test(frontend/builder): restore seed-message tests + guard empty messages array

- Re-add describe block for seed message sending (removed in 8b8eb80480):
  - verifies sendMessage is called with buildSeedPrompt when isGraphLoaded=true
  - verifies sendMessage is NOT called when isGraphLoaded=false (default)
  - verifies the hasSentSeedMessageRef guard fires only once per session
- Add test for empty messages guard in prepareSendMessagesRequest
- Guard messages.at(-1) in prepareSendMessagesRequest with an early throw
  so a runtime TypeError cannot occur if the AI SDK contract is violated

autogpt_platform/frontend/src/app/(platform)/build/components/BuilderChatPanel/__tests__/useBuilderChatPanel.test.ts

@@ -252,6 +252,58 @@ describe("useBuilderChatPanel – no auto-send on open", () => {
   });
 });
 
+describe("useBuilderChatPanel – seed message", () => {
+  it("sends seed message via sendMessage when session is available and isGraphLoaded=true", async () => {
+    mockPostV2CreateSession.mockResolvedValue({
+      status: 200,
+      data: { id: "sess-seed" },
+    });
+    mockNodes.push({ id: "n1", data: { title: "Search", description: "" } });
+
+    const { result } = renderHook(() =>
+      useBuilderChatPanel({ isGraphLoaded: true }),
+    );
+
+    await openAndFlush(() => result.current.handleToggle());
+
+    expect(mockSendMessage).toHaveBeenCalledOnce();
+    const callArg = mockSendMessage.mock.calls[0][0] as { text: string };
+    expect(typeof callArg.text).toBe("string");
+    expect(callArg.text).toContain("I'm building an agent");
+  });
+
+  it("does NOT send seed message when isGraphLoaded is false (default)", async () => {
+    mockPostV2CreateSession.mockResolvedValue({
+      status: 200,
+      data: { id: "sess-no-seed" },
+    });
+
+    const { result } = renderHook(() => useBuilderChatPanel());
+
+    await openAndFlush(() => result.current.handleToggle());
+
+    expect(mockSendMessage).not.toHaveBeenCalled();
+  });
+
+  it("sends seed message only once even when sessionId and isGraphLoaded deps re-run (hasSentSeedMessageRef guard)", async () => {
+    mockPostV2CreateSession.mockResolvedValue({
+      status: 200,
+      data: { id: "sess-once" },
+    });
+
+    const { result, rerender } = renderHook(() =>
+      useBuilderChatPanel({ isGraphLoaded: true }),
+    );
+
+    await openAndFlush(() => result.current.handleToggle());
+    expect(mockSendMessage).toHaveBeenCalledOnce();
+
+    rerender();
+
+    expect(mockSendMessage).toHaveBeenCalledOnce();
+  });
+});
+
 describe("useBuilderChatPanel – flowID reset", () => {
   it("resets appliedActionKeys when flowID changes", () => {
     mockNodes.push({ id: "n1", data: { hardcodedValues: {} } });
@@ -1208,6 +1260,31 @@ describe("useBuilderChatPanel – transport prepareSendMessagesRequest", () => {
       ctorArg.prepareSendMessagesRequest({ messages }),
     ).rejects.toThrow("Authentication failed");
   });
+
+  it("throws when messages array is empty (empty messages guard)", async () => {
+    const { DefaultChatTransport } = await import("ai");
+    const MockTransport = DefaultChatTransport as ReturnType<typeof vi.fn>;
+
+    mockPostV2CreateSession.mockResolvedValue({
+      status: 200,
+      data: { id: "sess-empty-msg" },
+    });
+
+    const { result } = renderHook(() => useBuilderChatPanel());
+
+    await openAndFlush(() => result.current.handleToggle());
+
+    const ctorArg = MockTransport.mock.calls[
+      MockTransport.mock.calls.length - 1
+    ][0] as {
+      prepareSendMessagesRequest: (args: {
+        messages: unknown[];
+      }) => Promise<unknown>;
+    };
+    await expect(
+      ctorArg.prepareSendMessagesRequest({ messages: [] }),
+    ).rejects.toThrow("No message to send");
+  });
 });
 
 describe("useBuilderChatPanel – handleKeyDown empty input guard", () => {

autogpt_platform/frontend/src/app/(platform)/build/components/BuilderChatPanel/useBuilderChatPanel.ts

@@ -207,7 +207,11 @@ export function useBuilderChatPanel({
         ? new DefaultChatTransport({
             api: `${environment.getAGPTServerBaseUrl()}/api/chat/sessions/${sessionId}/stream`,
             prepareSendMessagesRequest: async ({ messages }) => {
-              const last = messages[messages.length - 1];
+              const last = messages.at(-1);
+              if (!last)
+                throw new Error(
+                  "No message to send — messages array is empty.",
+                );
               const { token, error } = await getWebSocketToken();
               if (error || !token)
                 throw new Error(