diff --git a/ui/src/ui/chat/history-merge.test.ts b/ui/src/ui/chat/history-merge.test.ts
new file mode 100644
index 00000000000..9fc08f902a6
--- /dev/null
+++ b/ui/src/ui/chat/history-merge.test.ts
@@ -0,0 +1,54 @@
+import { describe, expect, it } from "vitest";
+import { preserveOptimisticTailMessages } from "../controllers/chat.ts";
+
+describe("preserveOptimisticTailMessages", () => {
+  it("keeps optimistic tail messages while history is stale", () => {
+    const persistedUser = {
+      role: "user",
+      content: [{ type: "text", text: "first" }],
+      __openclaw: { seq: 1 },
+    };
+    const optimisticUser = {
+      role: "user",
+      content: [{ type: "text", text: "latest ask" }],
+      timestamp: 10,
+    };
+    const optimisticAssistant = {
+      role: "assistant",
+      content: [{ type: "text", text: "latest answer" }],
+      timestamp: 11,
+    };
+
+    expect(
+      preserveOptimisticTailMessages(
+        [persistedUser],
+        [persistedUser, optimisticUser, optimisticAssistant],
+      ),
+    ).toEqual([persistedUser, optimisticUser, optimisticAssistant]);
+  });
+
+  it("drops streamed assistant tail when final history has caught up past the shared user", () => {
+    const persistedUser = {
+      role: "user",
+      content: [{ type: "text", text: "latest ask" }],
+      __openclaw: { seq: 1 },
+    };
+    const streamedAssistant = {
+      role: "assistant",
+      content: [{ type: "text", text: "partial streamed answer" }],
+      timestamp: 10,
+    };
+    const historyAssistant = {
+      role: "assistant",
+      content: [{ type: "text", text: "complete persisted answer" }],
+      __openclaw: { seq: 2 },
+    };
+
+    expect(
+      preserveOptimisticTailMessages(
+        [persistedUser, historyAssistant],
+        [persistedUser, streamedAssistant],
+      ),
+    ).toEqual([persistedUser, historyAssistant]);
+  });
+});
diff --git a/ui/src/ui/controllers/chat.ts b/ui/src/ui/controllers/chat.ts
index b45fe6aa526..6c03716b531 100644
--- a/ui/src/ui/controllers/chat.ts
+++ b/ui/src/ui/controllers/chat.ts
@@ -264,7 +264,7 @@ function messageDisplaySignature(message: unknown): string | null {
   }
 }
 
-function preserveOptimisticTailMessages(
+export function preserveOptimisticTailMessages(
   historyMessages: unknown[],
   previousMessages: unknown[],
 ): unknown[] {
@@ -279,29 +279,37 @@ function preserveOptimisticTailMessages(
       ? previousMessages
       : historyMessages;
   }
-  const historySignatures = new Set(
-    historyMessages
-      .map((message) => messageDisplaySignature(message))
-      .filter((signature): signature is string => Boolean(signature)),
-  );
+  const historySignatureIndexes = new Map<string, number>();
+  historyMessages.forEach((message, index) => {
+    const signature = messageDisplaySignature(message);
+    if (signature) {
+      historySignatureIndexes.set(signature, index);
+    }
+  });
   let sharedPreviousIndex = -1;
+  let sharedHistoryIndex = -1;
   for (let index = previousMessages.length - 1; index >= 0; index--) {
     const signature = messageDisplaySignature(previousMessages[index]);
-    if (signature && historySignatures.has(signature)) {
+    const historyIndex = signature ? historySignatureIndexes.get(signature) : undefined;
+    if (typeof historyIndex === "number") {
       sharedPreviousIndex = index;
+      sharedHistoryIndex = historyIndex;
       break;
     }
   }
   if (sharedPreviousIndex < 0) {
     return historyMessages;
   }
+  if (sharedHistoryIndex < historyMessages.length - 1) {
+    return historyMessages;
+  }
   const optimisticTail: unknown[] = [];
   for (const message of previousMessages.slice(sharedPreviousIndex + 1)) {
     if (!isLocallyOptimisticHistoryMessage(message) || shouldHideHistoryMessage(message)) {
       return historyMessages;
     }
     const signature = messageDisplaySignature(message);
-    if (!signature || historySignatures.has(signature)) {
+    if (!signature || historySignatureIndexes.has(signature)) {
       return historyMessages;
     }
     optimisticTail.push(message);
@@ -544,6 +552,9 @@ export async function sendChatMessage(
   if (!msg && !hasAttachments) {
     return null;
   }
+  if (state.chatSending) {
+    return state.chatRunId;
+  }
 
   const now = Date.now();