Python: Fix DevUI streaming memory growth and add cross-platform regression coverage (#5221)

* fix for memory leak in devui

* update async sleep

* remove old func

python/packages/devui/frontend/src/App.tsx

@@ -3,7 +3,7 @@
  * Features: Entity selection, layout management, debug coordination
  */
 
-import { useEffect, useCallback, useState } from "react";
+import { useEffect, useCallback, useRef, useState } from "react";
 import { AppHeader, DebugPanel, SettingsModal, DeploymentModal } from "@/components/layout";
 import { GalleryView } from "@/components/features/gallery";
 import { AgentView } from "@/components/features/agent";
@@ -15,17 +15,22 @@ import type {
   AgentInfo,
   WorkflowInfo,
   ExtendedResponseStreamEvent,
+  ResponseTextDeltaEvent,
 } from "@/types";
 import { Button } from "./components/ui/button";
 import { Input } from "./components/ui/input";
 import { useDevUIStore } from "@/stores";
 
+const DEBUG_TEXT_EVENT_FLUSH_INTERVAL_MS = 50;
+
 export default function App() {
   // Local state for auth handling
   const [authRequired, setAuthRequired] = useState(false);
   const [authToken, setAuthToken] = useState("");
   const [isTestingToken, setIsTestingToken] = useState(false);
   const [authError, setAuthError] = useState("");
+  const bufferedDebugTextRef = useRef<ResponseTextDeltaEvent | null>(null);
+  const lastBufferedDebugFlushAtRef = useRef(0);
 
   // Entity state from Zustand
   const agents = useDevUIStore((state) => state.agents);
@@ -303,16 +308,63 @@ export default function App() {
     [selectEntity, updateAgent, updateWorkflow, addToast]
   );
 
+  const flushBufferedDebugText = useCallback(() => {
+    const bufferedEvent = bufferedDebugTextRef.current;
+    if (!bufferedEvent) {
+      return;
+    }
+
+    bufferedDebugTextRef.current = null;
+    lastBufferedDebugFlushAtRef.current = performance.now();
+    addDebugEvent(bufferedEvent);
+  }, [addDebugEvent]);
+
   // Handle debug events from active view
   const handleDebugEvent = useCallback(
     (event: ExtendedResponseStreamEvent | "clear") => {
       if (event === "clear") {
+        bufferedDebugTextRef.current = null;
         clearDebugEvents();
-      } else {
-        addDebugEvent(event);
+        return;
       }
+
+      if (
+        event.type === "response.output_text.delta" &&
+        "delta" in event &&
+        typeof event.delta === "string" &&
+        event.delta.length > 0
+      ) {
+        const bufferedEvent = bufferedDebugTextRef.current;
+        const isSameOutput =
+          bufferedEvent !== null &&
+          bufferedEvent.item_id === event.item_id &&
+          bufferedEvent.output_index === event.output_index &&
+          bufferedEvent.content_index === event.content_index;
+
+        if (isSameOutput && bufferedEvent) {
+          bufferedDebugTextRef.current = {
+            ...bufferedEvent,
+            delta: bufferedEvent.delta + event.delta,
+            sequence_number: event.sequence_number ?? bufferedEvent.sequence_number,
+          };
+        } else {
+          flushBufferedDebugText();
+          bufferedDebugTextRef.current = { ...event } as ResponseTextDeltaEvent;
+        }
+
+        if (
+          performance.now() - lastBufferedDebugFlushAtRef.current >=
+          DEBUG_TEXT_EVENT_FLUSH_INTERVAL_MS
+        ) {
+          flushBufferedDebugText();
+        }
+        return;
+      }
+
+      flushBufferedDebugText();
+      addDebugEvent(event);
     },
-    [addDebugEvent, clearDebugEvents]
+    [addDebugEvent, clearDebugEvents, flushBufferedDebugText]
   );
 
   // Show loading state while initializing

python/packages/devui/frontend/src/components/features/agent/agent-view.tsx

@@ -44,6 +44,8 @@ import { loadStreamingState } from "@/services/streaming-state";
 
 type DebugEventHandler = (event: ExtendedResponseStreamEvent | "clear") => void;
 
+const ASSISTANT_TEXT_RENDER_INTERVAL_MS = 50;
+
 interface AgentViewProps {
   selectedAgent: AgentInfo;
   onDebugEvent: DebugEventHandler;
@@ -309,6 +311,71 @@ export function AgentView({ selectedAgent, onDebugEvent }: AgentViewProps) {
   } | null>(null);
   const userJustSentMessage = useRef<boolean>(false);
   const accumulatedTextRef = useRef<string>("");
+  const lastAssistantTextRenderAt = useRef(0);
+
+  const renderAssistantStreamingText = useCallback(
+    (
+      assistantMessageId: string,
+      status: "in_progress" | "completed" | "incomplete" = "in_progress",
+      force: boolean = false
+    ) => {
+      const now = performance.now();
+      if (
+        !force &&
+        now - lastAssistantTextRenderAt.current < ASSISTANT_TEXT_RENDER_INTERVAL_MS
+      ) {
+        return;
+      }
+
+      const currentItems = useDevUIStore.getState().chatItems;
+      let changed = false;
+      const nextItems = currentItems.map((item) => {
+        if (item.id !== assistantMessageId || item.type !== "message") {
+          return item;
+        }
+
+        const nextText = accumulatedTextRef.current;
+        const existingTextContent = item.content.find(
+          (content) => content.type === "text" || content.type === "output_text"
+        );
+        const currentText =
+          existingTextContent && "text" in existingTextContent
+            ? existingTextContent.text
+            : "";
+
+        if (currentText === nextText && item.status === status) {
+          return item;
+        }
+
+        changed = true;
+        const existingNonTextContent = item.content.filter(
+          (content) => content.type !== "text" && content.type !== "output_text"
+        );
+
+        return {
+          ...item,
+          content: nextText
+            ? [
+                ...existingNonTextContent,
+                {
+                  type: "text",
+                  text: nextText,
+                } as import("@/types/openai").MessageTextContent,
+              ]
+            : existingNonTextContent,
+          status,
+        };
+      });
+
+      if (changed) {
+        lastAssistantTextRenderAt.current = now;
+        setChatItems(nextItems);
+      } else if (force) {
+        lastAssistantTextRenderAt.current = now;
+      }
+    },
+    [setChatItems]
+  );
 
