From 8f9dd96355ed77ba3749fe1f89ed766471bb02ec Mon Sep 17 00:00:00 2001
From: Mohamed Boudra <boudra.moha@gmail.com>
Date: Sat, 1 Aug 2026 01:33:16 +0200
Subject: [PATCH] fix(timeline): restore prompt identity on resume

---
 .../timeline/session-stream-reducers.test.ts  | 68 ++++++++++++++++++-
 .../src/timeline/session-stream-reducers.ts   | 42 ++++++++++--
 2 files changed, 104 insertions(+), 6 deletions(-)

diff --git a/packages/app/src/timeline/session-stream-reducers.test.ts b/packages/app/src/timeline/session-stream-reducers.test.ts
index 3fe8742e79..7cb734ccaf 100644
--- a/packages/app/src/timeline/session-stream-reducers.test.ts
+++ b/packages/app/src/timeline/session-stream-reducers.test.ts
@@ -258,9 +258,73 @@ const baseStreamInput: ProcessAgentStreamEventInput = {
 // ---------------------------------------------------------------------------
 
 describe("processTimelineResponse", () => {
+  it("applies a missed same-sequence provider identity revision from an unchanged resume tail", () => {
+    const canonical = createUserMessage({
+      id: "canonical-prompt",
+      clientMessageId: "client-message",
+      text: "local prompt",
+      timestamp: new Date(1000),
+      timelineCursor: { epoch: "epoch-1", seq: 5 },
+    });
+    const unrelatedTail = makeAssistantItem("unrelated tail", "unrelated-tail");
+    const currentTail = [canonical, unrelatedTail];
+    const currentHead = [makeAssistantItem("live head", "live-head")];
+    const currentCursor: TimelineCursor = { epoch: "epoch-1", startSeq: 1, endSeq: 8 };
+
+    const result = processTimelineResponse({
+      ...baseTimelineInput,
+      currentTail,
+      currentHead,
+      currentCursor,
+      payload: {
+        ...baseTimelineInput.payload,
+        direction: "tail",
+        window: { minSeq: 1, maxSeq: 8, nextSeq: 9 },
+        startCursor: { seq: 5 },
+        endCursor: { seq: 8 },
+        entries: [
+          {
+            ...makeTimelineEntry(5, "provider prompt", "user_message"),
+            item: {
+              type: "user_message",
+              text: "provider prompt",
+              messageId: "provider-message",
+              clientMessageId: "client-message",
+            },
+          },
+        ],
+      },
+    });
+
+    expect(result.commit).toBe("apply");
+    expect(result.tail).not.toBe(currentTail);
+    expect(result.tail[0]).toEqual(
+      expect.objectContaining({
+        id: "canonical-prompt",
+        clientMessageId: "client-message",
+        messageId: "provider-message",
+        text: "local prompt",
+        timestamp: canonical.timestamp,
+        timelineCursor: { epoch: "epoch-1", seq: 5 },
+      }),
+    );
+    expect(result.tail[1]).toBe(unrelatedTail);
+    expect(result.head).toBe(currentHead);
+    expect(result.cursor).toBe(currentCursor);
+    expect(result.cursorChanged).toBe(false);
+    expect(result.acknowledgedClientMessageIds).toEqual(["client-message"]);
+  });
+
   it("discards an unchanged resume tail without replacing timeline state", () => {
-    const submitted = makeSubmittedUserMessage("local prompt", "client-message");
-    const currentTail = [submitted, makeAssistantItem("existing tail", "existing-tail")];
+    const canonical = createUserMessage({
+      id: "canonical-prompt",
+      clientMessageId: "client-message",
+      messageId: "provider-message",
+      text: "local prompt",
+      timestamp: new Date(1000),
+      timelineCursor: { epoch: "epoch-1", seq: 1 },
+    });
+    const currentTail = [canonical, makeAssistantItem("existing tail", "existing-tail")];
     const currentHead = [makeAssistantItem("existing head", "existing-head")];
 
     const result = processTimelineResponse({
diff --git a/packages/app/src/timeline/session-stream-reducers.ts b/packages/app/src/timeline/session-stream-reducers.ts
index 87d43ea325..5a694584b5 100644
--- a/packages/app/src/timeline/session-stream-reducers.ts
+++ b/packages/app/src/timeline/session-stream-reducers.ts
@@ -782,6 +782,20 @@ function deriveCanonicalAcknowledgements(params: {
   return [...acknowledged];
 }
 
+function shouldDiscardUnchangedTimelineResult(params: {
+  discardPolicy: boolean;
+  currentTail: StreamItem[];
+  currentHead: StreamItem[];
+  nextTail: StreamItem[];
+  nextHead: StreamItem[];
+}): boolean {
+  return (
+    params.discardPolicy &&
+    params.nextTail === params.currentTail &&
+    params.nextHead === params.currentHead
+  );
+}
+
 function applyTimelineIncrementalPath(args: {
   timelineUnits: TimelineUnit[];
   payload: ProcessTimelineResponseInput["payload"];
@@ -964,9 +978,20 @@ export function processTimelineResponse(
   const discard = resumeTailPolicy.kind === "discard";
   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: currentTail,
-      head: currentHead,
+      tail: revision.tail,
+      head: revision.head,
       cursor: currentCursor,
       cursorChanged: false,
       older: "unchanged",
@@ -1036,9 +1061,18 @@ export function processTimelineResponse(
     timelineResponseComplete;
 
   const initResolution: "resolve" | "reject" | null = shouldResolveDeferredInit ? "resolve" : null;
+  const commit = shouldDiscardUnchangedTimelineResult({
+    discardPolicy: discard,
+    currentTail,
+    currentHead,
+    nextTail,
+    nextHead,
+  })
+    ? "discard"
+    : "apply";
 
   return {
-    commit: discard ? "discard" : "apply",
+    commit,
     tail: nextTail,
     head: nextHead,
     cursor: nextCursor,
@@ -1135,7 +1169,7 @@ function applyCanonicalUserMessageRevision(input: {
       messageId: event.item.messageId,
       text: event.item.text,
       timestamp: input.timestamp,
-      timelineCursor: { epoch, seq },
+      timelineCursor: existing.timelineCursor,
     }),
     insert: "none",
     presentation: "existing",