Messages and Content Blocks

Understanding message types and content blocks in the Claude Agent SDK

The Claude Agent SDK uses a structured message system to represent conversations. Messages contain content blocks that represent different types of information.

Message Types

All messages are part of the Message union type:

Message = UserMessage | AssistantMessage | SystemMessage | ResultMessage | StreamEvent

UserMessage

Represents a message from the user to Claude.

@dataclass
class UserMessage:
    """User message."""
    
    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

The message content. Can be a simple string or a list of content blocks.

uuid · str | None

Unique identifier for the message. Only present when extra_args={"replay-user-messages": None} is set in options.

parent_tool_use_id · str | None

ID of the parent tool use if this is a follow-up message within a tool execution context.

tool_use_result · dict[str, Any] | None

Result data if this message is responding to a tool use.

Example

from claude_agent_sdk import ClaudeSDKClient

async with ClaudeSDKClient() as client:
    await client.query("What is the capital of France?")
    
    async for msg in client.receive_messages():
        if isinstance(msg, UserMessage):
            print(f"User said: {msg.content}")
            if msg.uuid:
                print(f"Message ID: {msg.uuid}")

AssistantMessage

Represents a message from Claude (the assistant).

@dataclass
class AssistantMessage:
    """Assistant message with content blocks."""
    
    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.).

model · str · required

The model used to generate this message (e.g., “claude-sonnet-4-5”).

parent_tool_use_id · str | None

ID of the parent tool use if this message is part of a tool execution.

error · AssistantMessageError | None

Error type if the message generation failed. Possible values:

• "authentication_failed"
• "billing_error"
• "rate_limit"
• "invalid_request"
• "server_error"
• "unknown"

Example

async with ClaudeSDKClient() as client:
    await client.query("Explain async/await")
    
    async for msg in client.receive_response():
        if isinstance(msg, AssistantMessage):
            print(f"Model: {msg.model}")
            for block in msg.content:
                if isinstance(block, TextBlock):
                    print(f"Text: {block.text}")
                elif isinstance(block, ToolUseBlock):
                    print(f"Using tool: {block.name}")

SystemMessage

Represents system-level messages with metadata about the conversation.

@dataclass
class SystemMessage:
    """System message with metadata."""
    
    subtype: str
    data: dict[str, Any]

subtype · str · required

The type of system message. Common subtypes:

• "task_started" - A task has started
• "task_progress" - Task progress update
• "task_notification" - Task completion/failure/stopped
• "mcp_status" - MCP server status
• "tool_execution" - Tool execution details

data · dict[str, Any] · required

Additional data specific to the message subtype.

Specialized System Messages

The SDK provides typed subclasses for common system message subtypes:

TaskStartedMessage

@dataclass
class TaskStartedMessage(SystemMessage):
    """System message emitted when a task starts."""
    
    task_id: str
    description: str
    uuid: str
    session_id: str
    tool_use_id: str | None = None
    task_type: str | None = None

TaskProgressMessage

@dataclass
class TaskProgressMessage(SystemMessage):
    """System message emitted while a task is in progress."""
    
    task_id: str
    description: str
    usage: TaskUsage
    uuid: str
    session_id: str
    tool_use_id: str | None = None
    last_tool_name: str | None = None

TaskNotificationMessage

@dataclass
class TaskNotificationMessage(SystemMessage):
    """System message emitted when a task completes, fails, or is stopped."""
    
    task_id: str
    status: TaskNotificationStatus  # "completed" | "failed" | "stopped"
    output_file: str
    summary: str
    uuid: str
    session_id: str
    tool_use_id: str | None = None
    usage: TaskUsage | None = None

Example

async with ClaudeSDKClient() as client:
    await client.query("Start a long-running task")
    
    async for msg in client.receive_messages():
        if isinstance(msg, TaskStartedMessage):
            print(f"Task started: {msg.description}")
        elif isinstance(msg, TaskProgressMessage):
            print(f"Progress: {msg.usage['total_tokens']} tokens used")
        elif isinstance(msg, TaskNotificationMessage):
            print(f"Task {msg.status}: {msg.summary}")

ResultMessage

Represents the final result of a conversation with cost and usage information.

@dataclass
class ResultMessage:
    """Result message with cost and usage information."""
    
    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

duration_ms · int · required

Total duration in milliseconds.

duration_api_ms · int · required

API call duration in milliseconds.

is_error · bool · required

Whether the conversation ended with an error.

num_turns · int · required

Number of conversation turns.

session_id · str · required

Session identifier.

stop_reason · str | None

Reason the conversation stopped (e.g., “end_turn”, “max_tokens”).

total_cost_usd · float | None

Total cost in USD.

usage · dict[str, Any] | None

Token usage statistics.

structured_output · Any

Structured output if output_format was specified in options.

Example

async with ClaudeSDKClient() as client:
    await client.query("What is 2+2?")
    
    async for msg in client.receive_response():
        if isinstance(msg, ResultMessage):
            print(f"Cost: ${msg.total_cost_usd:.4f}")
            print(f"Duration: {msg.duration_ms}ms")
            print(f"Turns: {msg.num_turns}")
            print(f"Tokens: {msg.usage}")

StreamEvent

Represents partial message updates during streaming (when include_partial_messages=True).

