Looks good to me! Posting a few comments from AI, would be good to resolve them but not required. Approving and you can decide how to proceed.

Issues

1. JSDoc inaccurately describes toolCalls behavior — CopilotChatMessageView.tsx:308-313
   
   The JSDoc says "while keeping the latest toolCalls" but the implementation (lines 332-334) uses ?? which recovers earlier toolCalls when the latest is undefined. The inline comment on lines 330-331 is accurate and directly contradicts the JSDoc summary. Since the JSDoc is what consumers read first on this exported function, it should be corrected to say something like: "and similarly recovers toolCalls from earlier occurrences if the latest is undefined."

2. Test "uses latest content" does not assert content value — CopilotChatMessageView.test.tsx:136-165
   
   The test "uses latest content when all assistant duplicates have non-empty content" verifies message counts and absence of duplicate-key warnings, but never asserts which content value was kept. The test name claims latest content wins but nothing proves it. Should add:

expect(assistantMessages[0].textContent).toContain("Full response from the assistant.");

3. No test for empty-array [] vs undefined toolCalls — CopilotChatMessageView.test.tsx
   
   The ?? operator for toolCalls is the critical design choice distinguishing "explicitly no tool calls" ([]) from "field not provided" (undefined). No test locks in this behavior. If someone later changes ?? to ||, the regression would go undetected. Suggested test:

it("keeps empty toolCalls array from later chunk (does not fall back)", () => {
  const messages: Message[] = [
    assistantMsg("a-1", "", [toolCall("tc-1", "fn")]),
    assistantMsg("a-1", "Done.", []),
  ];
  const result = deduplicateMessages(messages);
  expect(result).toHaveLength(1);
  expect((result[0] as AssistantMessage).toolCalls).toEqual([]);
});

Suggestions

1. Add test for content: undefined (distinct from "")
   
   All tests use "" to represent wiped content, but AssistantMessage.content is string | undefined. A test with explicitly undefined content would document that || handles both cases and guard against a future || → ?? refactor.
2. Redundant as AssistantMessage type assertions
   
   Lines 328-334. Since Message is a ZodDiscriminatedUnion on role, checking message.role === "assistant" already narrows the type. The casts on lines 328-329 and 333-334 are redundant (the one on line 340 for acc.set() is justified due to TypeScript spread-type limitations).
3. Consider @internal annotation on export
   
   deduplicateMessages is exported for testability but not re-exported through the barrel file. Adding @internal to the JSDoc would signal this isn't part of the public API.(1) Missing changeset — CopilotChatMessageView.tsx
4. No .changeset/*.md file is present.
   
   The changeset bot has flagged this. This is a patch-level bug fix for @copilotkit/react-core and must be added before merge.