Streaming chat completions

Request stream: true to receive JsonServerSentEventStream<ChatResponse>. Each value is a JSON SSE event: read event.data.choices, not event.choices. Deltas are incremental, not the final accumulated message.

Extraction and termination

Export	Contract
DoneDetector(event: ServerSentEvent): boolean	True only when raw event.data equals [DONE] exactly; no trimming/case folding
CompletionStreamResultExtractor(exchange)	Requires a FetchResponse and readable body; returns requiredJsonEventStream(DoneDetector)

The terminal marker is recognized before JSON parsing and is not emitted as a ChatResponse. Other malformed JSON fails stream consumption. A successful request Promise means a stream was obtained; errors can still occur later while reading. The extractor does not assemble choices, execute tool calls, estimate usage, or reconnect.

Complete streaming function

ChatClient does not accept a per-call signal parameter. Use the underlying Fetcher request API with CompletionStreamResultExtractor when you need a per-request signal. An AbortController can stop the pending request; cancel the acquired reader when finishing early, and always release its lock.

import { Fetcher } from '@ahoo-wang/fetcher';
import {
  CompletionStreamResultExtractor,
  type ChatResponse,
} from '@ahoo-wang/fetcher-openai';
import type { JsonServerSentEventStream } from '@ahoo-wang/fetcher-eventstream';

export async function readAnswer(
  baseURL: string,
  model: string,
  signal: AbortSignal,
) {
  const fetcher = new Fetcher({ baseURL });
  const stream = await fetcher.post<JsonServerSentEventStream<ChatResponse>>(
    '/chat/completions',
    {
      signal,
      body: {
        model,
        messages: [{ role: 'user', content: 'Say hello.' }],
        stream: true,
      },
    },
    { resultExtractor: CompletionStreamResultExtractor },
  );
  const reader = stream.getReader();
  let answer = '';
  let finished = false;
  try {
    while (true) {
      const { done, value } = await reader.read();
      if (done) {
        finished = true;
        break;
      }
      answer += value.data.choices[0]?.delta?.content ?? '';
    }
    return answer;
  } finally {
    try {
      if (!finished) await reader.cancel();
    } finally {
      reader.releaseLock();
    }
  }
}

The example expects an authenticated same-application proxy or a server-side configured transport. It does not call an external provider during documentation checks. See EventStream cancellation for iteration alternatives.

DoneDetector — packages/openai/src/chat/completionStreamResultExtractor.ts:39

CompletionStreamResultExtractor — packages/openai/src/chat/completionStreamResultExtractor.ts:88