   // Auto-scroll to bottom when new items arrive
   useEffect(() => {
@@ -382,6 +449,7 @@ export function AgentView({ selectedAgent, onDebugEvent }: AgentViewProps) {
           undefined,  // No abort signal for resume
           storedState.responseId  // Pass response ID for resume
         );
+        lastAssistantTextRenderAt.current = 0;
 
         for await (const openAIEvent of streamGenerator) {
           // Pass all events to debug panel
@@ -412,6 +480,12 @@ export function AgentView({ selectedAgent, onDebugEvent }: AgentViewProps) {
                 : JSON.stringify(error)
               : "Request failed";
 
+            if (accumulatedTextRef.current) {
+              renderAssistantStreamingText(assistantMessage.id, "incomplete", true);
+              setIsStreaming(false);
+              return;
+            }
+
             const currentItems = useDevUIStore.getState().chatItems;
             setChatItems(currentItems.map((item) =>
               item.id === assistantMessage.id && item.type === "message"
@@ -434,6 +508,7 @@ export function AgentView({ selectedAgent, onDebugEvent }: AgentViewProps) {
           // Handle function approval request events
           if (openAIEvent.type === "response.function_approval.requested") {
             const approvalEvent = openAIEvent as import("@/types/openai").ResponseFunctionApprovalRequestedEvent;
+            renderAssistantStreamingText(assistantMessage.id, "in_progress", true);
             setPendingApprovals([
               ...useDevUIStore.getState().pendingApprovals,
               {
@@ -458,6 +533,12 @@ export function AgentView({ selectedAgent, onDebugEvent }: AgentViewProps) {
             const errorEvent = openAIEvent as ExtendedResponseStreamEvent & { message?: string };
             const errorMessage = errorEvent.message || "An error occurred";
 
+            if (accumulatedTextRef.current) {
+              renderAssistantStreamingText(assistantMessage.id, "incomplete", true);
+              setIsStreaming(false);
+              return;
+            }
+
             const currentItems = useDevUIStore.getState().chatItems;
             setChatItems(currentItems.map((item) =>
               item.id === assistantMessage.id && item.type === "message"
@@ -484,27 +565,13 @@ export function AgentView({ selectedAgent, onDebugEvent }: AgentViewProps) {
             openAIEvent.delta
           ) {
             accumulatedTextRef.current += openAIEvent.delta;
-
-            const currentItems = useDevUIStore.getState().chatItems;
-            setChatItems(currentItems.map((item) =>
-              item.id === assistantMessage.id && item.type === "message"
-                ? {
-                    ...item,
-                    content: [
-                      {
-                        type: "text",
-                        text: accumulatedTextRef.current,
-                      } as import("@/types/openai").MessageTextContent,
-                    ],
-                    status: "in_progress" as const,
-                  }
-                : item
-            ));
+            renderAssistantStreamingText(assistantMessage.id);
           }
         }
 
         // Stream ended - mark as complete
         const finalUsage = currentMessageUsage.current;
+        renderAssistantStreamingText(assistantMessage.id, "in_progress", true);
 
         const currentItems = useDevUIStore.getState().chatItems;
         setChatItems(currentItems.map((item) =>
@@ -721,11 +788,12 @@ export function AgentView({ selectedAgent, onDebugEvent }: AgentViewProps) {
     setIsStreaming(false);
     setCurrentConversation(undefined);
     accumulatedTextRef.current = "";
+    lastAssistantTextRenderAt.current = 0;
 
     loadConversations();
     // currentConversation is intentionally excluded - this effect should only run when agent changes
     // eslint-disable-next-line react-hooks/exhaustive-deps
-  }, [selectedAgent, onDebugEvent, setChatItems, setIsStreaming, setLoadingConversations, setAvailableConversations, setCurrentConversation, setPendingApprovals, updateConversationUsage]);
+  }, [selectedAgent, onDebugEvent, renderAssistantStreamingText, setChatItems, setIsStreaming, setLoadingConversations, setAvailableConversations, setCurrentConversation, setPendingApprovals, updateConversationUsage]);
 
   // Removed old input handling functions - now handled by ChatMessageInput component
 
@@ -1118,6 +1186,7 @@ export function AgentView({ selectedAgent, onDebugEvent }: AgentViewProps) {
 
         // Clear text accumulator for new response
         accumulatedTextRef.current = "";
+        lastAssistantTextRenderAt.current = 0;
 
         // Create new AbortController for this request
         const signal = createAbortSignal();
@@ -1167,6 +1236,12 @@ export function AgentView({ selectedAgent, onDebugEvent }: AgentViewProps) {
             }
 
             // Update assistant message with error
+            if (accumulatedTextRef.current) {
+              renderAssistantStreamingText(assistantMessage.id, "incomplete", true);
+              setIsStreaming(false);
+              return; // Exit stream processing on failure
+            }
+
             const currentItems = useDevUIStore.getState().chatItems;
             setChatItems(currentItems.map((item) =>
               item.id === assistantMessage.id && item.type === "message"
@@ -1189,6 +1264,7 @@ export function AgentView({ selectedAgent, onDebugEvent }: AgentViewProps) {
           // Handle function approval request events
           if (openAIEvent.type === "response.function_approval.requested") {
             const approvalEvent = openAIEvent as import("@/types/openai").ResponseFunctionApprovalRequestedEvent;
+            renderAssistantStreamingText(assistantMessage.id, "in_progress", true);
 
