diff --git a/packages/ui/src/stores/message-v2/bridge.ts b/packages/ui/src/stores/message-v2/bridge.ts
index 6642f107..7b1b35be 100644
--- a/packages/ui/src/stores/message-v2/bridge.ts
+++ b/packages/ui/src/stores/message-v2/bridge.ts
@@ -5,7 +5,7 @@ import { getQuestionCallId, getQuestionMessageId } from "../../types/question"
 import type { Message, MessageInfo, ClientPart } from "../../types/message"
 import type { Session } from "../../types/session"
 import { messageStoreBus } from "./bus"
-import type { MessageStatus, SessionRevertState } from "./types"
+import type { MessageStatus, ReplaceMessageIdOptions, SessionRevertState } from "./types"
 
 interface SessionMetadata {
   id: string
@@ -121,10 +121,10 @@ export function applyPartDeltaV2(
   })
 }
 
-export function replaceMessageIdV2(instanceId: string, oldId: string, newId: string): void {
+export function replaceMessageIdV2(instanceId: string, oldId: string, newId: string, options?: Omit<ReplaceMessageIdOptions, "oldId" | "newId">): void {
   if (!oldId || !newId || oldId === newId) return
   const store = messageStoreBus.getOrCreate(instanceId)
-  store.replaceMessageId({ oldId, newId })
+  store.replaceMessageId({ oldId, newId, ...(options ?? {}) })
 }
 
 function extractPermissionMessageId(permission: PermissionRequestLike): string | undefined {
diff --git a/packages/ui/src/stores/message-v2/instance-store.ts b/packages/ui/src/stores/message-v2/instance-store.ts
index bc2bb1f3..12f70bc3 100644
--- a/packages/ui/src/stores/message-v2/instance-store.ts
+++ b/packages/ui/src/stores/message-v2/instance-store.ts
@@ -586,10 +586,10 @@ export function createInstanceMessageStore(instanceId: string, hooks?: MessageSt
       bufferPendingPart({ messageId: input.messageId, part: input.part, receivedAt: Date.now() })
       return
     }
-  
+
     const partId = ensurePartId(input.messageId, input.part, message.partIds.length)
     const cloned = clonePart(input.part)
-  
+
     setState(
       "messages",
       input.messageId,
@@ -792,6 +792,8 @@ export function createInstanceMessageStore(instanceId: string, hooks?: MessageSt
       id: options.newId,
       isEphemeral: false,
       updatedAt: Date.now(),
+      partIds: options.clearParts ? [] : existing.partIds,
+      parts: options.clearParts ? {} : existing.parts,
     }
 
     setState("messages", options.newId, cloned)
diff --git a/packages/ui/src/stores/message-v2/types.ts b/packages/ui/src/stores/message-v2/types.ts
index 4d41cabc..986990a3 100644
--- a/packages/ui/src/stores/message-v2/types.ts
+++ b/packages/ui/src/stores/message-v2/types.ts
@@ -152,6 +152,7 @@ export interface PartUpdateInput {
 export interface ReplaceMessageIdOptions {
   oldId: string
   newId: string
+  clearParts?: boolean
 }
 
 export interface ScrollCacheKey {
diff --git a/packages/ui/src/stores/session-actions.ts b/packages/ui/src/stores/session-actions.ts
index f94f6ce7..8e193c6d 100644
--- a/packages/ui/src/stores/session-actions.ts
+++ b/packages/ui/src/stores/session-actions.ts
@@ -94,7 +94,7 @@ async function sendMessage(
   }
 
   const messageId = createId("msg")
-  const textPartId = createId("part")
+  const textPartId = createId("prt")
 
   const resolvedPrompt = resolvePastedPlaceholders(prompt, attachments)
 
@@ -110,7 +110,6 @@ async function sendMessage(
 
   const requestParts: any[] = [
     {
-      id: textPartId,
       type: "text" as const,
       text: resolvedPrompt,
     },
@@ -120,9 +119,8 @@ async function sendMessage(
     for (const att of attachments) {
       const source = att.source
       if (source.type === "file") {
-        const partId = createId("part")
+        const partId = createId("prt")
         requestParts.push({
-          id: partId,
           type: "file" as const,
           url: att.url,
           mime: source.mime,
@@ -148,9 +146,8 @@ async function sendMessage(
           continue
         }
 
-        const partId = createId("part")
+        const partId = createId("prt")
         requestParts.push({
-          id: partId,
           type: "text" as const,
           text: value,
         })
@@ -184,7 +181,6 @@ async function sendMessage(
   })
 
   const requestBody = {
-    messageID: messageId,
     parts: requestParts,
     ...(session.agent && { agent: session.agent }),
     ...(session.model.providerId &&
diff --git a/packages/ui/src/stores/session-events.ts b/packages/ui/src/stores/session-events.ts
index 0d6ed470..710c7dab 100644
--- a/packages/ui/src/stores/session-events.ts
+++ b/packages/ui/src/stores/session-events.ts
@@ -240,19 +240,22 @@ function resolveMessageRole(info?: MessageInfo | null): MessageRole {
   return info?.role === "user" ? "user" : "assistant"
 }
 
-function findPendingMessageId(
+function findPendingSyntheticMessageId(
   store: InstanceMessageStore,
   sessionId: string,
   role: MessageRole,
 ): string | undefined {
   const messageIds = store.getSessionMessageIds(sessionId)
-  const lastId = messageIds[messageIds.length - 1]
-  if (!lastId) return undefined
-  const record = store.getMessage(lastId)
-  if (!record) return undefined
-  if (record.sessionId !== sessionId) return undefined
-  if (record.role !== role) return undefined
-  return record.status === "sending" ? record.id : undefined
+  for (const messageId of messageIds) {
+    const record = store.getMessage(messageId)
+    if (!record) continue
+    if (record.sessionId !== sessionId) continue
+    if (record.role !== role) continue
+    if (record.status !== "sending") continue
+    if (!record.isEphemeral) continue
+    return record.id
+  }
+  return undefined
 }
 
 function handleMessageUpdate(instanceId: string, event: MessageUpdateEvent | MessagePartUpdatedEvent): void {
@@ -282,9 +285,9 @@ function handleMessageUpdate(instanceId: string, event: MessageUpdateEvent | Mes
 
     let record = store.getMessage(messageId)
     if (!record) {
-      const pendingId = findPendingMessageId(store, sessionId, role)
+      const pendingId = findPendingSyntheticMessageId(store, sessionId, role)
       if (pendingId && pendingId !== messageId) {
-        replaceMessageIdV2(instanceId, pendingId, messageId)
+        replaceMessageIdV2(instanceId, pendingId, messageId, { clearParts: role === "user" })
         record = store.getMessage(messageId)
       }
     }
@@ -345,9 +348,9 @@ function handleMessageUpdate(instanceId: string, event: MessageUpdateEvent | Mes
 
     let record = store.getMessage(messageId)
     if (!record) {
-      const pendingId = findPendingMessageId(store, sessionId, role)
+      const pendingId = findPendingSyntheticMessageId(store, sessionId, role)
       if (pendingId && pendingId !== messageId) {
-        replaceMessageIdV2(instanceId, pendingId, messageId)
+        replaceMessageIdV2(instanceId, pendingId, messageId, { clearParts: role === "user" })
         record = store.getMessage(messageId)
       }
     }