fix(core): add explicit empty log guard in A2A pushMessage (#26198)

packages/core/src/agents/a2aUtils.test.ts

@@ -585,5 +585,27 @@ describe('a2aUtils', () => {
         status: 'completed',
       });
     });
+
+    it('should correctly push the first message when messageLog is empty (Issue #24894)', () => {
+      const reassembler = new A2AResultReassembler();
+
+      const message: Message = {
+        kind: 'message',
+        role: 'agent',
+        messageId: 'm1',
+        parts: [{ kind: 'text', text: 'First message' }],
+      };
+
+      reassembler.update({
+        kind: 'status-update',
+        contextId: 'ctx1',
+        status: {
+          state: 'working',
+          message,
+        },
+      } as unknown as SendMessageResult);
+
+      expect(reassembler.toString()).toBe('First message');
+    });
   });
 });

packages/core/src/agents/a2aUtils.ts

@@ -126,7 +126,7 @@ export class A2AResultReassembler {
     if (!message) return;
     if (message.role === 'user') return; // Skip user messages reflected by server
     const text = extractPartsText(message.parts, '');
-    if (text && this.messageLog[this.messageLog.length - 1] !== text) {
+    if (text && this.messageLog.at(-1) !== text) {
       this.messageLog.push(text);
     }
   }