sse-stream-parser

Incremental Server-Sent Events parser for LLM streaming — spec-correct multi-line fields, comments, CRLF/LF, chunk-boundary splits, [DONE] convention, and JSON-delta accumulation. Zero runtime dependencies, strict TypeScript.

Quickstart

import { SSEParser, isDoneEvent, createDeltaAccumulator } from "sse-stream-parser";

const parser = new SSEParser();
const accumulator = createDeltaAccumulator(
  (parsed) => parsed?.choices?.[0]?.delta?.content ?? null
);

// Feed raw chunks as they arrive from your HTTP stream reader — any
// alignment, including split mid-event.
for await (const chunk of responseBodyChunks) {
  for (const event of parser.feed(chunk)) {
    if (isDoneEvent(event)) break;
    accumulator.feed(event);
  }
}

console.log(accumulator.text); // the fully reassembled text

API

class SSEParser

• feed(chunk: string): SSEEvent[] — feed one raw chunk, get back the event(s) that completed as a result.
• flush(): SSEEvent[] — flush any buffered partial event at stream end, for sources that don't send a trailing blank line.

SSEEvent

interface SSEEvent {
  data: string;       // multi-line data: fields joined with "\n"
  event: string;       // defaults to "message" per spec
  id?: string;
  retry?: number;
}

isDoneEvent(event): boolean

True if event.data.trim() === "[DONE]" — the OpenAI-style stream-end sentinel.

createDeltaAccumulator(extract): DeltaAccumulator

Builds a running-text accumulator for JSON-delta streams. extract pulls the delta string out of the parsed JSON for one event (return null/undefined for events with no text delta, e.g. role markers). Malformed JSON in data is skipped rather than throwing.

interface DeltaAccumulator {
  feed(event: SSEEvent): string | null; // returns the delta appended, or null
  text: string;                          // full accumulated text
  reset(): void;
}

Limits

• This is an SSE parser, not an HTTP client — you supply the chunks (e.g. from fetch's ReadableStream, EventSource is not used since it doesn't expose raw chunk boundaries the way this library is designed to stress-test).
• id: fields containing a space are ignored, per the SSE spec (id must not contain U+0000 or whitespace in strict implementations — this parser is lenient elsewhere but enforces this one).
• createDeltaAccumulator assumes each event's data is a full JSON object; it does not reassemble JSON that itself spans multiple SSE events.

Part of the ferrow-toolkit collection · Sponsored by Ferrow