fix(sse): recognize bare delta.reasoning in combo streaming quality-gate peek (#13620) (#13806)

Merged in the 2026-09-16 sweep of the maintainer's own open PRs, at the owner's explicit instruction. No push was made to the PR branch: the merge took the head as the owning session left it (verified OPEN, non-draft and MERGEABLE against the release tip immediately before merging).

changelog.d/fixes/13620-combo-reasoning-only-sse-burst.md

@@ -0,0 +1 @@
+- **fix(sse):** stream reasoning deltas from combo targets incrementally instead of buffering them into a single burst, and stop rejecting reasoning-only streams as an empty completion (#13620) — thanks @NaNomicon

open-sse/utils/streamHelpers.ts

@@ -202,11 +202,7 @@ export function createSSEDataLineNormalizer(): SSEDataLineNormalizer {
         const normalizedLine = line.replace(CR_STRIP_RE, "");
         const trimmed = normalizedLine.trim();
 
-        if (
-          trimmed &&
-          SSE_FIELD_RE.test(trimmed) &&
-          hasSelfDescribingPendingDataPayload()
-        ) {
+        if (trimmed && SSE_FIELD_RE.test(trimmed) && hasSelfDescribingPendingDataPayload()) {
           flush(output);
         }
 
@@ -220,7 +216,9 @@ export function createSSEDataLineNormalizer(): SSEDataLineNormalizer {
   };
 }
 
-export function createSSEEventPrefixBuffer(options?: { forwardEvent?: boolean }): SSEEventPrefixBuffer {
+export function createSSEEventPrefixBuffer(options?: {
+  forwardEvent?: boolean;
+}): SSEEventPrefixBuffer {
   let lines: string[] = [];
   let emitted = false;
   // The `event:` line is only part of the SSE framing for protocols that define
@@ -277,12 +275,7 @@ function hasOpenAICompatibleStreamValue(parsed: Record<string, unknown>): boolea
     const delta = isRecord(choice.delta) ? choice.delta : null;
     if (!delta) return false;
     if (typeof delta.content === "string" && delta.content.length > 0) return true;
-    if (typeof delta.reasoning_content === "string" && delta.reasoning_content.length > 0) {
-      return true;
-    }
-    if (typeof delta.reasoning_text === "string" && delta.reasoning_text.length > 0) {
-      return true;
-    }
+    if (hasAnyReasoningSignal(delta)) return true;
     return Array.isArray(delta.tool_calls) && delta.tool_calls.length > 0;
   });
 }

