Cover timeline epoch reset behavior

packages/app/src/timeline/session-stream-reducers.test.ts

@@ -388,6 +388,41 @@ describe("processTimelineResponse", () => {
     expect(getUserTexts(result.tail)).toHaveLength(0);
   });
 
+  it("treats a stale epoch reset as a replacement with the new epoch window", () => {
+    const oldTail: StreamItem[] = [makeAssistantItem("old epoch message", "old-assistant")];
+    const oldHead: StreamItem[] = [makeAssistantItem("old live head", "old-head")];
+
+    const result = processTimelineResponse({
+      ...baseTimelineInput,
+      currentTail: oldTail,
+      currentHead: oldHead,
+      currentCursor: { epoch: "old-epoch", startSeq: 1, endSeq: 95 },
+      payload: {
+        ...baseTimelineInput.payload,
+        reset: true,
+        epoch: "new-epoch",
+        startCursor: { seq: 101 },
+        endCursor: { seq: 105 },
+        hasOlder: true,
+        entries: [
+          makeTimelineEntry(101, "new one"),
+          makeTimelineEntry(102, "new two"),
+          makeTimelineEntry(103, "new three"),
+          makeTimelineEntry(104, "new four"),
+          makeTimelineEntry(105, "new five"),
+        ],
+      },
+    });
+
+    expect(getAssistantTexts(result.tail)).toEqual(["new onenew twonew threenew fournew five"]);
+    expect(result.head).toEqual([]);
+    expect(result.cursor).toEqual({ epoch: "new-epoch", startSeq: 101, endSeq: 105 });
+    expect(result.sideEffects).not.toContainEqual({
+      type: "catch_up",
+      cursor: { epoch: "old-epoch", endSeq: 95 },
+    });
+  });
+
   it("performs bootstrap tail init with catch-up side effect", () => {
     const result = processTimelineResponse({
       ...baseTimelineInput,

packages/server/src/server/agent/agent-manager.test.ts

@@ -2214,6 +2214,20 @@ test("fetchTimeline returns a bounded reset window when cursor epoch is stale",
   expect(result.rows[0]?.seq).toBe(3);
   expect(result.rows[result.rows.length - 1]?.seq).toBe(3);
   expect(result.hasOlder).toBe(true);
+
+  const older = manager.fetchTimeline(snapshot.id, {
+    direction: "before",
+    cursor: {
+      epoch: result.epoch,
+      seq: result.rows[0]?.seq ?? 0,
+    },
+    limit: 1,
+  });
+
+  expect(older.reset).toBe(false);
+  expect(older.rows).toHaveLength(1);
+  expect(older.rows[0]?.seq).toBe(2);
+  expect(older.hasOlder).toBe(true);
 });
 
 test("getTimelineRows falls back to the in-memory timeline when no durable store is configured", async () => {