@dataclass
class StreamEvent:
    """Stream event for partial message updates during streaming."""
    
    uuid: str
    session_id: str
    event: dict[str, Any]  # The raw Anthropic API stream event
    parent_tool_use_id: str | None = None

event · dict[str, Any] · required

Raw Anthropic API stream event (e.g., content_block_start, content_block_delta, etc.).

Content Blocks

Content blocks represent different types of content within messages.

ContentBlock = TextBlock | ThinkingBlock | ToolUseBlock | ToolResultBlock

TextBlock

Represents text content from Claude.

@dataclass
class TextBlock:
    """Text content block."""
    
    text: str

text · str · required

The text content.

Example

for block in assistant_message.content:
    if isinstance(block, TextBlock):
        print(block.text)

ThinkingBlock

Represents Claude’s extended thinking process (when thinking is enabled).

@dataclass
class ThinkingBlock:
    """Thinking content block."""
    
    thinking: str
    signature: str

thinking · str · required

Claude’s internal reasoning and thought process.

signature · str · required

Signature for the thinking block.

Example

options = ClaudeAgentOptions(
    thinking={"type": "adaptive"}
)

async with ClaudeSDKClient(options) as client:
    await client.query("Solve this complex problem")
    
    async for msg in client.receive_response():
        if isinstance(msg, AssistantMessage):
            for block in msg.content:
                if isinstance(block, ThinkingBlock):
                    print(f"Claude's thinking: {block.thinking}")

ToolUseBlock

Represents Claude’s intent to use a tool.

@dataclass
class ToolUseBlock:
    """Tool use content block."""
    
    id: str
    name: str
    input: dict[str, Any]

id · str · required

Unique identifier for this tool use.

name · str · required

Name of the tool being used (e.g., “Bash”, “Read”, “Write”).

input · dict[str, Any] · required

Input parameters for the tool.

Example

async for msg in client.receive_messages():
    if isinstance(msg, AssistantMessage):
        for block in msg.content:
            if isinstance(block, ToolUseBlock):
                print(f"Using tool: {block.name}")
                print(f"Tool ID: {block.id}")
                print(f"Input: {block.input}")

ToolResultBlock

Represents the result of a tool execution.

@dataclass
class ToolResultBlock:
    """Tool result content block."""
    
    tool_use_id: str
    content: str | list[dict[str, Any]] | None = None
    is_error: bool | None = None

tool_use_id · str · required

ID of the tool use this result corresponds to.

content · str | list[dict[str, Any]] | None

The result content from the tool execution.

is_error · bool | None

Whether the tool execution resulted in an error.

Example

async for msg in client.receive_messages():
    if isinstance(msg, AssistantMessage):
        for block in msg.content:
            if isinstance(block, ToolResultBlock):
                print(f"Tool result for {block.tool_use_id}")
                if block.is_error:
                    print(f"Error: {block.content}")
                else:
                    print(f"Success: {block.content}")

Complete Example

from claude_agent_sdk import (
    ClaudeSDKClient,
    ClaudeAgentOptions,
    AssistantMessage,
    UserMessage,
    SystemMessage,
    ResultMessage,
    TextBlock,
    ThinkingBlock,
    ToolUseBlock,
    ToolResultBlock,
)

async def process_conversation():
    options = ClaudeAgentOptions(
        thinking={"type": "adaptive"},
        extra_args={"replay-user-messages": None}
    )
    
    async with ClaudeSDKClient(options) as client:
        await client.query("Help me analyze this codebase")
        
        async for msg in client.receive_messages():
            # Handle user messages
            if isinstance(msg, UserMessage):
                print(f"User [{msg.uuid}]: {msg.content}")
            
            # Handle assistant messages
            elif isinstance(msg, AssistantMessage):
                print(f"Assistant (model: {msg.model}):")
                for block in msg.content:
                    if isinstance(block, TextBlock):
                        print(f"  Text: {block.text}")
                    elif isinstance(block, ThinkingBlock):
                        print(f"  Thinking: {block.thinking[:100]}...")
                    elif isinstance(block, ToolUseBlock):
                        print(f"  Using tool: {block.name}")
                        print(f"    Input: {block.input}")
                    elif isinstance(block, ToolResultBlock):
                        status = "error" if block.is_error else "success"
                        print(f"  Tool result ({status}): {block.content}")
            
            # Handle system messages
            elif isinstance(msg, SystemMessage):
                print(f"System [{msg.subtype}]: {msg.data}")
            
            # Handle result
            elif isinstance(msg, ResultMessage):
                print(f"\nConversation complete!")
                print(f"  Duration: {msg.duration_ms}ms")
                print(f"  Turns: {msg.num_turns}")
                print(f"  Cost: ${msg.total_cost_usd:.4f}")
                print(f"  Stop reason: {msg.stop_reason}")
                break

Type Checking

Use isinstance() to check message and content block types:

if isinstance(msg, AssistantMessage):
    for block in msg.content:
        if isinstance(block, TextBlock):
            # Handle text
            pass
        elif isinstance(block, ToolUseBlock):
            # Handle tool use
            pass

receive_response() is a convenience method that automatically stops after receiving a ResultMessage. Use receive_messages() for full control over message processing.