Message & Event - AgentScope

Message and Event are the two fundamental data structures in AgentScope.

Message — the unit of inter-agent communication and persistence. Each Msg represents a complete conversation turn that is stored in context and exchanged between agents.
Event — the unit of frontend interaction and streaming. Events carry incremental progress updates (text tokens, tool call fragments, permission requests) and drive real-time UIs and human-in-the-loop workflows.

A sequence of events produced during a single reply call accumulates into exactly one assistant Msg. This guarantees that the complete message state is always recoverable from its event stream.

Message

A Msg represents a single turn in a conversation — a user input, an assistant response, or a system instruction, carrying structured content as a list of typed blocks.

One assistant message corresponds to one complete reply cycle of the agent (repeated reasoning and acting until a final response is produced).

Structure

The Msg class has the following core fields:

Field	Type	Description
`id`	`str`	Unique message identifier
`name`	`str`	Name of the sender
`role`	`"user" \| "assistant" \| "system"`	The sender’s role
`content`	`list[ContentBlock]`	Ordered list of content blocks
`metadata`	`dict`	Arbitrary key-value metadata
`created_at`	`str`	ISO 8601 timestamp of creation
`finished_at`	`str \| None`	ISO 8601 timestamp when the message was finalized
`usage`	`Usage`	Token usage statistics (for assistant messages)

Content Blocks

Message content is composed of typed blocks. Each block represents a distinct piece of information:

Block	Description	Allowed In
`TextBlock`	Plain text content	user, assistant, system
`DataBlock`	Binary data (images, audio) via base64 or URL	user, assistant
`ThinkingBlock`	Model reasoning (chain-of-thought)	assistant
`ToolCallBlock`	A tool invocation with name, input, and state	assistant
`ToolResultBlock`	The output of a tool execution	assistant
`HintBlock`	Instructions injected into the loop as user context	assistant

Role constraints are enforced at construction: user messages can only contain TextBlock and DataBlock; system messages can only contain TextBlock; assistant messages can contain all block types.

Create Messages

AgentScope provides three shortcut factory functions for quickly creating messages with the correct role, without manually specifying role or wrapping content into blocks:

Factory	Role	Allowed Content
`UserMsg(name, content)`	`user`	`str` or `list[TextBlock \| DataBlock]`
`AssistantMsg(name, content)`	`assistant`	`str` or `list[ContentBlock]`
`SystemMsg(name, content)`	`system`	`str` or `list[TextBlock]`

When content is a plain string, it is automatically wrapped into a TextBlock.

from agentscope.message import UserMsg, AssistantMsg, SystemMsg

# User message — text and optional images
user_msg = UserMsg(name="user", content="What's in this image?")

# User message with multimodal content
from agentscope.message import TextBlock, DataBlock, Base64Source
user_msg = UserMsg(
    name="user",
    content=[
        TextBlock(text="Describe this image:"),
        DataBlock(source=Base64Source(data="...", media_type="image/png")),
    ],
)

# System message — text only
system_msg = SystemMsg(name="system", content="You are a helpful assistant.")

# Assistant message — all block types allowed
assistant_msg = AssistantMsg(name="agent", content="Here is the result...")

Access Content

Msg provides helper methods to extract specific block types:

Method	Returns
`get_text_content(separator="\n")`	Concatenated text from all `TextBlock`s, or `None`
`get_content_blocks(block_type)`	Filtered list of blocks by type
`has_content_blocks(block_type)`	`True` if blocks of the given type exist

# Get all text content
text = msg.get_text_content()

# Get all tool calls
tool_calls = msg.get_content_blocks("tool_call")

# Check if message has tool results
if msg.has_content_blocks("tool_result"):
    ...

Event

Events are the streaming counterpart of messages. While the agent executes, it yields a sequence of AgentEvent objects that represent incremental progress — text tokens arriving, tool calls being constructed, results streaming back. Each event is lightweight and self-contained.

Event Lifecycle

Every event carries a reply_id that links it to the message being constructed. Within a reply, block_id or tool_call_id identifies which content block an event belongs to. Events follow a start → delta → end pattern for each content block: All events within the same reply share the same reply_id. Within a reply, use block_id to correlate text/thinking/data block events, and tool_call_id to correlate tool call and tool result events.

