diff --git a/packages/ui/src/stores/session-events.ts b/packages/ui/src/stores/session-events.ts
index 90430b1f5..0df24cc6e 100644
--- a/packages/ui/src/stores/session-events.ts
+++ b/packages/ui/src/stores/session-events.ts
@@ -327,7 +327,7 @@ function findPendingSyntheticMessageId(
     if (!record) continue
     if (record.sessionId !== sessionId) continue
     if (record.role !== role) continue
-    if (record.status !== "sending") continue
+    if (record.status !== "sending" && record.status !== "sent") continue
     if (!record.isEphemeral) continue
     return record.id
   }
@@ -384,6 +384,12 @@ function handleMessageUpdate(instanceId: string, event: MessageUpdateEvent | Mes
       upsertMessageInfoV2(instanceId, messageInfo, { status: "streaming" })
     }
   
+    // Clear any pending deltas for this part before applying the full part update.
+    // The part update contains the complete state from the server, so accumulated
+    // deltas would be stale and cause duplication if flushed later.
+    if (part.id) {
+      clearPendingDeltasForPart(instanceId, messageId, part.id)
+    }
     applyPartUpdateV2(instanceId, { ...part, sessionID: sessionId, messageID: messageId })
     handleConversationAssistantPartUpdated(instanceId, { ...part, sessionID: sessionId, messageID: messageId }, messageInfo)
 
@@ -402,6 +408,14 @@ function handleMessageUpdate(instanceId: string, event: MessageUpdateEvent | Mes
     const messageId = typeof info.id === "string" ? info.id : undefined
     if (!sessionId || !messageId) return
 
+    // Flush any pending deltas for this message before applying the update.
+    // Deltas are buffered for up to 50ms; if message.updated arrives before
+    // the buffer flushes, the message could be marked complete/error with
+    // stale text mutations still pending. Flushing first preserves the
+    // server's event ordering: all delta content is applied, then the
+    // message status/metadata update runs on the complete content.
+    flushPendingDeltasForMessage(instanceId, messageId)
+
     const timeInfo = (info.time ?? {}) as { created?: number; updated?: number; end?: number }
     const nextUpdated =
       typeof timeInfo.end === "number" && timeInfo.end > 0
@@ -453,12 +467,67 @@ function handleMessageUpdate(instanceId: string, event: MessageUpdateEvent | Mes
   }
 }
 
+const DELTA_FLUSH_INTERVAL = 50
+
+const pendingDeltas = new Map<string, { instanceId: string; messageId: string; partId: string; field: string; delta: string }>()
+let deltaFlushTimer: ReturnType<typeof setTimeout> | null = null
+
+function enqueueDelta(instanceId: string, messageId: string, partId: string, field: string, delta: string) {
+  const key = `${instanceId}:${messageId}:${partId}:${field}`
+  const existing = pendingDeltas.get(key)
+  const accumulated = existing ? existing.delta + delta : delta
+  pendingDeltas.set(key, { instanceId, messageId, partId, field, delta: accumulated })
+  if (deltaFlushTimer === null) {
+    deltaFlushTimer = setTimeout(flushDeltas, DELTA_FLUSH_INTERVAL)
+  }
+}
+
+function clearPendingDeltasForPart(instanceId: string, messageId: string, partId: string) {
+  const keysToDelete: string[] = []
+  for (const key of pendingDeltas.keys()) {
+    if (key.startsWith(`${instanceId}:${messageId}:${partId}:`)) {
+      keysToDelete.push(key)
+    }
+  }
+  for (const key of keysToDelete) {
+    pendingDeltas.delete(key)
+  }
+}
+
+function flushPendingDeltasForMessage(instanceId: string, messageId: string): void {
+  const prefix = `${instanceId}:${messageId}:`
+  for (const key of pendingDeltas.keys()) {
+    if (key.startsWith(prefix)) {
+      const pending = pendingDeltas.get(key)
+      if (pending) {
+        applyPartDeltaV2(instanceId, {
+          messageId: pending.messageId,
+          partId: pending.partId,
+          field: pending.field,
+          delta: pending.delta,
+        })
+        pendingDeltas.delete(key)
+      }
+    }
+  }
+}
+
+function flushDeltas() {
+  deltaFlushTimer = null
+  if (pendingDeltas.size === 0) return
+  const batch = Array.from(pendingDeltas.values())
+  pendingDeltas.clear()
+  for (const { instanceId, messageId, partId, field, delta } of batch) {
+    applyPartDeltaV2(instanceId, { messageId, partId, field, delta })
+  }
+}
+
 function handleMessagePartDelta(instanceId: string, event: MessagePartDeltaEvent): void {
   const props = event.properties
   if (!props) return
   const { messageID, partID, field, delta } = props
   if (!messageID || !partID || !field || typeof delta !== "string") return
-  applyPartDeltaV2(instanceId, { messageId: messageID, partId: partID, field, delta })
+  enqueueDelta(instanceId, messageID, partID, field, delta)
 }
 
 function handleSessionUpdate(instanceId: string, event: EventSessionUpdated): void {