[ntdatt812]

This currently fails CI on every open PR in the repo, including ones that touch nothing near it.

#12179 gave GLM a 64 KB stream buffer by adding a 16th argument to createSSETransformStreamWithLogger:

// GLM is a high-throughput provider — use a larger stream buffer (64KB) to
// keep provider → client pacing ahead of the model's token emission rate.
const transform = createSSETransformStreamWithLogger(
  …,
  suppressThinkClose,
  undefined,
  undefined,
  65536          // ← the 16th
);

The signature takes 15. Two things followed.

The visible one. open-sse/executors/glm.ts(244,5): error TS2554: Expected 2-15 arguments, but got 16. The API Route Typecheck gate is "reject new errors not covered by the frozen baseline", so it reports 1 new/regressed TypeScript error and fails — on any PR, since nobody's branch removes it. I found it while auditing my own open PRs: #12896 fails the identical 8 checks after a clean rebase onto release/v3.8.51, with a test that passes locally, and #12919 (not mine) fails the same 8.

The quiet one. The extra argument was simply dropped, and createSSEStream hard-coded the queue budget:

{ highWaterMark: 16384 },
{ highWaterMark: 16384 }

So the 64 KB that call site asks for never reached the stream. The perf change that hunk describes has not been in effect since it landed — the comment above it has been describing something that does not happen.

The change

StreamOptions gains streamBufferBytes, defaulting to the 16384 every provider used before it was configurable; the convenience wrapper takes it as a 16th parameter and forwards it; both queues use it. glm.ts is unchanged — its existing call now compiles and takes effect, which is the smallest edit that makes the original intent true rather than deleting it.

The default is a parameter default rather than || DEFAULT, so a deliberate 0 — a queue that applies backpressure immediately — is honoured instead of silently replaced. There is a test for exactly that, because || here is the easy way to write it and the wrong one.

Verification

tsc -p tsconfig.typecheck-api.json: 299 → 298 errors. The one removed is glm.ts TS2554, the one the gate names. Nothing new appears; the remaining stream.ts TS2353 about cancel is in the frozen baseline (it sat at line 2986 before this change and 2998 after — same error, shifted by the added lines).

tests/unit/sse-stream-buffer-bytes.test.ts: 5 passed. A TransformStream's writable queue starts with desiredSize === highWaterMark, so the assertions measure the queue that was actually built rather than standing in for it.

Each claim checked by breaking it:

reverted	fails
queues back to the hard-coded 16384	the three budget assertions
wrapper stops forwarding the argument	only the 16-positional-argument test
default written as || DEFAULT	only the streamBufferBytes: 0 test

I have not touched the buffer size for any other provider: without the option, every stream still gets 16384.