fix: resolve streaming not displaying for ReAct/simple agents

service/app/core/chat/langchain.py

@@ -467,6 +467,14 @@ async def _handle_messages_mode(data: Any, ctx: StreamContext) -> AsyncGenerator
         return
 
     if not ctx.is_streaming:
+        # Emit synthetic node_start if no node was detected
+        # This handles prebuilt agents (like ReAct) that don't include langgraph_node metadata
+        if ctx.event_ctx and not ctx.current_node:
+            node_start_event = AgentEventStreamHandler.create_node_start_event(ctx, "agent")
+            if node_start_event:
+                logger.info("[AgentEvent/Messages] Emitting synthetic node_start for 'agent'")
+                yield node_start_event
+
         logger.debug("Emitting streaming_start for stream_id=%s", ctx.stream_id)
         ctx.is_streaming = True
         yield StreamingEventHandler.create_streaming_start(ctx.stream_id)

web/src/components/layouts/components/ChatBubble.tsx

@@ -229,12 +229,39 @@ function ChatBubble({ message }: ChatBubbleProps) {
               {isLoading ? (
                 <LoadingMessage size="medium" className="text-sm" />
               ) : (
-                // Show markdownContent when:
-                // 1. No agentExecution (regular chat)
-                // 2. agentExecution is react type (simple agent without timeline)
-                // For multi-phase agents, content is shown in timeline phases
-                (!agentExecution || agentExecution.agentType === "react") &&
-                markdownContent
+                // Show content based on agent type:
+                // 1. No agentExecution: show message.content
+                // 2. React/simple agents with phases: show phase.streamedContent
+                // 3. React agents without phases: show message.content (fallback)
+                // 4. Multi-phase agents: content is shown in timeline phases
+                (() => {
+                  // No agent execution - regular chat
+                  if (!agentExecution) {
+                    return markdownContent;
+                  }
+
+                  // React/simple agents - show streaming content from phase or message.content
+                  if (agentExecution.agentType === "react") {
+                    // If we have phases with streamedContent, show that
+                    if (agentExecution.phases.length > 0) {
+                      const activePhase = agentExecution.phases.find(
+                        (p) => p.status === "running",
+                      );
+                      const phaseContent =
+                        activePhase?.streamedContent ||
+                        agentExecution.phases[agentExecution.phases.length - 1]
+                          ?.streamedContent;
+                      if (phaseContent) {
+                        return <Markdown content={phaseContent} />;
+                      }
+                    }
+                    // Fallback to message.content
+                    return markdownContent;
+                  }
+
+                  // Multi-phase agents - content is shown in timeline, don't show here
+                  return null;
+                })()
               )}
 
               {/* For multi-phase agents, show final report below timeline when completed */}

web/src/store/slices/chatSlice.ts

@@ -522,7 +522,25 @@ export const createChatSlice: StateCreator<
                 const agentMsgIndex = channel.messages.findLastIndex(
                   (m) => m.agentExecution?.status === "running",
                 );
+
                 if (agentMsgIndex !== -1) {
+                  const execution =
+                    channel.messages[agentMsgIndex].agentExecution;
+
+                  // Create default "Response" phase if no phases exist
+                  // This handles prebuilt agents (like ReAct) that don't emit node_start events
+                  if (execution && execution.phases.length === 0) {
+                    execution.phases.push({
+                      id: "response",
+                      name: "Response",
+                      status: "running",
+                      startedAt: Date.now(),
+                      nodes: [],
+                      streamedContent: "",
+                    });
+                    execution.currentNode = "response";
+                  }
+
                   // Agent execution exists, mark it as streaming but don't create a new message
                   // Content will be routed to phase.streamedContent in streaming_chunk
                   channel.messages[agentMsgIndex] = {