refactor(react-core): extract deduplicateMessages, add useMemo, expand test coverage

- Extract dedup logic into exported deduplicateMessages() pure function so
  it can be unit-tested directly
- Wrap with useMemo([messages]) to avoid allocating a new Map on every frame
- Collapse three-way branch to two-way (the two arms were identical)
- Use { ...existing, ...message, content } for a true merge so fields present
  only in an earlier occurrence are not silently dropped
- Expand comment to explain why || treats empty string as falsy
- Add deduplicateMessages unit tests: toolCalls assertion, reverse scenario,
  non-assistant keep-last behavior
- Rename "keeping the last occurrence" test — no longer accurate for assistant
  messages which now merge rather than keep-last

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: marthakelly

packages/react-core/src/v2/components/chat/CopilotChatMessageView.tsx

@@ -1,4 +1,4 @@
-import React, { useEffect, useReducer, useState } from "react";
+import React, { useEffect, useMemo, useReducer, useState } from "react";
 import { WithSlots, renderSlot, isReactComponentType } from "../../lib/slots";
 import CopilotChatAssistantMessage from "./CopilotChatAssistantMessage";
 import CopilotChatUserMessage from "./CopilotChatUserMessage";
@@ -273,6 +273,40 @@ const MemoizedCustomMessage = React.memo(
   },
 );
 
+/**
+ * Deduplicates messages by ID. For assistant messages, merges occurrences:
+ * recovers non-empty content from any earlier occurrence if the latest wiped it
+ * (empty string means the streaming update cleared the field, not blank text),
+ * while keeping the latest toolCalls and all other fields. For all other roles,
+ * keeps the last entry.
+ */
+export function deduplicateMessages(messages: Message[]): Message[] {
+  const acc = new Map<string, Message>();
+  for (const message of messages) {
+    const existing = acc.get(message.id);
+    if (
+      existing &&
+      message.role === "assistant" &&
+      existing.role === "assistant"
+    ) {
+      // Empty string means the streaming update cleared the field — fall back to
+      // any non-empty content seen earlier. Use { ...existing, ...message } so
+      // fields present only in an earlier occurrence are not silently dropped.
+      const content =
+        (message as AssistantMessage).content ||
+        (existing as AssistantMessage).content;
+      acc.set(message.id, {
+        ...existing,
+        ...message,
+        content,
+      } as AssistantMessage);
+    } else {
+      acc.set(message.id, message);
+    }
+  }
+  return [...acc.values()];
+}
+
 export type CopilotChatMessageViewProps = Omit<
   WithSlots<
     {
@@ -362,24 +396,10 @@ export function CopilotChatMessageView({
     );
   };
 
-  // Deduplicate messages by id. During streaming, the same message ID can appear
-  // multiple times (e.g. a text chunk followed by tool-call updates that clear
-  // the content field). For assistant messages we merge: keep the latest
-  // toolCalls (they accumulate) but recover non-empty content from any earlier
-  // occurrence if the latest wiped it. For all other roles, keep the last entry.
-  const messageAccumulator = new Map<string, Message>();
-  for (const message of messages) {
-    const existing = messageAccumulator.get(message.id);
-    if (!existing) {
-      messageAccumulator.set(message.id, message);
-    } else if (message.role === "assistant" && existing.role === "assistant") {
-      const content = message.content || existing.content;
-      messageAccumulator.set(message.id, { ...message, content });
-    } else {
-      messageAccumulator.set(message.id, message);
-    }
-  }
-  const deduplicatedMessages = [...messageAccumulator.values()];
+  const deduplicatedMessages = useMemo(
+    () => deduplicateMessages(messages),
+    [messages],
+  );
 
   if (
     process.env.NODE_ENV === "development" &&

packages/react-core/src/v2/components/chat/__tests__/CopilotChatMessageView.test.tsx

@@ -4,7 +4,9 @@ import { z } from "zod";
 import { vi } from "vitest";
 import { CopilotKitProvider } from "../../../providers/CopilotKitProvider";
 import { CopilotChatConfigurationProvider } from "../../../providers/CopilotChatConfigurationProvider";
-import CopilotChatMessageView from "../CopilotChatMessageView";
+import CopilotChatMessageView, {
+  deduplicateMessages,
+} from "../CopilotChatMessageView";
 import type {
   ActivityMessage,
   AssistantMessage,
@@ -131,7 +133,7 @@ describe("CopilotChatMessageView duplicate message deduplication", () => {
     expect(assistantMessages[0].textContent).toContain("Let me record that...");
   });
 
-  it("deduplicates messages with the same id, keeping the last occurrence", () => {
+  it("uses latest content when all assistant duplicates have non-empty content", () => {
     const messages: Message[] = [
       userMsg("user-1", "Hello"),
       assistantMsg("assistant-1", "Partial response..."),
@@ -162,7 +164,7 @@ describe("CopilotChatMessageView duplicate message deduplication", () => {
     consoleSpy.mockRestore();
   });
 
-  it("preserves order of unique messages", () => {
+  it("preserves order of unique messages (no duplicates)", () => {
     const messages: Message[] = [
       userMsg("user-1", "First question"),
       assistantMsg("assistant-1", "First answer"),
@@ -180,3 +182,50 @@ describe("CopilotChatMessageView duplicate message deduplication", () => {
     expect(assistantMessages).toHaveLength(2);
   });
 });
+
+describe("deduplicateMessages", () => {
+  it("recovers non-empty content and keeps latest toolCalls when later duplicate clears content", () => {
+    const messages: Message[] = [
+      assistantMsg("assistant-1", "Let me record that..."),
+      assistantMsg("assistant-1", "", [toolCall("tc-1", "captureData")]),
+      assistantMsg("assistant-1", "", [
+        toolCall("tc-1", "captureData"),
+        toolCall("tc-2", "updateMemory"),
+      ]),
+    ];
+
+    const result = deduplicateMessages(messages);
+
+    expect(result).toHaveLength(1);
+    const merged = result[0] as AssistantMessage;
+    // Content recovered from the first occurrence
+    expect(merged.content).toBe("Let me record that...");
+    // toolCalls from the latest occurrence (both tc-1 and tc-2)
+    expect(merged.toolCalls).toHaveLength(2);
+    expect(merged.toolCalls?.map((tc) => tc.id)).toEqual(["tc-1", "tc-2"]);
+  });
+
+  it("uses content from a later occurrence when early occurrence has empty content", () => {
+    const messages: Message[] = [
+      assistantMsg("assistant-1", ""),
+      assistantMsg("assistant-1", "Here is the result."),
+    ];
+
+    const result = deduplicateMessages(messages);
+
+    expect(result).toHaveLength(1);
+    expect((result[0] as AssistantMessage).content).toBe("Here is the result.");
+  });
+
+  it("keeps last entry for non-assistant roles", () => {
+    const messages: Message[] = [
+      userMsg("u-1", "Hello"),
+      userMsg("u-1", "Hello (updated)"),
+    ];
+
+    const result = deduplicateMessages(messages);
+
+    expect(result).toHaveLength(1);
+    expect((result[0] as UserMessage).content).toBe("Hello (updated)");
+  });
+});