tests/unit/issue-13620-combo-reasoning-only-sse-burst.test.ts

@@ -0,0 +1,126 @@
+/**
+ * Issue #13620 — combo quality-gate bounded peek buffers bare `delta.reasoning`
+ * SSE chunks (Ollama Cloud shape) because `hasOpenAICompatibleStreamValue()`
+ * (open-sse/utils/streamHelpers.ts) only recognizes `content`,
+ * `reasoning_content`, `reasoning_text`, and `tool_calls` — not bare
+ * `reasoning`. As a result the peek loop never exits on a reasoning-only
+ * chunk and keeps buffering every reasoning delta until the first real
+ * `content` delta arrives (client sees dead silence, then a single burst),
+ * and a stream that finishes with reasoning-only content is rejected as an
+ * "empty completion" (secondary 502 failure mode).
+ */
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { isKnownNonClaudeStreamPayload } = await import("../../open-sse/utils/streamHelpers.ts");
+const { validateResponseQuality } = await import("../../open-sse/services/combo.ts");
+
+const encoder = new TextEncoder();
+const silentLog = { warn: () => {} };
+
+test("#13620 root cause: isKnownNonClaudeStreamPayload must recognize bare delta.reasoning (Ollama Cloud shape)", () => {
+  const parsed = {
+    choices: [
+      { index: 0, delta: { role: "assistant", content: "", reasoning: "We" }, finish_reason: null },
+    ],
+  };
+  const recognized = isKnownNonClaudeStreamPayload(parsed);
+  assert.equal(
+    recognized,
+    true,
+    "bare delta.reasoning must be recognized as stream content so the combo peek stops buffering immediately, " +
+      "matching hasValuableContent()/hasAnyReasoningSignal() which already treat it as real signal"
+  );
+});
+
+// Emits ONE physical Uint8Array chunk per SSE frame (mirrors real network
+// framing, unlike a single enqueue() of the whole body) and counts how many
+// chunks the underlying stream has handed out by the time the caller stops
+// reading — this is what actually distinguishes "peek recognizes the first
+// reasoning delta and stops buffering there" from "peek keeps consuming
+// (buffering) every reasoning chunk until the content chunk arrives".
+function makeReasoningOnlyThenContentStream(): {
+  response: Response;
+  chunksPulled: { count: number };
+} {
+  const frames = [
+    { choices: [{ index: 0, delta: { role: "assistant", content: "", reasoning: "We" } }] },
+    { choices: [{ index: 0, delta: { content: "", reasoning: " need" } }] },
+    { choices: [{ index: 0, delta: { content: "", reasoning: " to think." } }] },
+    { choices: [{ index: 0, delta: { content: "Hello" } }] },
+    { choices: [{ index: 0, delta: {}, finish_reason: "stop" }] },
+  ].map((c) => encoder.encode(`data: ${JSON.stringify(c)}\n\n`));
+  frames.push(encoder.encode("data: [DONE]\n\n"));
+
+  const chunksPulled = { count: 0 };
+  let idx = 0;
+  const stream = new ReadableStream<Uint8Array>({
+    pull(controller) {
+      if (idx >= frames.length) {
+        controller.close();
+        return;
+      }
+      controller.enqueue(frames[idx++]);
+      chunksPulled.count = idx;
+    },
+  });
+  return {
+    response: new Response(stream, {
+      status: 200,
+      headers: { "content-type": "text/event-stream" },
+    }),
+    chunksPulled,
+  };
+}
+
+test("#13620 behavioral: peek must stop buffering at the first reasoning delta, not wait for content", async () => {
+  const { response, chunksPulled } = makeReasoningOnlyThenContentStream();
+  const out = await validateResponseQuality(response, true, silentLog);
+  assert.equal(out.valid, true, `expected valid, got reason: ${out.reason}`);
+  assert.ok(out.clonedResponse, "expected a clonedResponse to replay/forward the stream");
+
+  // Frame indices: 0-2 = reasoning-only deltas, 3 = content, 4 = finish_reason, 5 = [DONE].
+  // Correct: the peek recognizes frame 0 immediately via a single reader.read() call.
+  // The WHATWG ReadableStream spec itself speculatively pulls one extra chunk into its
+  // internal queue right after that read() resolves (default highWaterMark backpressure
+  // refill) — so up to 2 physical chunks may be pulled even when the peek loop performs
+  // exactly one logical read. That prefetch is unavoidable engine behavior, not something
+  // validateResponseQuality controls, so the threshold below allows for it.
+  // Bug: the peek loop itself keeps calling reader.read() through frames 0-2 (each
+  // triggering its own follow-up prefetch), only stopping at frame 3 -> 4-5 chunks pulled.
+  assert.ok(
+    chunksPulled.count <= 2,
+    "expected the peek to stop after at most 1 logical read (<=2 chunks pulled, accounting for " +
+      `ReadableStream's own one-chunk-ahead prefetch); actual: ${chunksPulled.count} chunks were ` +
+      "pulled before the peek recognized any content — this is the reasoning-prefix burst-buffering bug"
+  );
+});
+
+function makeReasoningOnlyTerminatedStream(): Response {
+  const chunks = [
+    { choices: [{ index: 0, delta: { role: "assistant", reasoning: "Deep" } }] },
+    { choices: [{ index: 0, delta: { reasoning: " thinking..." } }] },
+    { choices: [{ index: 0, delta: {}, finish_reason: "stop" }] },
+  ];
+  const body = chunks.map((c) => `data: ${JSON.stringify(c)}\n\n`).join("") + "data: [DONE]\n\n";
+  return new Response(
+    new ReadableStream<Uint8Array>({
+      start(controller) {
+        controller.enqueue(encoder.encode(body));
+        controller.close();
+      },
+    }),
+    { status: 200, headers: { "content-type": "text/event-stream" } }
+  );
+}
+
+test("#13620 secondary failure mode: reasoning-only stream that terminates cleanly must not be rejected as an empty completion", async () => {
+  const res = makeReasoningOnlyTerminatedStream();
+  const out = await validateResponseQuality(res, true, silentLog);
+  assert.equal(
+    out.valid,
+    true,
+    `expected valid (reasoning-only is real output, mirrors the non-streaming validator's ` +
+      `reasoning_content ?? reasoning acceptance) — got reason: ${out.reason}`
+  );
+});