feat: isDelta property on Message for streaming

Author: acensia

src/a2a/server/events/event_consumer.py

@@ -102,7 +102,7 @@ async def consume_all(self) -> AsyncGenerator[Event]:
 
                 is_final_event = (
                     (isinstance(event, TaskStatusUpdateEvent) and event.final)
-                    or isinstance(event, Message)
+                    or isinstance(event, Message) and not event.isDelta
                     or (
                         isinstance(event, Task)
                         and event.status.state

src/a2a/server/tasks/task_manager.py

@@ -134,8 +134,10 @@ async def save_task_event(
             if task.status.message:
                 if not task.history:
                     task.history = [task.status.message]
-                else:
+                elif task.history[-1].messageId != task.status.message.messageId:
                     task.history.append(task.status.message)
+                elif task.status.message.isDelta:
+                    task.history[-1].parts.extend(task.status.message.parts)
             if event.metadata:
                 if not task.metadata:
                     task.metadata = {}

src/a2a/server/tasks/task_updater.py

@@ -182,4 +182,5 @@ def new_agent_message(
             messageId=str(uuid.uuid4()),
             metadata=metadata,
             parts=parts,
+            isDelta=False
         )

src/a2a/types.py

@@ -1336,6 +1336,10 @@ class Message(BaseModel):
     """
     Identifier of task the message is related to
     """
+    isDelta: bool | None = None
+    """
+    Indicates if this is a delta message
+    """
 
 
 class MessageSendParams(BaseModel):