             // Add to pending approvals (for popup)
             setPendingApprovals([
@@ -1267,6 +1343,12 @@ export function AgentView({ selectedAgent, onDebugEvent }: AgentViewProps) {
             const errorMessage = errorEvent.message || "An error occurred";
 
             // Update assistant message with error and stop streaming
+            if (accumulatedTextRef.current) {
+              renderAssistantStreamingText(assistantMessage.id, "incomplete", true);
+              setIsStreaming(false);
+              return; // Exit stream processing early on error
+            }
+
             const currentItems = useDevUIStore.getState().chatItems;
             setChatItems(currentItems.map((item) =>
               item.id === assistantMessage.id && item.type === "message"
@@ -1290,6 +1372,7 @@ export function AgentView({ selectedAgent, onDebugEvent }: AgentViewProps) {
           if (openAIEvent.type === "response.output_item.added") {
             const outputItemEvent = openAIEvent as import("@/types/openai").ResponseOutputItemAddedEvent;
             const item = outputItemEvent.item;
+            renderAssistantStreamingText(assistantMessage.id, "in_progress", true);
 
             // Handle function calls as separate conversation items
             if (item.type === "function_call") {
@@ -1363,28 +1446,7 @@ export function AgentView({ selectedAgent, onDebugEvent }: AgentViewProps) {
             openAIEvent.delta
           ) {
             accumulatedTextRef.current += openAIEvent.delta;
-
-            // Update assistant message with accumulated content
-            // Preserve any existing non-text content (images, files, data)
-            const currentItems = useDevUIStore.getState().chatItems;
-            setChatItems(currentItems.map((item) => {
-              if (item.id === assistantMessage.id && item.type === "message") {
-                // Keep existing non-text content, update text content
-                const existingNonTextContent = item.content.filter(c => c.type !== "text");
-                return {
-                  ...item,
-                  content: [
-                    ...existingNonTextContent,
-                    {
-                      type: "text",
-                      text: accumulatedTextRef.current,
-                    } as import("@/types/openai").MessageTextContent,
-                  ],
-                  status: "in_progress" as const,
-                };
-              }
-              return item;
-            }));
+            renderAssistantStreamingText(assistantMessage.id);
           }
 
           // Handle completion/error by detecting when streaming stops
@@ -1394,6 +1456,7 @@ export function AgentView({ selectedAgent, onDebugEvent }: AgentViewProps) {
         // Stream ended - mark as complete
         // Usage is provided via response.completed event (OpenAI standard)
         const finalUsage = currentMessageUsage.current;
+        renderAssistantStreamingText(assistantMessage.id, "in_progress", true);
 
         const currentItems = useDevUIStore.getState().chatItems;
         setChatItems(currentItems.map((item) =>
@@ -1419,45 +1482,42 @@ export function AgentView({ selectedAgent, onDebugEvent }: AgentViewProps) {
         if (isAbortError(error)) {
           // User cancelled - mark as cancelled for UI feedback
           setWasCancelled(true);
-          // Mark the message as completed with what we have
-          const currentItems = useDevUIStore.getState().chatItems;
-          setChatItems(currentItems.map((item) =>
-            item.id === assistantMessage.id && item.type === "message"
-              ? {
-                  ...item,
-                  status: accumulatedTextRef.current ? "completed" as const : "incomplete" as const,
-                  // Keep whatever text we have accumulated
-                  content: item.content,
-                }
-              : item
-          ));
+          renderAssistantStreamingText(
+            assistantMessage.id,
+            accumulatedTextRef.current ? "completed" : "incomplete",
+            true
+          );
         } else {
           // Other errors - show error message
-          const currentItems = useDevUIStore.getState().chatItems;
-          setChatItems(currentItems.map((item) =>
-            item.id === assistantMessage.id && item.type === "message"
-              ? {
-                  ...item,
-                  content: [
-                    {
-                      type: "text",
-                      text: `Error: ${
-                        error instanceof Error
-                          ? error.message
-                          : "Failed to get response"
-                      }`,
-                    } as import("@/types/openai").MessageTextContent,
-                  ],
-                  status: "incomplete" as const,
-                }
-              : item
-          ));
+          if (accumulatedTextRef.current) {
+            renderAssistantStreamingText(assistantMessage.id, "incomplete", true);
+          } else {
+            const currentItems = useDevUIStore.getState().chatItems;
+            setChatItems(currentItems.map((item) =>
+              item.id === assistantMessage.id && item.type === "message"
+                ? {
+                    ...item,
+                    content: [
+                      {
+                        type: "text",
+                        text: `Error: ${
+                          error instanceof Error
+                            ? error.message
+                            : "Failed to get response"
+                        }`,
+                      } as import("@/types/openai").MessageTextContent,
+                    ],
+                    status: "incomplete" as const,
+                  }
+                : item
+            ));
+          }
         }
         setIsStreaming(false);
         resetCancelling();
       }
     },
-    [selectedAgent, currentConversation, onDebugEvent, setChatItems, setIsStreaming, setCurrentConversation, setAvailableConversations, setPendingApprovals, updateConversationUsage, createAbortSignal, resetCancelling]
+    [selectedAgent, currentConversation, onDebugEvent, renderAssistantStreamingText, setChatItems, setIsStreaming, setCurrentConversation, setAvailableConversations, setPendingApprovals, updateConversationUsage, createAbortSignal, resetCancelling]
   );
 
