feat: add real-time streaming of subagent AI messages

Enable task tool to capture and stream AI messages as they are generated by subagents. This replaces simple polling status updates with detailed message-level progress updates.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/src/subagents/executor.py

@@ -44,6 +44,7 @@ class SubagentResult:
         error: Error message (if failed).
         started_at: When execution started.
         completed_at: When execution completed.
+        ai_messages: List of complete AI messages (as dicts) generated during execution.
     """
 
     task_id: str
@@ -53,6 +54,12 @@ class SubagentResult:
     error: str | None = None
     started_at: datetime | None = None
     completed_at: datetime | None = None
+    ai_messages: list[dict[str, Any]] | None = None
+
+    def __post_init__(self):
+        """Initialize mutable defaults."""
+        if self.ai_messages is None:
+            self.ai_messages = []
 
 
 # Global storage for background task results
@@ -197,22 +204,28 @@ class SubagentExecutor:
 
         return state
 
-    def execute(self, task: str) -> SubagentResult:
+    def execute(self, task: str, result_holder: SubagentResult | None = None) -> SubagentResult:
         """Execute a task synchronously.
 
         Args:
             task: The task description for the subagent.
+            result_holder: Optional pre-created result object to update during execution.
 
         Returns:
             SubagentResult with the execution result.
         """
-        task_id = str(uuid.uuid4())[:8]
-        result = SubagentResult(
-            task_id=task_id,
-            trace_id=self.trace_id,
-            status=SubagentStatus.RUNNING,
-            started_at=datetime.now(),
-        )
+        if result_holder is not None:
+            # Use the provided result holder (for async execution with real-time updates)
+            result = result_holder
+        else:
+            # Create a new result for synchronous execution
+            task_id = str(uuid.uuid4())[:8]
+            result = SubagentResult(
+                task_id=task_id,
+                trace_id=self.trace_id,
+                status=SubagentStatus.RUNNING,
+                started_at=datetime.now(),
+            )
 
         try:
             agent = self._create_agent()
@@ -229,50 +242,74 @@ class SubagentExecutor:
 
             logger.info(f"[trace={self.trace_id}] Subagent {self.config.name} starting execution with max_turns={self.config.max_turns}")
 
-            # Run the agent using invoke for complete result
-            # Note: invoke() runs until completion or interruption
-            # Timeout is handled at the execute_async level, not here
-            final_state = agent.invoke(state, config=run_config, context=context)  # type: ignore[arg-type]
+            # Use stream instead of invoke to get real-time updates
+            # This allows us to collect AI messages as they are generated
+            final_state = None
+            for chunk in agent.stream(state, config=run_config, context=context, stream_mode="values"):  # type: ignore[arg-type]
+                final_state = chunk
+
+                # Extract AI messages from the current state
+                messages = chunk.get("messages", [])
+                if messages:
+                    last_message = messages[-1]
+                    # Check if this is a new AI message
+                    if isinstance(last_message, AIMessage):
+                        # Convert message to dict for serialization
+                        message_dict = last_message.model_dump()
+                        # Only add if it's not already in the list (avoid duplicates)
+                        # Check by comparing message IDs if available, otherwise compare full dict
+                        message_id = message_dict.get("id")
+                        is_duplicate = False
+                        if message_id:
+                            is_duplicate = any(msg.get("id") == message_id for msg in result.ai_messages)
+                        else:
+                            is_duplicate = message_dict in result.ai_messages
+
+                        if not is_duplicate:
+                            result.ai_messages.append(message_dict)
+                            logger.info(f"[trace={self.trace_id}] Subagent {self.config.name} captured AI message #{len(result.ai_messages)}")
 
             logger.info(f"[trace={self.trace_id}] Subagent {self.config.name} completed execution")
 
