Skip to main content

​
Overview

The Claude Agent SDK uses strongly-typed message classes to represent conversation turns, system events, and streaming updates. 
Message = UserMessage | AssistantMessage | SystemMessage | ResultMessage | StreamEvent

​
UserMessage

Represents a message from the user. 
@dataclass
class UserMessage:
    content: str | list[ContentBlock]
    uuid: str | None = None
    parent_tool_use_id: str | None = None
    tool_use_result: dict[str, Any] | None = None
​
content
str | list[ContentBlock]
required
Message content as a string or list of content blocks.
​
uuid
str | None
Unique message identifier.
​
parent_tool_use_id
str | None
ID of the parent tool use if this is a tool result message.
​
tool_use_result
dict[str, Any] | None
Tool result data if this message contains tool output.

​
Example

message = UserMessage(
    content="What files are in the current directory?",
    uuid="msg-123"
)

await client.send_message(message)

​
AssistantMessage

Represents a response from Claude. 
@dataclass
class AssistantMessage:
    content: list[ContentBlock]
    model: str
    parent_tool_use_id: str | None = None
    error: AssistantMessageError | None = None
​
content
list[ContentBlock]
required
List of content blocks (text, thinking, tool use, etc.).See Content Blocks for details.
​
model
str
required
Model that generated the response (e.g., "claude-sonnet-4-20250514").
​
parent_tool_use_id
str | None
ID of the parent tool use if this is from a sub-agent.
​
error
AssistantMessageError | None
Error type if the message failed.
  • "authentication_failed"
  • "billing_error"
  • "rate_limit"
  • "invalid_request"
  • "server_error"
  • "unknown"

​
Example

async for message in client:
    if isinstance(message, AssistantMessage):
        for block in message.content:
            if isinstance(block, TextBlock):
                print(block.text)
            elif isinstance(block, ToolUseBlock):
                print(f"Using tool: {block.name}")

​
SystemMessage

Represents system-level events and metadata. 
@dataclass
class SystemMessage:
    subtype: str
    data: dict[str, Any]
​
subtype
str
required
System message type (e.g., "task_started", "task_progress", "task_notification").
​
data
dict[str, Any]
required
Message-specific data payload.

​
Task Messages

The SDK provides specialized subclasses for task-related system messages:

​
Example

async for message in client:
    if isinstance(message, TaskStartedMessage):
        print(f"Task started: {message.description}")
    elif isinstance(message, TaskProgressMessage):
        print(f"Progress: {message.usage['total_tokens']} tokens used")
    elif isinstance(message, TaskNotificationMessage):
        print(f"Task {message.status}: {message.summary}")

​
ResultMessage

Final result message with cost and usage information. 
@dataclass
class ResultMessage:
    subtype: str
    duration_ms: int
    duration_api_ms: int
    is_error: bool
    num_turns: int
    session_id: str
    stop_reason: str | None = None
    total_cost_usd: float | None = None
    usage: dict[str, Any] | None = None
    result: str | None = None
    structured_output: Any = None
​
subtype
str
required
Result type (typically "result").
​
duration_ms
int
required
Total session duration in milliseconds.
​
duration_api_ms
int
required
API call duration in milliseconds.
​
is_error
bool
required
Whether the session ended with an error.
​
num_turns
int
required
Number of conversation turns.
​
session_id
str
required
Session identifier.
​
stop_reason
str | None
Reason for stopping (e.g., "max_turns", "end_turn").
​
total_cost_usd
float | None
Total cost in USD.
​
usage
dict[str, Any] | None
Token usage statistics.
​
result
str | None
Final result text.
​
structured_output
Any
Structured output if output_format was specified in options.

​
Example

result = await query("Analyze this codebase", options)

if isinstance(result, ResultMessage):
    print(f"Completed in {result.duration_ms}ms")
    print(f"Used {result.num_turns} turns")
    print(f"Cost: ${result.total_cost_usd:.4f}")
    print(f"Result: {result.result}")

​
StreamEvent

Partial message update during streaming (requires include_partial_messages=True). 
@dataclass
class StreamEvent:
    uuid: str
    session_id: str
    event: dict[str, Any]  # Raw Anthropic API stream event
    parent_tool_use_id: str | None = None
​
uuid
str
required
Message UUID being updated.
​
session_id
str
required
Session identifier.
​
event
dict[str, Any]
required
Raw Anthropic API stream event data.
​
parent_tool_use_id
str | None
Parent tool use ID if from a sub-agent.

​
Example

options = ClaudeAgentOptions(include_partial_messages=True)
client = ClaudeSDKClient(options)

async for message in client:
    if isinstance(message, StreamEvent):
        # Handle partial updates
        if message.event.get("type") == "content_block_delta":
            delta = message.event.get("delta", {})
            if "text" in delta:
                print(delta["text"], end="", flush=True)

​
Session History Types

Types for reading historical session data:

​
SDKSessionInfo

@dataclass
class SDKSessionInfo:
    session_id: str
    summary: str
    last_modified: int
    file_size: int
    custom_title: str | None = None
    first_prompt: str | None = None
    git_branch: str | None = None
    cwd: str | None = None
Returned by list_sessions() with session metadata.

​
SessionMessage

@dataclass
class SessionMessage:
    type: Literal["user", "assistant"]
    uuid: str
    session_id: str
    message: Any  # Raw Anthropic API message dict
    parent_tool_use_id: None = None
Returned by get_session_messages() for reading conversation history.

Build docs developers (and LLMs) love

Get started for freeTalk to us