diff --git a/packages/ui/src/components/chat/message/MessageBody.tsx b/packages/ui/src/components/chat/message/MessageBody.tsx
index fa0bbfa3..c6c09f46 100644
--- a/packages/ui/src/components/chat/message/MessageBody.tsx
+++ b/packages/ui/src/components/chat/message/MessageBody.tsx
@@ -726,10 +726,13 @@ const UserMessageBody = React.memo(({ messageId, parts, messageCreatedAt, isMobi
                 )}
                 style={useStickyScrollableUserContent ? { maxHeight: 'calc(var(--chat-scroll-height, 100dvh) * 0.4)' } : undefined}
             >
+                {/* Positional keys, not part ids: the server echo of a just-sent
+                    message swaps the optimistic part id, and id-based keys would
+                    remount the text subtree (blank frame + height jump). */}
                 {userContentParts.map((part, index) => {
                     if (isSubtaskPart(part)) {
                         return (
-                            <React.Fragment key={part.id ?? `user-subtask-${index}`}>
+                            <React.Fragment key={`user-subtask-${index}`}>
                                 <UserSubtaskPart part={part} />
                             </React.Fragment>
                         );
@@ -737,7 +740,7 @@ const UserMessageBody = React.memo(({ messageId, parts, messageCreatedAt, isMobi
 
                     if (isShellActionPart(part)) {
                         return (
-                            <React.Fragment key={part.id ?? `user-shell-${index}`}>
+                            <React.Fragment key={`user-shell-${index}`}>
                                 <UserShellActionPart part={part} />
                             </React.Fragment>
                         );
@@ -752,7 +755,7 @@ const UserMessageBody = React.memo(({ messageId, parts, messageCreatedAt, isMobi
                         }
                     }
                     return (
-                        <React.Fragment key={part.id ?? `user-text-${index}`}>
+                        <React.Fragment key={`user-text-${index}`}>
                             <UserTextPart
                                 part={part}
                                 messageId={messageId}
diff --git a/packages/ui/src/sync/__tests__/event-reducer.test.ts b/packages/ui/src/sync/__tests__/event-reducer.test.ts
index 59ffc74a..2dda4914 100644
--- a/packages/ui/src/sync/__tests__/event-reducer.test.ts
+++ b/packages/ui/src/sync/__tests__/event-reducer.test.ts
@@ -114,6 +114,22 @@ describe("applyDirectoryEvent", () => {
     expect(draft.part.msg_1).toEqual([legacyPart, currentPart])
   })
 
+  test("replaces an optimistic user part in place instead of appending it", () => {
+    const optimisticText = { id: "prt_optimistic_text", messageID: "msg_1", type: "text", text: "hi" } as Part
+    const optimisticFile = { id: "prt_optimistic_file", messageID: "msg_1", type: "file", filename: "a.png" } as Part
+    const serverText = { id: "prt_server_text", messageID: "msg_1", sessionID: "ses_1", type: "text", text: "hi" } as Part
+    const draft = state({
+      message: { ses_1: [{ id: "msg_1", sessionID: "ses_1", role: "user", time: { created: 1 } } as Message] },
+      part: { msg_1: [optimisticText, optimisticFile] },
+    })
+
+    expect(applyDirectoryEvent(draft, {
+      type: "message.part.updated",
+      properties: { part: serverText },
+    } as Event)).toBe(true)
+    expect(draft.part.msg_1).toEqual([serverText, optimisticFile])
+  })
+
   test("returns typed materialization when delta arrives before parts", () => {
     const result = applyDirectoryEvent(state(), deltaEvent())
 
diff --git a/packages/ui/src/sync/event-reducer.ts b/packages/ui/src/sync/event-reducer.ts
index 0eaf4e71..13057258 100644
--- a/packages/ui/src/sync/event-reducer.ts
+++ b/packages/ui/src/sync/event-reducer.ts
@@ -441,9 +441,12 @@ export function applyDirectoryEvent(
           ? next.findIndex((p) => p.type === part.type && !(p as { sessionID?: string }).sessionID)
           : -1
         if (optimisticIndex >= 0) {
-          next.splice(optimisticIndex, 1)
+          // Replace in place: pushing to the end reorders text/file parts of a
+          // just-sent message and remounts its rendered subtree.
+          next[optimisticIndex] = part
+        } else {
+          next.push(part)
         }
-        next.push(part)
       }
       draft.part[messageID] = next
       return missingOwningMessage