   // Handle non-streaming message sending

python/packages/devui/frontend/src/services/api.ts

@@ -16,9 +16,10 @@ import type {
 import type { AgentFrameworkRequest } from "@/types/agent-framework";
 import type { ExtendedResponseStreamEvent } from "@/types/openai";
 import {
+  applyStreamingEventToState,
+  createStreamingState,
   loadStreamingState,
-  updateStreamingState,
-  markStreamingCompleted,
+  saveStreamingState,
   clearStreamingState,
 } from "./streaming-state";
 import { isAbortError } from "@/hooks";
@@ -72,6 +73,7 @@ const DEFAULT_API_BASE_URL =
 // Retry configuration for streaming
 const RETRY_INTERVAL_MS = 1000; // Base retry interval (will use exponential backoff)
 const MAX_RETRY_ATTEMPTS = 10; // Max 10 retries (~30 seconds with exponential backoff)
+const STREAMING_STATE_SAVE_INTERVAL_MS = 250;
 
 // Get backend URL from localStorage or default
 function getBackendUrl(): string {
@@ -223,7 +225,7 @@ class ApiClient {
           chat_client_type: entity.chat_client_type,
           context_provider: entity.context_provider,
           middleware: entity.middleware,
-        };
+        } as AgentInfo;
       } else {
         // Workflow - prefer executors field, fall back to tools for backward compatibility
         const executorList = entity.executors || entity.tools || [];
@@ -263,7 +265,7 @@ class ApiClient {
           input_type_name: entity.input_type_name || "Input",
           start_executor_id: startExecutorId,
           tools: [],
-        };
+        } as WorkflowInfo;
       }
     });
 
@@ -484,31 +486,65 @@ class ApiClient {
     let hasYieldedAnyEvent = false;
     let currentResponseId: string | undefined = resumeResponseId;
     let lastMessageId: string | undefined = undefined;
+    let lastStreamingStateSaveAt = 0;
+    let storedState = conversationId ? loadStreamingState(conversationId) : null;
+    let streamingState = storedState ? { ...storedState } : null;
+
+    const persistStreamingState = (force: boolean = false): void => {
+      if (!conversationId || !streamingState) {
+        return;
+      }
+
+      const now = Date.now();
+      if (!force && now - lastStreamingStateSaveAt < STREAMING_STATE_SAVE_INTERVAL_MS) {
+        return;
+      }
+
+      lastStreamingStateSaveAt = now;
+      saveStreamingState({
+        ...streamingState,
+        timestamp: now,
+      });
+    };
+
+    const recordStreamingEvent = (event: ExtendedResponseStreamEvent): void => {
+      if (!conversationId || !currentResponseId) {
+        return;
+      }
+
+      streamingState = applyStreamingEventToState(
+        streamingState ?? createStreamingState({
+          conversationId,
+          responseId: currentResponseId,
+          lastMessageId,
+          lastSequenceNumber,
+          accumulatedText: storedState?.accumulatedText,
+        }),
+        event,
+        currentResponseId,
+        lastMessageId
+      );
+
+      const isTextDelta =
+        event.type === "response.output_text.delta" &&
+        "delta" in event &&
+        typeof event.delta === "string" &&
+        event.delta.length > 0;
+      persistStreamingState(!isTextDelta);
+    };
 
     // Try to resume from stored state if conversation ID is provided
-    if (conversationId) {
-      const storedState = loadStreamingState(conversationId);
-      if (storedState) {
-        // Use stored response ID if no explicit one provided
-        if (!resumeResponseId) {
-          currentResponseId = storedState.responseId;
-        }
-
-        lastSequenceNumber = storedState.lastSequenceNumber;
-        lastMessageId = storedState.lastMessageId;
-
-        // Replay stored events only if we're not explicitly resuming
-        // (explicit resume means the caller already has the events)
-        if (!resumeResponseId) {
-          for (const event of storedState.events) {
-            hasYieldedAnyEvent = true;
-            yield event;
-          }
-        } else {
-          // Mark that we've already seen events up to this sequence number
-          hasYieldedAnyEvent = storedState.events.length > 0;
-        }
+    if (storedState) {
+      // Use stored response ID if no explicit one provided
+      if (!resumeResponseId) {
+        currentResponseId = storedState.responseId;
       }
+
+      lastSequenceNumber = storedState.lastSequenceNumber;
+      lastMessageId = storedState.lastMessageId;
+      hasYieldedAnyEvent =
+        storedState.lastSequenceNumber >= 0 ||
+        Boolean(storedState.accumulatedText);
     }
 
     while (retryCount <= MAX_RETRY_ATTEMPTS) {
@@ -621,7 +657,8 @@ class ApiClient {
             if (done) {
               // Stream completed successfully
               if (conversationId) {
-                markStreamingCompleted(conversationId);
+                clearStreamingState(conversationId);
+                streamingState = null;
               }
               return;
             }
@@ -640,7 +677,8 @@ class ApiClient {
                 // Handle [DONE] signal
                 if (dataStr === "[DONE]") {
                   if (conversationId) {
-                    markStreamingCompleted(conversationId);
+                    clearStreamingState(conversationId);
+                    streamingState = null;
                   }
                   return;
                 }
@@ -676,6 +714,9 @@ class ApiClient {
                       if (conversationId) {
                         clearStreamingState(conversationId);
                       }
+                      storedState = null;
+                      streamingState = null;
+                      lastStreamingStateSaveAt = 0;
                       yield {
                         type: "error",
                         message: "Connection lost - previous response failed. Starting new response.",
@@ -684,9 +725,7 @@ class ApiClient {
                       hasYieldedAnyEvent = true;
 
                       // Save new event to storage
-                      if (conversationId && currentResponseId) {
-                        updateStreamingState(conversationId, openAIEvent, currentResponseId, lastMessageId);
-                      }
+                      recordStreamingEvent(openAIEvent);
 
                       yield openAIEvent;
                     }
@@ -698,9 +737,7 @@ class ApiClient {
                       hasYieldedAnyEvent = true;
 
