fix(chat): reap replaced stream buffers

src/lib/server/chat-stream-buffer.test.ts

@@ -107,6 +107,30 @@ test("a follow-up turn owns the shared conversation key; unknown keys return nul
   resetRunBuffersForTest();
 });
 
+test("a follow-up preserves the predecessor reap timer", (t) => {
+  resetRunBuffersForTest();
+  t.mock.timers.enable({ apis: ["setTimeout"] });
+
+  const first = openRunBuffer(["run-old", "conv-shared"]);
+  first.record({ kind: "assistant_chunk", text: "old transcript" });
+  first.finish();
+
+  openRunBuffer(["run-new", "conv-shared"]);
+  t.mock.timers.tick(2 * 60_000);
+
+  assert.equal(
+    hasRunBuffer("run-old"),
+    false,
+    "the finished predecessor is reaped under its unique run id",
+  );
+  assert.equal(
+    hasRunBuffer("conv-shared"),
+    true,
+    "the predecessor timer never deletes the replacement conversation mapping",
+  );
+  resetRunBuffersForTest();
+});
+
 test("getRunBufferStatus returns payload-free metadata without side effects", () => {
   resetRunBuffersForTest();
   let attachCount = 0;

src/lib/server/chat-stream-buffer.ts

@@ -80,8 +80,10 @@ export function openRunBuffer(
   };
   for (const key of keys) {
     if (!key) continue;
-    const stale = buffers.get(key);
-    if (stale && stale.reapTimer) clearTimeout(stale.reapTimer);
+    // A finished predecessor can still be reachable through another key
+    // (normally its unique run id). Keep its reap timer armed: the callback
+    // already checks map identity, so it will remove only predecessor
+    // mappings and leave this replacement untouched.
     buffers.set(key, buffer);
     buffer.keys.push(key);
   }