Event Types

All events inherit from EventBase which provides common fields:

Field	Type	Description
`id`	`str`	Unique event identifier
`created_at`	`str`	ISO 8601 timestamp

Events are grouped by category below. Every event also carries a reply_id field (except where noted) that links it to the message being constructed.

Lifecycle Events

ReplyStartEvent — Agent begins a new reply.

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`session_id`	`str`	ID of the session
`name`	`str`	Agent name
`role`	`str`	Agent role (default `"assistant"`)

ReplyEndEvent — Agent finishes the reply.

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`session_id`	`str`	ID of the session

ExceedMaxItersEvent — Agent reached the maximum reasoning-acting iterations.

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`name`	`str`	Agent name

Text Streaming Events

TextBlockStartEvent — A new text block begins.

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`block_id`	`str`	Unique identifier of the text block

TextBlockDeltaEvent — Incremental text content arrives.

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`block_id`	`str`	Unique identifier of the text block
`delta`	`str`	Incremental text content

TextBlockEndEvent — The text block is complete.

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`block_id`	`str`	Unique identifier of the text block

Thinking Streaming Events

ThinkingBlockStartEvent — A new thinking block begins.

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`block_id`	`str`	Unique identifier of the thinking block

ThinkingBlockDeltaEvent — Incremental thinking content arrives.

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`block_id`	`str`	Unique identifier of the thinking block
`delta`	`str`	Incremental thinking text

ThinkingBlockEndEvent — The thinking block is complete.

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`block_id`	`str`	Unique identifier of the thinking block

Data Streaming Events

DataBlockStartEvent — A new data block begins (image, audio, etc.).

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`block_id`	`str`	Unique identifier of the data block
`media_type`	`str`	MIME type (e.g. `"image/png"`)

DataBlockDeltaEvent — Incremental binary data arrives.

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`block_id`	`str`	Unique identifier of the data block
`data`	`str`	Incremental base64-encoded data
`media_type`	`str`	MIME type

DataBlockEndEvent — The data block is complete.

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`block_id`	`str`	Unique identifier of the data block

Tool Call Streaming Events

ToolCallStartEvent — The agent begins a tool call.

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`tool_call_id`	`str`	Unique identifier of the tool call
`tool_call_name`	`str`	Name of the tool being called

ToolCallDeltaEvent — Incremental tool call input arrives.

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`tool_call_id`	`str`	Unique identifier of the tool call
`delta`	`str`	Incremental JSON fragment of tool input

ToolCallEndEvent — The tool call input is complete.

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`tool_call_id`	`str`	Unique identifier of the tool call

Tool Result Streaming Events

ToolResultStartEvent — Tool execution begins.

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`tool_call_id`	`str`	ID of the corresponding tool call
`tool_call_name`	`str`	Name of the tool

ToolResultTextDeltaEvent — Incremental text output from the tool.

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`tool_call_id`	`str`	ID of the corresponding tool call
`delta`	`str`	Incremental text content

ToolResultDataDeltaEvent — Binary data output from the tool.

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`tool_call_id`	`str`	ID of the corresponding tool call
`block_id`	`str`	Unique identifier of the data block
`media_type`	`str`	MIME type of the content
`data`	`str \| None`	Base64-encoded data (mutually exclusive with `url`)
`url`	`str \| None`	URL pointing to the content (mutually exclusive with `data`)

ToolResultEndEvent — Tool execution is complete.

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`tool_call_id`	`str`	ID of the corresponding tool call
`state`	`ToolResultState`	Final state: `SUCCESS`, `ERROR`, `INTERRUPTED`, `DENIED`, or `RUNNING`

Model Call Events

ModelCallStartEvent — A model API call begins.

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`model_name`	`str`	Name of the model being called

ModelCallEndEvent — A model API call completes.

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`input_tokens`	`int`	Number of input tokens consumed
`output_tokens`	`int`	Number of output tokens generated

Human-in-the-Loop Events

RequireUserConfirmEvent — Agent pauses for user confirmation.

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`tool_calls`	`list[ToolCallBlock]`	Tool calls pending user confirmation

RequireExternalExecutionEvent — Agent pauses for external execution.

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`tool_calls`	`list[ToolCallBlock]`	Tool calls to be executed externally

UserConfirmResultEvent — User provides confirmation results (input event).

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`confirm_results`	`list[ConfirmResult]`	Confirmation results for each pending tool call

ExternalExecutionResultEvent — External system provides execution results (input event).

