# OpenAI streaming events — Responses SSE, Responses WebSocket, Chat Completions chunks

Status: DOCUMENTED (59 SSE event types from the OpenAPI ResponseStreamEvent union, 2 GA + 1 beta WebSocket client events, 62 GA + 2 beta server events) · LIVE_VERIFIED for the 11 lifecycle/text events observed on 2026-09-18 and for Chat Completions chunks. Machine-readable: generated/fragments/streaming-events/openai-responses.json, openai-responses-websocket.json, openai-chat-completions.json.

Sources

• https://developers.openai.com/api/reference/resources/responses/streaming-events · https://developers.openai.com/api/reference/resources/responses/websocket-events · https://developers.openai.com/api/reference/resources/beta/subresources/responses/streaming-events
• https://developers.openai.com/api/reference/resources/chat/subresources/completions/streaming-events
• https://developers.openai.com/api/docs/guides/streaming-responses · https://developers.openai.com/api/docs/guides/websocket-mode · https://developers.openai.com/api/docs/guides/background#streaming-a-background-response
• OpenAPI ResponseStreamEvent, ResponsesClientEvent, ResponsesServerEvent, CreateChatCompletionStreamResponse

Last verified: 2026-09-18

# 1. Responses SSE (stream: true)

Transport: Content-Type: text/event-stream. Each event is

event: response.output_text.delta
data: {"type":"response.output_text.delta","item_id":"msg_…","output_index":0,"content_index":0,"delta":"OK","logprobs":[],"obfuscation":"tcI6fDMDcuuDp0","sequence_number":4}

Invariants (live): event: name equals data.type; sequence_number starts at 0 and increments by 1 per event; delta events carry an obfuscation pad (disable with stream_options.include_obfuscation:false); lifecycle events embed the full Response object (response.created has status:"in_progress", response.completed carries final usage). No data: [DONE] sentinel — the stream ends after the terminal event (response.completed | response.failed | response.incomplete | error).

# Observed sequences (gpt-5.4-nano, "Reply with OK.", max_output_tokens 32)

# Complete SSE event catalogue (59)

Not in the SSE union but present as output items / other tools' events: apply_patch has no dedicated streaming events in this spec snapshot (its calls appear via output_item.*). The beta SSE union (BetaResponseStreamEvent) is identical to the GA list.

Handling guidance: branch on type; accumulate output_text.delta per (output_index,content_index); treat output_item.done as the canonical item (its encrypted_content is complete, output_item.added may be partial); read usage and prompt_cache_diagnostics from response.completed.response; persist the last sequence_number for background resume.

# 2. Responses WebSocket events (wss://api.openai.com/v1/responses)

Client → server

Server → client: every SSE event above (with an optional stream_id echo for named lanes) plus:

Concurrency model: same stream_id → FIFO; different lanes → concurrent (≤16 in flight). previous_response_id = lineage (fork a lane from another lane's response; wait for the fork's response.in_progress before advancing the parent lane with store:false). Connection cache is lost on reconnect.

# 3. Chat Completions chunks (stream: true)

Data-only SSE (no event: line): data: {chat.completion.chunk} … data: [DONE].

Observed (gpt-4.1-nano, stream_options.include_usage:true):

data: {"id":"chatcmpl-…","object":"chat.completion.chunk","created":…,"model":"gpt-4.1-nano-2025-04-14","service_tier":"default","system_fingerprint":"fp_…","choices":[{"index":0,"delta":{"role":"assistant","content":"","refusal":null},"logprobs":null,"finish_reason":null}],"usage":null,"obfuscation":"GO7oM1GH"}
data: {…,"choices":[{"index":0,"delta":{"content":"OK"},"logprobs":null,"finish_reason":null}],"usage":null,"obfuscation":"4EFX0fab"}
data: {…,"choices":[{"index":0,"delta":{},"logprobs":null,"finish_reason":"stop"}],"usage":null,"obfuscation":"9Pwv"}
data: {…,"choices":[],"usage":{"prompt_tokens":11,"completion_tokens":1,"total_tokens":12,"prompt_tokens_details":{…},"completion_tokens_details":{…}}}
data: [DONE]

• delta fields: role (first chunk), content, refusal, tool_calls[]{index, id, type, function:{name, arguments}} (arguments streamed as string fragments to merge by index), function_call (deprecated), audio{id, data, transcript, expires_at}.
• finish_reason non-null only on the last content chunk; usage chunk has choices: []; obfuscation present unless include_obfuscation:false; moderation results (if requested) arrive on a final moderation chunk.
• Legacy /v1/completions streams the same text_completion object per chunk (choices[].text), usage chunk with choices:[], then [DONE].

# 4. Responses vs Chat streaming at a glance