-            # Extract the final message - find the last AIMessage
-            messages = final_state.get("messages", [])
-            logger.info(f"[trace={self.trace_id}] Subagent {self.config.name} final messages count: {len(messages)}")
-
-            # Find the last AIMessage in the conversation
-            last_ai_message = None
-            for msg in reversed(messages):
-                if isinstance(msg, AIMessage):
-                    last_ai_message = msg
-                    break
-
-            if last_ai_message is not None:
-                content = last_ai_message.content
-                logger.info(f"[trace={self.trace_id}] Subagent {self.config.name} last AI message content type: {type(content)}")
-
-                # Handle both str and list content types
-                if isinstance(content, str):
-                    result.result = content
-                elif isinstance(content, list):
-                    # Extract text from list of content blocks
-                    text_parts = []
-                    for block in content:
-                        if isinstance(block, str):
-                            text_parts.append(block)
-                        elif isinstance(block, dict) and "text" in block:
-                            text_parts.append(block["text"])
-                    result.result = "\n".join(text_parts) if text_parts else "No text content in response"
-                else:
-                    result.result = str(content)
-            elif messages:
-                # Fallback: use the last message if no AIMessage found
-                last_message = messages[-1]
-                logger.warning(f"[trace={self.trace_id}] Subagent {self.config.name} no AIMessage found, using last message: {type(last_message)}")
-                result.result = str(last_message.content) if hasattr(last_message, "content") else str(last_message)
-            else:
-                logger.warning(f"[trace={self.trace_id}] Subagent {self.config.name} no messages in final state")
+            if final_state is None:
+                logger.warning(f"[trace={self.trace_id}] Subagent {self.config.name} no final state")
                 result.result = "No response generated"
+            else:
+                # Extract the final message - find the last AIMessage
+                messages = final_state.get("messages", [])
+                logger.info(f"[trace={self.trace_id}] Subagent {self.config.name} final messages count: {len(messages)}")
+
+                # Find the last AIMessage in the conversation
+                last_ai_message = None
+                for msg in reversed(messages):
+                    if isinstance(msg, AIMessage):
+                        last_ai_message = msg
+                        break
+
+                if last_ai_message is not None:
+                    content = last_ai_message.content
+                    # Handle both str and list content types for the final result
+                    if isinstance(content, str):
+                        result.result = content
+                    elif isinstance(content, list):
+                        # Extract text from list of content blocks for final result only
+                        text_parts = []
+                        for block in content:
+                            if isinstance(block, str):
+                                text_parts.append(block)
+                            elif isinstance(block, dict) and "text" in block:
+                                text_parts.append(block["text"])
+                        result.result = "\n".join(text_parts) if text_parts else "No text content in response"
+                    else:
+                        result.result = str(content)
+                elif messages:
+                    # Fallback: use the last message if no AIMessage found
+                    last_message = messages[-1]
+                    logger.warning(f"[trace={self.trace_id}] Subagent {self.config.name} no AIMessage found, using last message: {type(last_message)}")
+                    result.result = str(last_message.content) if hasattr(last_message, "content") else str(last_message)
+                else:
+                    logger.warning(f"[trace={self.trace_id}] Subagent {self.config.name} no messages in final state")
+                    result.result = "No response generated"
 
             result.status = SubagentStatus.COMPLETED
             result.completed_at = datetime.now()
@@ -316,10 +353,12 @@ class SubagentExecutor:
             with _background_tasks_lock:
                 _background_tasks[task_id].status = SubagentStatus.RUNNING
                 _background_tasks[task_id].started_at = datetime.now()
+                result_holder = _background_tasks[task_id]
 
             try:
                 # Submit execution to execution pool with timeout
-                execution_future: Future = _execution_pool.submit(self.execute, task)
+                # Pass result_holder so execute() can update it in real-time
+                execution_future: Future = _execution_pool.submit(self.execute, task, result_holder)
                 try:
                     # Wait for execution with timeout
                     exec_result = execution_future.result(timeout=self.config.timeout_seconds)
@@ -328,6 +367,7 @@ class SubagentExecutor:
                         _background_tasks[task_id].result = exec_result.result
                         _background_tasks[task_id].error = exec_result.error
                         _background_tasks[task_id].completed_at = datetime.now()
+                        _background_tasks[task_id].ai_messages = exec_result.ai_messages
                 except FuturesTimeoutError:
                     logger.error(
                         f"[trace={self.trace_id}] Subagent {self.config.name} execution timed out after {self.config.timeout_seconds}s"