Field	Type	Description
`reply_id`	`str`	ID of the reply message
`execution_results`	`list[ToolResultBlock]`	Results returned by the external executor

Reconstruct Messages from Events

Events and messages are not independent — they are two views of the same data. Every event produced by reply_stream can be applied to a Msg via append_event(), reconstructing the complete message incrementally. This guarantees that the final message state is fully recoverable from the event stream alone.

from agentscope.message import Msg, AssistantMsg

msg = None

# Accumulate events into the message
async for event in agent.reply_stream(user_msg):
	if isinstance(event, ReplyStartEvent):
		# Create a new message when the reply starts
		msg = AssistantMsg(name=event.name, content=[], id=event.reply_id)

	else:
		# For all other events, append to the message to reconstruct its state
        msg.append_event(event)

The append_event method handles all event types:

Event Type	Effect on Msg
`ReplyEndEvent`	Sets `finished_at` timestamp
`TextBlockStartEvent`	Appends a new empty `TextBlock`
`TextBlockDeltaEvent`	Concatenates `delta` to the block’s text
`DataBlockStartEvent`	Appends a new empty `DataBlock`
`DataBlockDeltaEvent`	Concatenates `data` to the block’s base64 content
`ThinkingBlockStartEvent`	Appends a new empty `ThinkingBlock`
`ThinkingBlockDeltaEvent`	Concatenates `delta` to the block’s thinking text
`ToolCallStartEvent`	Appends a new `ToolCallBlock` with empty input
`ToolCallDeltaEvent`	Concatenates `delta` to the tool call’s input
`ToolResultStartEvent`	Appends a new `ToolResultBlock` with empty output
`ToolResultTextDeltaEvent`	Appends text to the tool result’s output
`ToolResultDataDeltaEvent`	Appends a binary data block to the tool result’s output
`ToolResultEndEvent`	Sets the tool result’s final `state`
`RequireUserConfirmEvent`	Updates tool call states to `ASKING`
`ExternalExecutionResultEvent`	Appends `ToolResultBlock`s to content

This design enables flexible deployment: a backend can stream events over WebSocket to a frontend, which reconstructs the message client-side. If the connection drops, replaying the event sequence from any checkpoint restores the exact message state.

TypeScript Support

A TypeScript version of the message and event primitives is available on npm, so frontends can reconstruct messages from the event stream with the same appendEvent API:

pnpm install @agentscope-ai/agentscope

import { AssistantMsg, ReplyStartEvent } from "@agentscope-ai/agentscope/message";

let msg: AssistantMsg | null = null;

for await (const event of stream) {
    if (event.type === "REPLY_START") {
        msg = new AssistantMsg({ name: event.name, content: [], id: event.reply_id });
    } else {
        msg?.appendEvent(event);
    }
}

Example: Streaming UI

A typical pattern for building a streaming interface:

from agentscope.message import AssistantMsg, UserMsg
from agentscope.event import (
    ReplyStartEvent,
    TextBlockDeltaEvent,
    ToolCallStartEvent,
    ToolResultEndEvent,
    ReplyEndEvent,
)

msg = None

async for event in agent.reply_stream(UserMsg("user", "Fix the bug")):
    if isinstance(event, ReplyStartEvent):
        msg = AssistantMsg(name=event.name, content=[], id=event.reply_id)

    elif isinstance(event, TextBlockDeltaEvent):
        print(event.delta, end="", flush=True)

    elif isinstance(event, ToolCallStartEvent):
        print(f"\n[Calling {event.tool_call_name}...]")

    elif isinstance(event, ToolResultEndEvent):
        print(f"[Tool finished: {event.state}]")

    elif isinstance(event, ReplyEndEvent):
        print("\n[Done]")

    # Always accumulate into the message
    if msg is not None:
        msg.append_event(event)

# msg now contains the complete reply

Agent

How the agent produces events and messages in the ReAct loop

Context

How messages are stored, compressed, and offloaded

Documentation Index

​Message

​Structure

​Content Blocks

​Create Messages

​Access Content

​Event

​Event Lifecycle

​Event Types

​Reconstruct Messages from Events

​TypeScript Support

​Example: Streaming UI

​Further Reading

Agent

Context

Message

Structure

Content Blocks

Create Messages

Access Content

Event

Event Lifecycle

Event Types

Reconstruct Messages from Events

TypeScript Support

Example: Streaming UI

Further Reading