View source for Implementation:Anthropics Anthropic sdk python BetaRawContentBlockStartEvent

{{PageInfo|type=Implementation|title=Anthropics_Anthropic_sdk_python_BetaRawContentBlockStartEvent}}
{| class="wikitable" style="float:right; margin-left:1em; width:300px;"
|-
! Knowledge Sources
|
* [https://github.com/anthropics/anthropic-sdk-python Anthropic SDK Python]
|-
! Domains
| [[domain::API_Types]], [[domain::Beta_Features]]
|-
! Last Updated
| [[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 ===

<code>BetaRawContentBlockStartEvent</code> extends <code>BaseModel</code> 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 <code>ContentBlock</code> type alias, which is a discriminated union identical to <code>BetaContentBlock</code>, covering all 15 beta content block types. This union uses <code>PropertyInfo(discriminator="type")</code> for automatic deserialization based on the <code>type</code> 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 <code>"content_block_start"</code>

=== Usage ===

Use <code>BetaRawContentBlockStartEvent</code> 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:''' [https://github.com/anthropics/anthropic-sdk-python Anthropic SDK Python]
* '''File:''' <code>src/anthropic/types/beta/beta_raw_content_block_start_event.py</code>

=== Signature ===
<syntaxhighlight lang="python">
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"]
</syntaxhighlight>

=== Import ===
<syntaxhighlight lang="python">
from anthropic.types.beta import BetaRawContentBlockStartEvent
</syntaxhighlight>

== I/O Contract ==

{| class="wikitable"
|-
! Field !! Type !! Required !! Description
|-
| <code>content_block</code> || <code>ContentBlock</code> (discriminated union) || Yes || The initial content block object; uses the same 15-variant union as <code>BetaContentBlock</code>
|-
| <code>index</code> || <code>int</code> || Yes || Zero-based position of this block in the message content array
|-
| <code>type</code> || <code>Literal["content_block_start"]</code> || Yes || Always <code>"content_block_start"</code>
|}

=== ContentBlock Variants ===

{| class="wikitable"
|-
! Variant Type !! <code>type</code> Value
|-
| <code>BetaTextBlock</code> || <code>"text"</code>
|-
| <code>BetaThinkingBlock</code> || <code>"thinking"</code>
|-
| <code>BetaRedactedThinkingBlock</code> || <code>"redacted_thinking"</code>
|-
| <code>BetaToolUseBlock</code> || <code>"tool_use"</code>
|-
| <code>BetaServerToolUseBlock</code> || <code>"server_tool_use"</code>
|-
| <code>BetaWebSearchToolResultBlock</code> || <code>"web_search_tool_result"</code>
|-
| <code>BetaWebFetchToolResultBlock</code> || <code>"web_fetch_tool_result"</code>
|-
| <code>BetaCodeExecutionToolResultBlock</code> || <code>"code_execution_tool_result"</code>
|-
| <code>BetaBashCodeExecutionToolResultBlock</code> || <code>"bash_code_execution_tool_result"</code>
|-
| <code>BetaTextEditorCodeExecutionToolResultBlock</code> || <code>"text_editor_code_execution_tool_result"</code>
|-
| <code>BetaToolSearchToolResultBlock</code> || <code>"tool_search_tool_result"</code>
|-
| <code>BetaMCPToolUseBlock</code> || <code>"mcp_tool_use"</code>
|-
| <code>BetaMCPToolResultBlock</code> || <code>"mcp_tool_result"</code>
|-
| <code>BetaContainerUploadBlock</code> || <code>"container_upload"</code>
|-
| <code>BetaCompactionBlock</code> || <code>"compaction"</code>
|}

== Usage Examples ==

=== Processing Stream Events ===
<syntaxhighlight lang="python">
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}'")
</syntaxhighlight>

=== Type-Safe Block Handling ===
<syntaxhighlight lang="python">
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}")
</syntaxhighlight>

== Related Pages ==

* [[Anthropics_Anthropic_sdk_python_BetaContentBlock]] -- The response-level union type matching the local <code>ContentBlock</code> 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

[[Category:Implementations]]