diff --git a/packages/views/issues/components/thread-utils.test.ts b/packages/views/issues/components/thread-utils.test.ts
new file mode 100644
index 000000000..2c1f9c76c
--- /dev/null
+++ b/packages/views/issues/components/thread-utils.test.ts
@@ -0,0 +1,63 @@
+import { describe, expect, it } from "vitest";
+import type { TimelineEntry } from "@multica/core/types";
+import { collectThreadReplies } from "./thread-utils";
+
+function comment(id: string, createdAt: string, parentId: string | null): TimelineEntry {
+  return {
+    type: "comment",
+    id,
+    actor_type: "member",
+    actor_id: "user-1",
+    content: id,
+    parent_id: parentId,
+    created_at: createdAt,
+    updated_at: createdAt,
+    comment_type: "comment",
+  } as TimelineEntry;
+}
+
+function bucketByParent(entries: TimelineEntry[]): Map<string, TimelineEntry[]> {
+  const map = new Map<string, TimelineEntry[]>();
+  for (const e of entries) {
+    if (!e.parent_id) continue;
+    const list = map.get(e.parent_id) ?? [];
+    list.push(e);
+    map.set(e.parent_id, list);
+  }
+  return map;
+}
+
+describe("collectThreadReplies", () => {
+  it("orders a late nested reply after earlier sibling replies (#3691)", () => {
+    // R1 (50m ago) triggered a slow agent; R2 (30m) and R3 (10m) arrived while
+    // it ran; D (3m ago) is the agent's reply, forced to nest under R1. A
+    // depth-first walk yields R1-D-R2-R3; the thread must read R1-R2-R3-D.
+    const r1 = comment("r1", "2026-06-11T10:00:00Z", "root");
+    const r2 = comment("r2", "2026-06-11T10:20:00Z", "root");
+    const r3 = comment("r3", "2026-06-11T10:40:00Z", "root");
+    const d = comment("d", "2026-06-11T10:47:00Z", "r1");
+
+    const out = collectThreadReplies("root", bucketByParent([r1, r2, r3, d]));
+
+    expect(out.map((e) => e.id)).toEqual(["r1", "r2", "r3", "d"]);
+  });
+
+  it("still returns every descendant across nesting levels", () => {
+    const r1 = comment("r1", "2026-06-11T10:00:00Z", "root");
+    const d1 = comment("d1", "2026-06-11T10:05:00Z", "r1");
+    const d2 = comment("d2", "2026-06-11T10:10:00Z", "d1");
+
+    const out = collectThreadReplies("root", bucketByParent([r1, d1, d2]));
+
+    expect(out.map((e) => e.id)).toEqual(["r1", "d1", "d2"]);
+  });
+
+  it("breaks created_at ties by id so the order is deterministic", () => {
+    const b = comment("b", "2026-06-11T10:00:00Z", "root");
+    const a = comment("a", "2026-06-11T10:00:00Z", "b");
+
+    const out = collectThreadReplies("root", bucketByParent([b, a]));
+
+    expect(out.map((e) => e.id)).toEqual(["a", "b"]);
+  });
+});
diff --git a/packages/views/issues/components/thread-utils.ts b/packages/views/issues/components/thread-utils.ts
index ab821e8b8..a47074fb9 100644
--- a/packages/views/issues/components/thread-utils.ts
+++ b/packages/views/issues/components/thread-utils.ts
@@ -1,11 +1,19 @@
 import type { TimelineEntry } from "@multica/core/types";
+import { sortTimelineEntriesAsc } from "@multica/core/issues/timeline-sort";
 
 /**
  * Walks the parent_id graph rooted at `rootId` and returns every descendant in
- * traversal order. Shared between CommentCard (which renders the expanded
- * thread) and ResolvedThreadBar (which displays the collapsed count + author
- * list) so the two views stay in sync — direct-children-only counts diverge
- * once nested replies exist (see Emacs review on PR #2300).
+ * CHRONOLOGICAL order (created_at ASC, id tie-break). Shared between
+ * CommentCard (which renders the expanded thread) and ResolvedThreadBar
+ * (which displays the collapsed count + author list) so the two views stay in
+ * sync — direct-children-only counts diverge once nested replies exist (see
+ * Emacs review on PR #2300).
+ *
+ * Chronological, not depth-first: agent replies are forced to nest under the
+ * comment that triggered them, so a depth-first walk lets a slow agent's late
+ * reply render BEFORE earlier sibling replies (#3691). The server's --thread
+ * output the agent reads is already chronological (ListThreadCommentsForIssue
+ * in comment.sql); this keeps the UI on the same order.
  */
 export function collectThreadReplies(
   rootId: string,
@@ -20,7 +28,7 @@ export function collectThreadReplies(
     }
   };
   walk(rootId);
-  return out;
+  return sortTimelineEntriesAsc(out);
 }
 
 /**