feat: isDelta property on Message for streaming (#267)

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

@@ -78,6 +78,23 @@ async def get_task(self) -> Task | None:
             logger.debug('Task %s not found.', self.task_id)
         return self._current_task
 
+    async def add_history_message(self, task: Task) -> None:
+        """Adds a message to the task's history.
+
+        If the message is not a delta, it appends the message to the history.
+        If the message is a delta, but with the same messageId, it extends the last message in the history.
+        If the message is a delta, but with a different messageId, it appends the message to the history.
+
+        Args:
+            task: The task with the message to add to the history.
+        """
+        if not task.history:
+            task.history = [task.status.message]
+        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)
+
     async def save_task_event(
         self, event: Task | TaskStatusUpdateEvent | TaskArtifactUpdateEvent
     ) -> Task | None:
@@ -132,10 +149,7 @@ async def save_task_event(
                 'Updating task %s status to: %s', task.id, event.status.state
             )
             if task.status.message:
-                if not task.history:
-                    task.history = [task.status.message]
-                else:
-                    task.history.append(task.status.message)
+                await self.add_history_message(task)
             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):