fix(chat): SSE parser drops large events when chunk boundary lands between header and data

[oxyc]

Real bug surfaced by @oskaresh testing #55 locally: in a parallel run of three same-shape gds/content-read tool calls, the middle card got stuck on Running… while the other two flipped to Done. Audit log and persisted conversation both held the correct 100 KB result for the middle call; the UI just never saw it.

Root cause

parseSSE in use-runtime-adapter.ts declared eventType inside the network-chunk loop:

while (true) {
  const { value } = await reader.read();
  ...
  let eventType: string | null = null;  // ← reset every chunk
  for (const line of lines) { ... }
}

When the server's SSE frame is bigger than one TCP chunk, the event: line lands in chunk N and the matching data: <huge json>\n\n line lands in chunk N+1. The new iteration starts with eventType = null, the if (eventType && data) guard fails, and the event is silently dropped. Small results (the two siblings: ~30 KB and ~60 KB) fit in one chunk so they always work; the bug only fires above ~64 KB.

The middle call's tool_result payload was 100 169 chars. That's exactly large enough to straddle a frame.

Fix

Lifted eventType to the same scope as buffer so it persists across reader.read() iterations. The only writes that reset it are the legitimate end-of-event signals (a yielded data: line, a blank line per SSE spec) — both pre-existed and stay.

Refactor

Extracted parseSSE to its own hooks/parse-sse.ts so the regression test can import it without dragging @wordpress/element into the Jest sandbox. No behaviour change beyond the fix.

Regression test

hooks/__tests__/parseSSE.test.ts feeds parseSSE a hand-chopped ReadableStream<Uint8Array> to assert the right events come through:

Case	Coverage
Single complete event in one chunk	Sanity.
event: in chunk 1, data: in chunk 2	The primary bug.
Mid-data:-line split across three chunks	Multi-KB payloads.
Three back-to-back tool_results with middle straddling chunks	Mirrors the exact production scenario.
Blank line resets eventType	SSE spec compliance.
Malformed JSON skipped, not thrown	Defensive.

Test catches the regression: temporarily restoring the inner let eventType fails exactly the 3 chunk-straddling cases (Tests: 3 failed, 30 passed), passes 33/33 with the fix.

@types/node added to devDeps so the test can import node:stream/web / node:util for ReadableStream / TextEncoder / TextDecoder polyfills under wp-scripts' Jest config.

Verification

• npm run typecheck — clean
• npm run lint:js — 0 errors, 0 warnings
• npm run test:unit — 33/33
• npm run build — succeeds

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