Streamed responses through the proxy previously exposed no usable cost:
the x-litellm-response-cost header is unreadable mid-stream and the final
usage chunk carried only tokens, priced against an alias model name the
client cannot resolve. The include_cost_in_streaming_usage flag existed
but was off by default and only fixed the wire, not SDK clients.
Stamp usage.cost into the joined streaming response by default wherever a
final usage object is built: the chat-completions stream_chunk_builder,
the native /v1/responses RESPONSE_COMPLETED event, and synthetic response
events. Provider-reported cost always wins over the computed value, and
only positive computed costs are stamped so unpriceable alias responses
keep deferring to the logging object's own calculation. Per-chunk SSE
cost injection (/v1/messages, generateContent, passthrough) stays behind
the flag.
Also normalize non-litellm usage objects in stream_chunk_builder: openai
CompletionUsage lacks Usage.__contains__, so membership probes silently
returned False and client-side rebuilds dropped the wire cost and
recounted token usage locally. Wire token counts and cost now survive.
Resolves LIT-6427
Streaming pass-through for native Anthropic /v1/messages and the /v1/responses
streaming iterator never set logging_obj.completion_start_time, so
_success_handler_helper_fn fell back to completion_start_time = end_time.
Downstream TTFT consumers (Prometheus, OTEL, Langfuse, Admin UI, spend logs
completionStartTime) then reported time-to-first-token equal to total request
duration.
Stamp completion_start_time on the first chunk in PassThroughStreamingHandler.
chunk_processor and BaseResponsesAPIStreamingIterator._process_chunk, mirroring
CustomStreamWrapper for /chat/completions.
Resolves LIT-4185
Co-authored-by: yucheng <yucheng@yuchengs-MBP.localdomain>