                       // Save event to storage before yielding
-                      if (conversationId && currentResponseId) {
-                        updateStreamingState(conversationId, openAIEvent, currentResponseId, lastMessageId);
-                      }
+                      recordStreamingEvent(openAIEvent);
 
                       yield openAIEvent;
                     }
@@ -709,9 +746,7 @@ class ApiClient {
                     hasYieldedAnyEvent = true;
 
                     // Still save to storage if we have conversation context
-                    if (conversationId && currentResponseId) {
-                      updateStreamingState(conversationId, openAIEvent, currentResponseId, lastMessageId);
-                    }
+                    recordStreamingEvent(openAIEvent);
 
                     yield openAIEvent;
                   }
@@ -730,7 +765,8 @@ class ApiClient {
         // Don't retry on abort
         if (isAbortError(error)) {
           if (conversationId) {
-            markStreamingCompleted(conversationId); // Clean up state
+            clearStreamingState(conversationId);
+            streamingState = null;
           }
           throw error; // Re-throw abort error without retrying
         }

python/packages/devui/frontend/src/services/streaming-state.ts

@@ -3,7 +3,6 @@
  *
  * Manages browser storage of streaming response state to enable:
  * - Resume interrupted streams after page refresh
- * - Replay cached events before fetching new ones
  * - Graceful recovery from network disconnections
  */
 
@@ -14,7 +13,6 @@ export interface StreamingState {
   responseId: string;
   lastMessageId?: string;
   lastSequenceNumber: number;
-  events: ExtendedResponseStreamEvent[];
   timestamp: number; // When this state was last updated
   completed: boolean; // Whether the stream completed successfully
   accumulatedText?: string; // Accumulated text content for quick restoration
@@ -23,6 +21,14 @@ export interface StreamingState {
 const STORAGE_KEY_PREFIX = "devui_streaming_state_";
 const STATE_EXPIRY_MS = 24 * 60 * 60 * 1000; // 24 hours
 
+interface CreateStreamingStateOptions {
+  conversationId: string;
+  responseId: string;
+  lastMessageId?: string;
+  lastSequenceNumber?: number;
+  accumulatedText?: string;
+}
+
 /**
  * Storage key for a specific conversation
  */
@@ -31,16 +37,81 @@ function getStorageKey(conversationId: string): string {
 }
 
 /**
- * Extract accumulated text from events (for quick restoration)
+ * Read raw streaming state from storage, including completed entries.
  */
-function extractAccumulatedText(events: ExtendedResponseStreamEvent[]): string {
-  let text = "";
-  for (const event of events) {
-    if (event.type === "response.output_text.delta" && "delta" in event) {
-      text += event.delta;
-    }
+function readStreamingState(conversationId: string): StreamingState | null {
+  const key = getStorageKey(conversationId);
+  const data = localStorage.getItem(key);
+
+  if (!data) {
+    return null;
   }
-  return text;
+
+  const state: StreamingState = JSON.parse(data);
+
+  // Check if state has expired
+  const age = Date.now() - state.timestamp;
+  if (age > STATE_EXPIRY_MS) {
+    clearStreamingState(conversationId);
+    return null;
+  }
+
+  return state;
+}
+
+/**
+ * Create an initial streaming state snapshot.
+ */
+export function createStreamingState({
+  conversationId,
+  responseId,
+  lastMessageId,
+  lastSequenceNumber = -1,
+  accumulatedText,
+}: CreateStreamingStateOptions): StreamingState {
+  return {
+    conversationId,
+    responseId,
+    lastMessageId,
+    lastSequenceNumber,
+    timestamp: Date.now(),
+    completed: false,
+    accumulatedText,
+  };
+}
+
+/**
+ * Apply an incoming stream event to an in-memory streaming state snapshot.
+ */
+export function applyStreamingEventToState(
+  state: StreamingState,
+  event: ExtendedResponseStreamEvent,
+  responseId: string,
+  lastMessageId?: string
+): StreamingState {
+  const sequenceNumber = "sequence_number" in event ? event.sequence_number : undefined;
+  const nextState: StreamingState = {
+    ...state,
+    responseId,
+    lastMessageId,
+    timestamp: Date.now(),
+    completed: event.type === "response.completed" || event.type === "response.failed",
+  };
+
+  if (sequenceNumber !== undefined) {
+    nextState.lastSequenceNumber = sequenceNumber;
+  }
+
+  if (
+    event.type === "response.output_text.delta" &&
+    "delta" in event &&
+    typeof event.delta === "string" &&
+    event.delta.length > 0
+  ) {
+    nextState.accumulatedText = `${state.accumulatedText ?? ""}${event.delta}`;
+  }
+
+  return nextState;
 }
 
 /**
@@ -71,19 +142,8 @@ export function saveStreamingState(state: StreamingState): void {
  */
 export function loadStreamingState(conversationId: string): StreamingState | null {
   try {
-    const key = getStorageKey(conversationId);
-    const data = localStorage.getItem(key);
-
-    if (!data) {
-      return null;
-    }
-
-    const state: StreamingState = JSON.parse(data);
-
-    // Check if state has expired
-    const age = Date.now() - state.timestamp;
-    if (age > STATE_EXPIRY_MS) {
-      clearStreamingState(conversationId);
+    const state = readStreamingState(conversationId);
+    if (!state) {
       return null;
     }
 
@@ -99,54 +159,6 @@ export function loadStreamingState(conversationId: string): StreamingState | nul
   }
 }
 
