diff --git a/packages/app/src/timeline/session-stream-reducers.test.ts b/packages/app/src/timeline/session-stream-reducers.test.ts
index 88e512abea..7e57c53b96 100644
--- a/packages/app/src/timeline/session-stream-reducers.test.ts
+++ b/packages/app/src/timeline/session-stream-reducers.test.ts
@@ -362,18 +362,28 @@ describe("processTimelineResponse", () => {
   });
 
   it("appends only rows newer than the cursor from an overlapping resume tail", () => {
-    const loaded: StreamItem = {
-      kind: "user_message",
-      id: "loaded-30",
-      text: "already loaded",
+    const loaded = createUserMessage({
+      id: "local-prompt-30",
+      clientMessageId: "client-message-30",
+      text: "local presentation",
       timestamp: new Date(1030),
       timelineCursor: { epoch: "epoch-1", seq: 30 },
-    };
+      attachments: [{ type: "text", mimeType: "text/plain", text: "local attachment" }],
+    });
+    const unrelated = createUserMessage({
+      id: "unrelated-row",
+      messageId: "unrelated-provider-message",
+      text: "unrelated",
+      timestamp: new Date(1035),
+      timelineCursor: { epoch: "epoch-1", seq: 35 },
+    });
+    const currentTail = [loaded, unrelated];
+    const currentCursor: TimelineCursor = { epoch: "epoch-1", startSeq: 1, endSeq: 40 };
 
     const result = processTimelineResponse({
       ...baseTimelineInput,
-      currentTail: [loaded],
-      currentCursor: { epoch: "epoch-1", startSeq: 1, endSeq: 40 },
+      currentTail,
+      currentCursor,
       payload: {
         ...baseTimelineInput.payload,
         direction: "tail",
@@ -381,17 +391,35 @@ describe("processTimelineResponse", () => {
         startCursor: { seq: 30 },
         endCursor: { seq: 42 },
         entries: [
-          makeTimelineEntry(30, "already loaded", "user_message"),
-          makeTimelineEntry(41, "new 41", "user_message"),
-          makeTimelineEntry(42, "new 42", "user_message"),
+          {
+            ...makeTimelineEntry(30, "provider presentation", "user_message"),
+            item: {
+              type: "user_message",
+              text: "provider presentation",
+              clientMessageId: "client-message-30",
+              messageId: "provider-message-30",
+            },
+          },
+          makeTimelineEntry(41, "new output 41"),
+          makeTimelineEntry(42, "new output 42"),
         ],
       },
     });
 
     expect(result.commit).toBe("apply");
-    expect(result.tail[0]).toBe(loaded);
-    expect(getUserTexts(result.tail)).toEqual(["already loaded", "new 41", "new 42"]);
+    expect(result.tail[0]).toEqual({ ...loaded, messageId: "provider-message-30" });
+    expect(result.tail[1]).toBe(unrelated);
+    expect(
+      [...result.tail, ...result.head].filter(
+        (item) => item.kind === "user_message" && item.clientMessageId === "client-message-30",
+      ),
+    ).toHaveLength(1);
+    expect(getAssistantTexts([...result.tail, ...result.head])).toEqual([
+      "new output 41new output 42",
+    ]);
     expect(result.cursor).toEqual({ epoch: "epoch-1", startSeq: 1, endSeq: 42 });
+    expect(result.cursorChanged).toBe(true);
+    expect(result.acknowledgedClientMessageIds).toEqual(["client-message-30"]);
     expect(result.sideEffects).toEqual([{ type: "flush_pending_updates" }]);
   });
 
diff --git a/packages/app/src/timeline/session-stream-reducers.ts b/packages/app/src/timeline/session-stream-reducers.ts
index 5a694584b5..047026a6d0 100644
--- a/packages/app/src/timeline/session-stream-reducers.ts
+++ b/packages/app/src/timeline/session-stream-reducers.ts
@@ -185,8 +185,8 @@ function deriveBootstrapTailTimelinePolicy({
 
 type ResumeTailPolicy =
   | { kind: "not_resume" }
-  | { kind: "discard" }
-  | { kind: "append" }
+  | { kind: "discard"; coveredEndSeq: number }
+  | { kind: "append"; coveredEndSeq: number }
   | { kind: "replace"; preserveContinuity: boolean };
 
 function deriveResumeTailPolicy(input: {
@@ -205,13 +205,13 @@ function deriveResumeTailPolicy(input: {
     return { kind: "replace", preserveContinuity: false };
   }
   if (input.windowMaxSeq === input.currentCursor.endSeq) {
-    return { kind: "discard" };
+    return { kind: "discard", coveredEndSeq: input.currentCursor.endSeq };
   }
   if (input.windowMaxSeq < input.currentCursor.endSeq) {
     return { kind: "replace", preserveContinuity: false };
   }
   if (input.pageStartSeq !== null && input.pageStartSeq <= input.currentCursor.endSeq + 1) {
-    return { kind: "append" };
+    return { kind: "append", coveredEndSeq: input.currentCursor.endSeq };
   }
   return { kind: "replace", preserveContinuity: true };
 }
@@ -976,19 +976,28 @@ export function processTimelineResponse(
     bootstrapReplace: replace,
   });
   const discard = resumeTailPolicy.kind === "discard";
+  const coveredUnits = timelineUnits.filter(
+    (unit) => "coveredEndSeq" in resumeTailPolicy && unit.seqEnd <= resumeTailPolicy.coveredEndSeq,
+  );
+  const revision = processAgentStreamEvents({
+    events: coveredUnits.map((unit) => ({
+      event: unit.event,
+      seq: unit.seqEnd,
+      epoch: payload.epoch,
+      timestamp: unit.timestamp,
+    })),
+    currentTail,
+    currentHead,
+    currentCursor,
+  });
+  const coveredRevisionAcknowledgements = deriveCanonicalAcknowledgements({
+    units: coveredUnits,
+    epoch: payload.epoch,
+    currentTail,
+    currentHead,
+  });
   let timelineResult: TimelinePathResult;
   if (discard) {
-    const revision = processAgentStreamEvents({
-      events: timelineUnits.map((unit) => ({
-        event: unit.event,
-        seq: unit.seqEnd,
-        epoch: payload.epoch,
-        timestamp: unit.timestamp,
-      })),
-      currentTail,
-      currentHead,
-      currentCursor,
-    });
     timelineResult = {
       tail: revision.tail,
       head: revision.head,
@@ -996,12 +1005,7 @@ export function processTimelineResponse(
       cursorChanged: false,
       older: "unchanged",
       sideEffects: [],
-      acknowledgedClientMessageIds: deriveCanonicalAcknowledgements({
-        units: timelineUnits,
-        epoch: payload.epoch,
-        currentTail,
-        currentHead,
-      }),
+      acknowledgedClientMessageIds: [],
     };
   } else if (replace || resumeTailPolicy.kind === "replace") {
     const isResumeReplacement = resumeTailPolicy.kind === "replace";
@@ -1027,11 +1031,20 @@ export function processTimelineResponse(
     timelineResult = applyTimelineIncrementalPath({
       timelineUnits: incrementalUnits,
       payload,
-      currentTail,
-      currentHead,
+      currentTail: revision.tail,
+      currentHead: revision.head,
       currentCursor,
     });
   }
+  timelineResult = {
+    ...timelineResult,
+    acknowledgedClientMessageIds: [
+      ...new Set([
+        ...coveredRevisionAcknowledgements,
+        ...timelineResult.acknowledgedClientMessageIds,
+      ]),
+    ],
+  };
 
   const nextTail = timelineResult.tail;
   const nextHead = timelineResult.head;