Fix timeline catch-up for projected overlap

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

@@ -19,6 +19,7 @@ import {
 function makeTimelineEntry(seq: number, text: string, type: string = "assistant_message") {
   return {
     seqStart: seq,
+    seqEnd: seq,
     provider: "claude",
     item: { type, text },
     timestamp: new Date(1000 + seq).toISOString(),
@@ -318,6 +319,39 @@ describe("processTimelineResponse", () => {
     expect(result.cursorChanged).toBe(false);
   });
 
+  it("requests canonical catch-up when a projected entry overlaps unseen seqs", () => {
+    const existingCursor: TimelineCursor = {
+      epoch: "epoch-1",
+      startSeq: 1,
+      endSeq: 5,
+    };
+
+    const result = processTimelineResponse({
+      ...baseTimelineInput,
+      currentCursor: existingCursor,
+      payload: {
+        ...baseTimelineInput.payload,
+        epoch: "epoch-1",
+        entries: [
+          {
+            ...makeTimelineEntry(4, "merged assistant message"),
+            seqEnd: 8,
+          },
+        ],
+      },
+    });
+
+    expect(result.tail).toBe(baseTimelineInput.currentTail);
+    expect(result.cursorChanged).toBe(false);
+    expect(result.sideEffects).toContainEqual({
+      type: "catch_up",
+      cursor: {
+        epoch: "epoch-1",
+        endSeq: 5,
+      },
+    });
+  });
+
   it("drops entries with epoch mismatch", () => {
     const existingCursor: TimelineCursor = {
       epoch: "epoch-1",

packages/app/src/contexts/session-stream-reducers.ts

@@ -53,6 +53,7 @@ type InitRequestDirection = "tail" | "after";
 
 type TimelineResponseEntry = {
   seqStart: number;
+  seqEnd: number;
   provider: string;
   item: Record<string, unknown>;
   timestamp: string;
@@ -122,6 +123,7 @@ export function processTimelineResponse(
   // ------------------------------------------------------------------
   const timelineUnits = payload.entries.map((entry) => ({
     seq: entry.seqStart,
+    seqEnd: entry.seqEnd,
     event: {
       type: "timeline",
       provider: entry.provider,
@@ -200,7 +202,14 @@ export function processTimelineResponse(
         gapCursor = cursor ? { epoch: cursor.epoch, endSeq: cursor.endSeq } : null;
         break;
       }
-      if (decision === "drop_stale" || decision === "drop_epoch") {
+      if (decision === "drop_stale") {
+        if (cursor && unit.seqEnd > cursor.endSeq) {
+          gapCursor = { epoch: cursor.epoch, endSeq: cursor.endSeq };
+          break;
+        }
+        continue;
+      }
+      if (decision === "drop_epoch") {
         continue;
       }
 
@@ -209,7 +218,7 @@ export function processTimelineResponse(
         cursor = {
           epoch: payload.epoch,
           startSeq: unit.seq,
-          endSeq: unit.seq,
+          endSeq: unit.seqEnd,
         };
         continue;
       }
@@ -218,7 +227,7 @@ export function processTimelineResponse(
       }
       cursor = {
         ...cursor,
-        endSeq: unit.seq,
+        endSeq: unit.seqEnd,
       };
     }