-/**
- * Update streaming state with a new event
- */
-export function updateStreamingState(
-  conversationId: string,
-  event: ExtendedResponseStreamEvent,
-  responseId: string,
-  lastMessageId?: string
-): void {
-  try {
-    const existing = loadStreamingState(conversationId);
-    const sequenceNumber = "sequence_number" in event ? event.sequence_number : undefined;
-
-    const newEvents = existing ? [...existing.events, event] : [event];
-
-    const state: StreamingState = {
-      conversationId,
-      responseId,
-      lastMessageId,
-      lastSequenceNumber: sequenceNumber ?? (existing?.lastSequenceNumber ?? -1),
-      events: newEvents,
-      timestamp: Date.now(),
-      completed: event.type === "response.completed" || event.type === "response.failed",
-      accumulatedText: extractAccumulatedText(newEvents),
-    };
-
-    saveStreamingState(state);
-  } catch (error) {
-    console.error("Failed to update streaming state:", error);
-  }
-}
-
-/**
- * Mark streaming state as completed
- */
-export function markStreamingCompleted(conversationId: string): void {
-  try {
-    const existing = loadStreamingState(conversationId);
-    if (existing) {
-      existing.completed = true;
-      existing.timestamp = Date.now();
-      saveStreamingState(existing);
-    }
-  } catch (error) {
-    console.error("Failed to mark streaming as completed:", error);
-  }
-}
-
 /**
  * Clear streaming state for a conversation
  */

