feat(core): improve A2A content extraction (#20487)

Co-authored-by: Gal Zahavi <38544478+galz10@users.noreply.github.com>

packages/core/src/agents/a2aUtils.ts

@@ -74,6 +74,26 @@ export class A2AResultReassembler {
             ]);
           }
         }
+        // History Fallback: Some agent implementations do not populate the
+        // status.message in their final terminal response, instead archiving
+        // the final answer in the task's history array. To ensure we don't
+        // present an empty result, we fallback to the most recent agent message
+        // in the history only when the task is terminal and no other content
+        // (message log or artifacts) has been reassembled.
+        if (
+          isTerminalState(chunk.status?.state) &&
+          this.messageLog.length === 0 &&
+          this.artifacts.size === 0 &&
+          chunk.history &&
+          chunk.history.length > 0
+        ) {
+          const lastAgentMsg = [...chunk.history]
+            .reverse()
+            .find((m) => m.role?.toLowerCase().includes('agent'));
+          if (lastAgentMsg) {
+            this.pushMessage(lastAgentMsg);
+          }
+        }
         break;
 
       case 'message': {
@@ -126,7 +146,7 @@ export class A2AResultReassembler {
  * Handles Text, Data (JSON), and File parts.
  */
 export function extractMessageText(message: Message | undefined): string {
-  if (!message) {
+  if (!message || !message.parts || !Array.isArray(message.parts)) {
     return '';
   }
 
@@ -158,7 +178,6 @@ function extractPartText(part: Part): string {
   }
 
   if (isDataPart(part)) {
-    // Attempt to format known data types if metadata exists, otherwise JSON stringify
     return `Data: ${JSON.stringify(part.data)}`;
   }