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Reference

Streaming events

The event sequence each endpoint sends when stream is true.

Streams are server-sent events with content-type: text/event-stream. Events are relayed as they arrive.

Sequences

data: {"id":"gen-...","choices":[{"index":0,"delta":{"role":"assistant","content":""}}]}

data: {"id":"gen-...","choices":[{"index":0,"delta":{"content":"Hello"}}]}

data: {"id":"gen-...","choices":[{"index":0,"delta":{},"finish_reason":"stop"}]}

data: {"id":"gen-...","choices":[],"usage":{"prompt_tokens":14,"completion_tokens":7,"total_tokens":21}}

data: [DONE]

Chat Completions

Each chunk has choices[].delta. The last content chunk carries finish_reason. The last chunk before data: [DONE] carries usage. With stream_options.include_usage, that chunk has an empty choices array, as in OpenAI's API. Without it, the chunk keeps one choice with an empty delta.

Comment lines that begin with : may appear between events. Parsers skip them.

Responses

The event types are response.created, response.in_progress, response.output_item.added, response.content_part.added, response.output_text.delta, response.output_text.done, response.content_part.done, response.function_call_arguments.delta, response.function_call_arguments.done, response.output_item.done, response.completed, response.incomplete, response.failed and error. The stream ends with one of three final events, each carrying the final response object, with its usage when the provider reports it: response.completed when the answer finished, response.incomplete when it stopped early (the reason is in incomplete_details), or response.failed.

Messages

message_start opens with the input token count. Each content block sends content_block_start, one or more content_block_delta events, and content_block_stop. Delta types are text_delta, input_json_delta, thinking_delta and signature_delta. message_delta carries stop_reason and cumulative usage.output_tokens, then message_stop ends the stream.

ping events can appear anywhere, and an error event can appear mid-stream. Ignore event types you do not know.

Images

POST /v1/images streams only on models that support it. The events are image_generation.partial_image as the image forms, then image_generation.completed with the final image and usage, then data: [DONE]. An error event can replace either.

Errors

A failure after the stream starts keeps HTTP 200 and arrives as an event: a top-level error with finish_reason: "error" on Chat Completions, response.failed or error on Responses, and an error event on Messages.