python/packages/devui/tests/devui/test_ui_memory_regression.py

@@ -0,0 +1,750 @@
+# Copyright (c) Microsoft. All rights reserved.
+
+"""Browser-based regression test for DevUI streaming memory growth."""
+
+from __future__ import annotations
+
+import asyncio
+import contextlib
+import http.client
+import json
+import os
+import shutil
+import signal
+import socket
+import subprocess
+import sys
+import tempfile
+import threading
+import time
+from collections.abc import AsyncIterable, Awaitable, Generator
+from dataclasses import dataclass
+from pathlib import Path
+from typing import Any
+
+import pytest
+import uvicorn
+from agent_framework import (
+    AgentResponse,
+    AgentResponseUpdate,
+    AgentSession,
+    BaseAgent,
+    Content,
+    Message,
+    ResponseStream,
+)
+from websockets.asyncio.client import connect as websocket_connect
+
+from agent_framework_devui import DevServer
+
+_BROWSER_COMMANDS = (
+    "chrome",
+    "chrome.exe",
+    "google-chrome",
+    "google-chrome-stable",
+    "chromium",
+    "chromium-browser",
+    "microsoft-edge",
+    "msedge",
+    "msedge.exe",
+)
+_BROWSER_ENV_VARS = ("DEVUI_TEST_BROWSER", "CHROME_BIN", "BROWSER_BIN")
+_WINDOWS_PROCESS_QUERY = """
+$rows = @(
+    Get-CimInstance Win32_Process | ForEach-Object {
+        if (-not $_.CommandLine) {
+            return
+        }
+
+        try {
+            $process = Get-Process -Id $_.ProcessId -ErrorAction Stop
+            [PSCustomObject]@{
+                pid = [int]$_.ProcessId
+                parent_pid = [int]$_.ParentProcessId
+                rss_kb = [int][Math]::Round($process.WorkingSet64 / 1KB)
+                command = [string]$_.CommandLine
+            }
+        }
+        catch {
+        }
+    }
+)
+
+$rows | ConvertTo-Json -Compress
+""".strip()
+
+_STREAM_CHUNK_COUNT = 12_000
+_STREAM_CHUNK_SIZE = 128
+_POST_SEND_DELAY_S = 1.0
+_SAMPLE_INTERVAL_S = 0.5
+_SAMPLE_WINDOW_S = 12.0
+_MAX_RENDERER_GROWTH_MB = 500.0
+
+
+@dataclass(frozen=True)
+class _BrowserProcessRow:
+    pid: int
+    parent_pid: int
+    rss_kb: int
+    command: str
+
+
+class MemoryStressAgent(BaseAgent):
+    """Agent that emits many small streaming chunks."""
+
+    def __init__(self, *, chunk_count: int, chunk_size: int, delay_ms: float, **kwargs: Any) -> None:
+        super().__init__(**kwargs)
+        self._chunk_count = chunk_count
+        self._chunk_size = max(chunk_size, 24)
+        self._delay_s = max(delay_ms, 0.0) / 1000.0
+
+    def run(
+        self,
+        messages: str | Message | list[str] | list[Message] | None = None,
+        *,
+        stream: bool = False,
+        session: AgentSession | None = None,
+        **kwargs: Any,
+    ) -> Awaitable[AgentResponse] | ResponseStream[AgentResponseUpdate, AgentResponse]:
+        del messages, session, kwargs
+        if stream:
+            return self._run_stream()
+        return self._run()
+
+    async def _run(self) -> AgentResponse:
+        text = "".join(self._make_chunk(index) for index in range(self._chunk_count))
+        return AgentResponse(messages=[Message("assistant", [Content.from_text(text=text)])])
+
+    def _run_stream(self) -> ResponseStream[AgentResponseUpdate, AgentResponse]:
+        async def _iter() -> AsyncIterable[AgentResponseUpdate]:
+            for index in range(self._chunk_count):
+                yield AgentResponseUpdate(
+                    contents=[Content.from_text(text=self._make_chunk(index))],
+                    role="assistant",
+                )
+                if self._delay_s:
+                    await asyncio.sleep(self._delay_s)
+
+        return ResponseStream(_iter(), finalizer=AgentResponse.from_updates)
+
+    def _make_chunk(self, index: int) -> str:
+        prefix = f"[{index:06d}] "
+        payload_size = max(self._chunk_size - len(prefix), 1)
+        payload = ("x" * (payload_size - 1)) + ("\n" if index % 8 == 7 else " ")
+        return prefix + payload
+
+
+class _CDPClient:
+    """Minimal Chrome DevTools Protocol client for a single attached page."""
+
+    def __init__(self, websocket: Any) -> None:
+        self._websocket = websocket
+        self._next_id = 0
+
+    async def send(
+        self,
+        method: str,
+        params: dict[str, Any] | None = None,
+        *,
+        session_id: str | None = None,
+    ) -> dict[str, Any]:
+        self._next_id += 1
+        command_id = self._next_id
+        payload: dict[str, Any] = {"id": command_id, "method": method}
+        if params is not None:
+            payload["params"] = params
+        if session_id is not None:
+            payload["sessionId"] = session_id
+
+        await self._websocket.send(json.dumps(payload))
+
+        while True:
+            raw_message = await self._websocket.recv()
+            if isinstance(raw_message, bytes):
+                raw_message = raw_message.decode("utf-8")
+
+            message = json.loads(raw_message)
+            if message.get("id") != command_id:
+                continue
+
+            error = message.get("error")
+            if isinstance(error, dict):
+                raise RuntimeError(f"CDP command {method} failed: {error}")
+
+            result = message.get("result")
+            return result if isinstance(result, dict) else {}
+
+    async def evaluate(self, expression: str, *, session_id: str) -> Any:
+        result = await self.send(
+            "Runtime.evaluate",
+            {
+                "expression": expression,
+                "awaitPromise": True,
+                "returnByValue": True,
+            },
+            session_id=session_id,
+        )
+        remote_result = result.get("result")
+        if isinstance(remote_result, dict):
+            return remote_result.get("value")
+        return None
+
+
+def _get_browser_candidates() -> tuple[Path, ...]:
+    if sys.platform == "darwin":
+        return (
+            Path("/Applications/Microsoft Edge.app/Contents/MacOS/Microsoft Edge"),
+            Path("/Applications/Google Chrome.app/Contents/MacOS/Google Chrome"),
+            Path("/Applications/Chromium.app/Contents/MacOS/Chromium"),
+        )
+
+    if sys.platform == "win32":
+        windows_bases: list[Path] = []
+        for env_var in ("PROGRAMFILES", "PROGRAMFILES(X86)", "LOCALAPPDATA"):
+            raw_value = os.environ.get(env_var)
+            if raw_value:
+                windows_bases.append(Path(raw_value))
+
+        return tuple(
+            dict.fromkeys(
+                [base / "Microsoft/Edge/Application/msedge.exe" for base in windows_bases]
+                + [base / "Google/Chrome/Application/chrome.exe" for base in windows_bases]
+                + [base / "Chromium/Application/chrome.exe" for base in windows_bases]
+            )
+        )
+
+    return (
+        Path("/usr/bin/google-chrome"),
+        Path("/usr/bin/google-chrome-stable"),
+        Path("/usr/bin/chromium"),
+        Path("/usr/bin/chromium-browser"),
+        Path("/usr/bin/microsoft-edge"),
+        Path("/opt/google/chrome/chrome"),
+        Path("/opt/microsoft/msedge/msedge"),
+        Path("/snap/bin/chromium"),
+    )
+
+
+def _find_browser_executable() -> Path | None:
+    for env_var in _BROWSER_ENV_VARS:
+        configured_path = os.environ.get(env_var)
+        if not configured_path:
+            continue
+
+        candidate = Path(configured_path).expanduser()
+        if candidate.exists():
+            return candidate
+
+    for candidate in _get_browser_candidates():
+        if candidate.exists():
+            return candidate
+
+    for command in _BROWSER_COMMANDS:
+        resolved = shutil.which(command)
+        if resolved is not None:
+            return Path(resolved)
+
+    return None
+
+
+def _find_available_port() -> int:
+    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as sock:
+        sock.bind(("127.0.0.1", 0))
+        sock.listen(1)
+        return int(sock.getsockname()[1])
+
+
+def _get_json_response(*, host: str, port: int, path: str) -> dict[str, Any]:
