEndEvent

​ReasoningMessageChunkEvent

​ReasoningEndEvent

​ReasoningEncryptedValueEvent

​Deprecated Events

​Thinking Events (Deprecated)

​Event Discrimination

​TextMessageChunkEvent

​ToolCallChunkEvent

Events

EventType Enum

BaseEvent

Lifecycle Events

RunStartedEvent

RunFinishedEvent

RunErrorEvent

StepStartedEvent

StepFinishedEvent

Text Message Events

TextMessageStartEvent

TextMessageContentEvent

TextMessageEndEvent

Tool Call Events

ToolCallStartEvent

ToolCallArgsEvent

ToolCallEndEvent

ToolCallResultEvent

State Management Events

StateSnapshotEvent

StateDeltaEvent

MessagesSnapshotEvent

ActivitySnapshotEvent

ActivityDeltaEvent

Special Events

RawEvent

CustomEvent

Reasoning Events

ReasoningStartEvent

ReasoningMessageStartEvent

ReasoningMessageContentEvent

ReasoningMessageEndEvent

ReasoningMessageChunkEvent

ReasoningEndEvent

ReasoningEncryptedValueEvent

Deprecated Events

Thinking Events (Deprecated)

Event Discrimination

TextMessageChunkEvent

ToolCallChunkEvent