fix(sse): strip Composer <｜final｜> sentinel markers leaking after Composer reasoning (#4842)

Integrated into release/v3.8.37 — cherry-picked defining commit onto release tip; CHANGELOG re-merged; tests green.

open-sse/executors/cursor.ts

@@ -325,11 +325,32 @@ export function isComposerModel(model: string | undefined | null): boolean {
   return /^composer(?:-|$)/i.test(id ?? "");
 }
 
+// Composer's protobuf sometimes wraps the visible suffix in sentinel tags:
+// `<｜final｜>` (full-width pipes) or `<|final|>` (ASCII), optionally closed
+// with a matching `<｜/final｜>` / `<|/final|>`. These are protocol-internal
+// and must never leak to OpenAI-compatible clients (decolua/9router#1316).
+const COMPOSER_OPEN_MARKER = /^\s*<[｜|]\s*final\s*[｜|]>\s*/i;
+const COMPOSER_CLOSE_MARKER = /\s*<[｜|]\s*\/\s*final\s*[｜|]>\s*$/i;
+const COMPOSER_PARTIAL_OPEN = /^\s*<(?![｜|/])/;
+const COMPOSER_PARTIAL_OPEN_PIPE = /^\s*<[｜|][^>]*$/;
+
 export function visibleComposerContentFromThinking(thinking: string): string {
   if (!thinking) return "";
   const endIdx = thinking.lastIndexOf(COMPOSER_THINK_END);
   if (endIdx < 0) return "";
-  return thinking.slice(endIdx + COMPOSER_THINK_END.length).trimStart();
+  let visible = thinking.slice(endIdx + COMPOSER_THINK_END.length).trimStart();
+  if (COMPOSER_OPEN_MARKER.test(visible)) {
+    visible = visible.replace(COMPOSER_OPEN_MARKER, "");
+  } else if (
+    COMPOSER_PARTIAL_OPEN.test(visible) ||
+    COMPOSER_PARTIAL_OPEN_PIPE.test(visible)
+  ) {
+    // A streamed chunk delivered only a partial opening marker (e.g. `<` or
+    // `<｜fin`). Hold back everything until more data arrives so the marker
+    // fragment never leaks as content.
+    return "";
+  }
+  return visible.replace(COMPOSER_CLOSE_MARKER, "").trim();
 }
 
 export function composerReasoningRemainder(thinking: string): string {

tests/unit/cursor-composer-thinking.test.ts

@@ -71,6 +71,41 @@ test("visibleComposerContentFromThinking returns suffix after last </think> (tri
   assert.equal(visibleComposerContentFromThinking("ends with</think>"), "");
 });
 
+test("visibleComposerContentFromThinking strips `<｜final｜>` sentinel markers (full-width + ASCII)", () => {
+  // Full-width pipe sentinels (decolua/9router#1316).
+  assert.equal(
+    visibleComposerContentFromThinking("reasoning</think><｜final｜>OK_PR1316<｜/final｜>"),
+    "OK_PR1316"
+  );
+  // ASCII pipe sentinels.
+  assert.equal(
+    visibleComposerContentFromThinking("reasoning</think><|final|>HELLO<|/final|>"),
+    "HELLO"
+  );
+  // Open marker without a closing tag still gets stripped.
+  assert.equal(
+    visibleComposerContentFromThinking("r</think><｜final｜>just open"),
+    "just open"
+  );
+  // No sentinel — plain suffix is unchanged.
+  assert.equal(
+    visibleComposerContentFromThinking("reasoning</think>plain answer"),
+    "plain answer"
+  );
+});
+
+test("visibleComposerContentFromThinking holds back a partial opening marker until complete", () => {
+  // A streamed chunk delivered only the start of the sentinel — emit nothing yet.
+  assert.equal(visibleComposerContentFromThinking("r</think><"), "");
+  assert.equal(visibleComposerContentFromThinking("r</think><｜fin"), "");
+  assert.equal(visibleComposerContentFromThinking("r</think><|fin"), "");
+  // Once the full marker + payload arrive the real content surfaces.
+  assert.equal(
+    visibleComposerContentFromThinking("r</think><｜final｜>NOW_VISIBLE"),
+    "NOW_VISIBLE"
+  );
+});
+
 test("composerReasoningRemainder returns only the hidden portion before last </think>", () => {
   assert.equal(
     composerReasoningRemainder("private reasoning</think>OK"),
@@ -135,6 +170,27 @@ test("Composer non-streaming aggregation: thinking with </think> populates total
   assert.equal(ctx.totalText, "OK");
 });
 
+test("Composer streaming: partial `<｜final｜>` sentinel split across chunks never leaks", () => {
+  const chunks: string[] = [];
+  const ctx: StreamCtx = newStreamCtx("cu/composer-2.5", (c) => chunks.push(c));
+  // The sentinel arrives byte-fragmented across three thinking-delta frames.
+  processFrame(buildThinkingDeltaPayload("reasoning</think><｜fina"), ctx, new Set());
+  processFrame(buildThinkingDeltaPayload("l｜>OK_S"), ctx, new Set());
+  processFrame(buildThinkingDeltaPayload("TREAM"), ctx, new Set());
+
+  const events = parseSSE(chunks.join(""));
+  const content = events
+    .map((e) => {
+      const choices = (e as { choices?: Array<{ delta?: { content?: string } }> }).choices;
+      return choices?.[0]?.delta?.content ?? "";
+    })
+    .join("");
+  assert.equal(content, "OK_STREAM");
+  assert.equal(ctx.totalText, "OK_STREAM");
+  assert.ok(!chunks.join("").includes("final"), "sentinel literal must not leak");
+  assert.ok(!chunks.join("").includes("｜"), "full-width pipe must not leak");
+});
+
 test("Non-Composer model: thinking field stays in reasoning_content (unchanged contract)", () => {
   const chunks: string[] = [];
   const ctx: StreamCtx = newStreamCtx("gpt-5.3-codex", (c) => chunks.push(c));