deepseek stream first chunk return empty asistent choice

[FeiDeng]

For stream case, it seems always return this one as first chunk.

data: {"id":"3588d74098294703a64504506ffe196e","object":"chat.completion.chunk","created":1748046366,"model":"deepseek-v3-0324","choices":[{"index":0,"delta":{"role":"assistant","content":null,"reasoning_content":null,"tool_calls":null},"logprobs":null,"finish_reason":null,"matched_stop":null}],"usage":null}'

from adapter I saw this related code
if is_first:
# First chunk with role
is_first = False
delta = DeltaMessage(role="assistant")
choice_data = ChatCompletionResponseStreamChoice(
index=index,
delta=delta,
finish_reason=finish_reason_type,
matched_stop=(
finish_reason["matched"]
if finish_reason and "matched" in finish_reason
else None
),
logprobs=choice_logprobs,
)
chunk = ChatCompletionStreamResponse(
id=content["meta_info"]["id"],
created=created,
choices=[choice_data],
model=request.model,
)

Just wonder why we need this？ Seems other chat engine won't do the same.

Thanks