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2026-07-26-eventsource-parser-for-deepseek-sse.md

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Agent Note: Replace the hand-rolled SSE parser in llm-deepseek with eventsource-parser

Status: implemented Archived: 2026-08-07

English | 中文

Problem

packages/llm/llm-deepseek/src/sse.ts hand-implemented Server-Sent Events parsing: a streaming TextDecoder, event-block splitting on \r?\n\r?\n, data: payload extraction and joining, comment/field skipping, the [DONE] sentinel, a STREAM_CLOSED error on EOF without it, and a flush of a final unterminated event block. The file was ~67 lines with ~108 lines of dedicated tests (tests/sse.spec.ts) re-proving SSE spec behavior — UTF-8 split across chunks, CRLF handling, multi-data: joining, no-space-after-colon — that a maintained parser already guarantees. Its only consumer is adapter.ts (yield* translate(parseSse(response.body))).

This is exactly the surface eventsource-parser owns: the de-facto standard SSE parser (it underlies the Vercel AI SDK and the MCP SDK), zero-dependency, actively maintained, and already present in this repo's lockfile transitively via @modelcontextprotocol/sdk — so adopting it directly adds no new supply-chain surface in practice.

Decision

sse.ts delegates SSE framing to EventSourceParserStream from eventsource-parser/stream: parseSse pipes the response body through new TextDecoderStream() then new EventSourceParserStream() and keeps only the DeepSeek protocol shim — yield each event's data, terminate on [DONE], and throw LlmError('STREAM_CLOSED') when the stream ends without the sentinel. All required builtins (TextDecoderStream, pipeThrough, async-iterable ReadableStream) exist at the Node ^22.19 engine floor. The spec-conformance tests are gone; tests/sse.spec.ts pins only the [DONE]/STREAM_CLOSED/EOF contract. eventsource-parser is llm-deepseek's second runtime dependency after schemastery. The twin-adapters note and the dsh-llm JSDoc that branded this adapter "hand-rolled fetch + SSE parsing" now describe it as direct fetch with library-framed SSE.

The library also strips a leading BOM (the hand-rolled parser would fail to match data: after one) and offers maxBufferSize hardening the hand-rolled parser lacked.

Alternatives considered

  • Keep the hand-rolled parser. Defensible under the twin-adapters decision: the adapter is deliberately the hand-rolled design-verification twin of the pi-ai adapter. But that note's load-bearing distinction is owning the fetch/translate internals versus delegating to a full provider SDK; a ~700-byte SSE micro-parser is transport plumbing, not the design under verification. The twin-adapters note now states that reading explicitly.
  • createParser({onEvent}) callback API instead of the stream. Works fed by a manual TextDecoder loop, but the pipeThrough composition deletes more of the hand-rolled code.

Consequences

  • The remaining shim only encodes the DeepSeek [DONE]/STREAM_CLOSED protocol; SSE framing edge cases are eventsource-parser's contract and are no longer re-proven here.
  • One deliberate robustness deviation is dropped: the hand-rolled parser flushed a final event block that lacked its terminating blank line, so a trailing data: [DONE] without \n\n still yielded DONE. eventsource-parser is spec-strict and only dispatches on the blank line, so that shape is now STREAM_CLOSED. Real providers and dsh-llm-mock-server always terminate events properly — the flush was a robustness nicety, not an observed provider shape — and tests/sse.spec.ts pins the new truncation verdict for that tail.
  • The documented "hand-rolled" identity of the twin adapter narrows to the fetch/translate internals; the twin-adapters note was updated in the same change rather than leaving the claim stale.