Implementation:Anthropics Anthropic sdk python BetaRawContentBlockStartEvent

Knowledge Sources	Anthropic SDK Python
Domains	API_Types, Beta_Features
Last Updated	2026-02-15 12:00 GMT

Overview

BetaRawContentBlockStartEvent is a Pydantic model representing the streaming event emitted when a new content block begins during a beta API streaming response. It carries the initial state of the content block along with its positional index.

Description

BetaRawContentBlockStartEvent extends BaseModel and is emitted as a Server-Sent Event (SSE) during streaming responses from the beta Messages API. It signals the start of a new content block and provides the full initial block object.

The file also defines a local ContentBlock type alias, which is a discriminated union identical to BetaContentBlock, covering all 15 beta content block types. This union uses PropertyInfo(discriminator="type") for automatic deserialization based on the type field.

Key fields:

• content_block -- The full initial content block object (one of the 15 block types)
• index -- The zero-based position of this block within the message's content array
• type -- Always "content_block_start"

Usage

Use BetaRawContentBlockStartEvent when processing raw streaming events from the beta API. This event tells you that a new content block has begun and provides its initial state. You typically listen for this event to initialize tracking of a new block, then accumulate updates via subsequent delta events.

Code Reference

Source Location

• Repository: Anthropic SDK Python
• File: src/anthropic/types/beta/beta_raw_content_block_start_event.py

Signature

ContentBlock: TypeAlias = Annotated[
    Union[
        BetaTextBlock,
        BetaThinkingBlock,
        BetaRedactedThinkingBlock,
        BetaToolUseBlock,
        BetaServerToolUseBlock,
        BetaWebSearchToolResultBlock,
        BetaWebFetchToolResultBlock,
        BetaCodeExecutionToolResultBlock,
        BetaBashCodeExecutionToolResultBlock,
        BetaTextEditorCodeExecutionToolResultBlock,
        BetaToolSearchToolResultBlock,
        BetaMCPToolUseBlock,
        BetaMCPToolResultBlock,
        BetaContainerUploadBlock,
        BetaCompactionBlock,
    ],
    PropertyInfo(discriminator="type"),
]


class BetaRawContentBlockStartEvent(BaseModel):
    content_block: ContentBlock
    index: int
    type: Literal["content_block_start"]

Import

from anthropic.types.beta import BetaRawContentBlockStartEvent

I/O Contract

Field	Type	Required	Description
content_block	ContentBlock (discriminated union)	Yes	The initial content block object; uses the same 15-variant union as BetaContentBlock
index	int	Yes	Zero-based position of this block in the message content array
type	Literal["content_block_start"]	Yes	Always "content_block_start"

ContentBlock Variants

Variant Type	type Value
BetaTextBlock	"text"
BetaThinkingBlock	"thinking"
BetaRedactedThinkingBlock	"redacted_thinking"
BetaToolUseBlock	"tool_use"
BetaServerToolUseBlock	"server_tool_use"
BetaWebSearchToolResultBlock	"web_search_tool_result"
BetaWebFetchToolResultBlock	"web_fetch_tool_result"
BetaCodeExecutionToolResultBlock	"code_execution_tool_result"
BetaBashCodeExecutionToolResultBlock	"bash_code_execution_tool_result"
BetaTextEditorCodeExecutionToolResultBlock	"text_editor_code_execution_tool_result"
BetaToolSearchToolResultBlock	"tool_search_tool_result"
BetaMCPToolUseBlock	"mcp_tool_use"
BetaMCPToolResultBlock	"mcp_tool_result"
BetaContainerUploadBlock	"container_upload"
BetaCompactionBlock	"compaction"

Usage Examples

Processing Stream Events

import anthropic

client = anthropic.Anthropic()

with client.beta.messages.stream(
    model="claude-sonnet-4-20250514",
    max_tokens=1024,
    messages=[{"role": "user", "content": "Write a haiku."}],
    betas=["beta-feature-flag"],
) as stream:
    for event in stream:
        if event.type == "content_block_start":
            print(f"Block {event.index} started: {event.content_block.type}")
            if event.content_block.type == "text":
                print(f"  Initial text: '{event.content_block.text}'")

Type-Safe Block Handling

from anthropic.types.beta import BetaRawContentBlockStartEvent

def handle_block_start(event: BetaRawContentBlockStartEvent) -> None:
    block = event.content_block
    if block.type == "tool_use":
        print(f"Tool use started at index {event.index}: {block.name}")
    elif block.type == "thinking":
        print(f"Thinking block started at index {event.index}")
    elif block.type == "text":
        print(f"Text block started at index {event.index}")

Related Pages

• Anthropics_Anthropic_sdk_python_BetaContentBlock -- The response-level union type matching the local ContentBlock alias
• Anthropics_Anthropic_sdk_python_BetaRawMessageDeltaEvent -- Message-level delta events emitted during streaming
• Anthropics_Anthropic_sdk_python_BetaMessage -- The complete message model assembled from streaming events
• Anthropics_Anthropic_sdk_python_Stream_Event_Processing -- General stream event processing patterns