+    connection = http.client.HTTPConnection(host, port, timeout=5)
+    try:
+        connection.request("GET", path)
+        response = connection.getresponse()
+        if response.status != 200:
+            raise RuntimeError(f"Request to {path} failed with status {response.status}")
+        payload = response.read().decode("utf-8")
+    finally:
+        connection.close()
+
+    data = json.loads(payload)
+    if isinstance(data, dict):
+        return data
+    raise RuntimeError(f"Expected JSON object from {path}, got: {type(data).__name__}")
+
+
+async def _get_devtools_websocket_url(port: int) -> str:
+    deadline = time.monotonic() + 10.0
+    while time.monotonic() < deadline:
+        with contextlib.suppress(Exception):
+            version_data = _get_json_response(host="127.0.0.1", port=port, path="/json/version")
+            websocket_url = version_data.get("webSocketDebuggerUrl")
+            if isinstance(websocket_url, str) and websocket_url:
+                return websocket_url
+        await asyncio.sleep(0.1)
+
+    raise RuntimeError(f"Timed out waiting for DevTools on port {port}")
+
+
+def _wait_for_server_details(server_instance: uvicorn.Server) -> tuple[int, str]:
+    deadline = time.monotonic() + 10.0
+    actual_port: int | None = None
+
+    while time.monotonic() < deadline:
+        if hasattr(server_instance, "servers") and server_instance.servers:
+            for uvicorn_server in server_instance.servers:
+                sockets = getattr(uvicorn_server, "sockets", None)
+                if not sockets:
+                    continue
+                actual_port = int(sockets[0].getsockname()[1])
+                break
+
+        if actual_port is not None:
+            with contextlib.suppress(Exception):
+                health = _get_json_response(host="127.0.0.1", port=actual_port, path="/health")
+                if health.get("status") == "healthy":
+                    entities = _get_json_response(host="127.0.0.1", port=actual_port, path="/v1/entities")
+                    entity_list = entities.get("entities")
+                    if isinstance(entity_list, list) and entity_list:
+                        entity = entity_list[0]
+                        if isinstance(entity, dict) and isinstance(entity.get("id"), str):
+                            return actual_port, entity["id"]
+        time.sleep(0.1)
+
+    raise RuntimeError("Timed out waiting for DevUI server startup")
+
+
+def _parse_posix_process_rows(output: str) -> list[_BrowserProcessRow]:
+    rows: list[_BrowserProcessRow] = []
+    for line in output.splitlines():
+        parts = line.strip().split(None, 3)
+        if len(parts) != 4:
+            continue
+
+        pid_text, parent_pid_text, rss_text, command = parts
+        with contextlib.suppress(ValueError):
+            rows.append(
+                _BrowserProcessRow(
+                    pid=int(pid_text),
+                    parent_pid=int(parent_pid_text),
+                    rss_kb=int(rss_text),
+                    command=command,
+                )
+            )
+
+    return rows
+
+
+def _parse_windows_process_rows(output: str) -> list[_BrowserProcessRow]:
+    text = output.strip()
+    if not text:
+        return []
+
+    payload = json.loads(text)
+    items = payload if isinstance(payload, list) else [payload]
+
+    rows: list[_BrowserProcessRow] = []
+    for item in items:
+        if not isinstance(item, dict):
+            continue
+
+        pid = item.get("pid")
+        parent_pid = item.get("parent_pid")
+        rss_kb = item.get("rss_kb")
+        command = item.get("command")
+        if not all(isinstance(value, int) for value in (pid, parent_pid, rss_kb)):
+            continue
+        if not isinstance(command, str):
+            continue
+
+        rows.append(
+            _BrowserProcessRow(
+                pid=pid,
+                parent_pid=parent_pid,
+                rss_kb=rss_kb,
+                command=command,
+            )
+        )
+
+    return rows
+
+
+def _read_process_rows() -> list[_BrowserProcessRow]:
+    if sys.platform == "win32":
+        result = subprocess.run(
+            ["powershell", "-NoProfile", "-Command", _WINDOWS_PROCESS_QUERY],
+            capture_output=True,
+            text=True,
+            check=True,
+            encoding="utf-8",
+        )
+        return _parse_windows_process_rows(result.stdout)
+
+    result = subprocess.run(
+        ["ps", "-axo", "pid=,ppid=,rss=,command="],
+        capture_output=True,
+        text=True,
+        check=True,
+    )
+    return _parse_posix_process_rows(result.stdout)
+
+
+def _collect_process_tree(root_pids: set[int], process_rows: list[_BrowserProcessRow]) -> list[_BrowserProcessRow]:
+    process_by_pid = {row.pid: row for row in process_rows}
+    child_pids_by_parent: dict[int, list[int]] = {}
+    for row in process_rows:
+        child_pids_by_parent.setdefault(row.parent_pid, []).append(row.pid)
+
+    collected_rows: list[_BrowserProcessRow] = []
+    seen_pids: set[int] = set()
+    pending_pids = list(root_pids)
+
+    while pending_pids:
+        pid = pending_pids.pop()
+        if pid in seen_pids:
+            continue
+
+        seen_pids.add(pid)
+        process_row = process_by_pid.get(pid)
+        if process_row is None:
+            continue
+
+        collected_rows.append(process_row)
+        pending_pids.extend(child_pids_by_parent.get(pid, []))
+
+    return collected_rows
+
+
+def _collect_browser_process_rows(root_pid: int, profile_dir: str) -> list[_BrowserProcessRow]:
+    process_rows = _read_process_rows()
+    normalized_profile_dir = profile_dir.casefold()
+    matched_root_pids = {row.pid for row in process_rows if normalized_profile_dir in row.command.casefold()}
+    matched_root_pids.add(root_pid)
+    return _collect_process_tree(matched_root_pids, process_rows)
+
+
+def _sample_peak_renderer_rss_mb(root_pid: int, profile_dir: str) -> float:
+    renderer_rss_kb = [
+        row.rss_kb
+        for row in _collect_browser_process_rows(root_pid, profile_dir)
+        if "--type=renderer" in row.command.casefold()
+    ]
+    return round((max(renderer_rss_kb, default=0)) / 1024, 2)
+
+
+def _terminate_browser_processes(root_pid: int, profile_dir: str) -> None:
+    browser_rows = _collect_browser_process_rows(root_pid, profile_dir)
+    browser_pids = sorted({row.pid for row in browser_rows} | {root_pid}, reverse=True)
+
+    if sys.platform == "win32":
+        for pid in browser_pids:
+            with contextlib.suppress(subprocess.CalledProcessError):
+                subprocess.run(
+                    ["taskkill", "/PID", str(pid), "/T", "/F"],
+                    stdout=subprocess.DEVNULL,
+                    stderr=subprocess.DEVNULL,
+                    check=True,
+                )
+        return
+
+    for pid in browser_pids:
+        with contextlib.suppress(ProcessLookupError):
+            os.kill(pid, signal.SIGTERM)
+
+
+def _launch_browser_process(*, browser_path: Path, debug_port: int, profile_dir: str) -> subprocess.Popen[str]:
+    return subprocess.Popen(
+        [
+            str(browser_path),
+            "--headless=new",
+            f"--remote-debugging-port={debug_port}",
+            "--remote-debugging-address=127.0.0.1",
+            f"--user-data-dir={profile_dir}",
+            "--no-first-run",
+            "--no-default-browser-check",
+            "--disable-background-networking",
+            "--disable-sync",
+            "--disable-renderer-backgrounding",
+            "--hide-scrollbars",
+            "--mute-audio",
+            "--enable-precise-memory-info",
+            "--no-sandbox",
+            "about:blank",
+        ],
+        stdout=subprocess.DEVNULL,
+        stderr=subprocess.DEVNULL,
+        text=True,
+    )
+
+
+def _shutdown_browser_process(browser_process: subprocess.Popen[str], *, profile_dir: str) -> None:
+    with contextlib.suppress(Exception):
+        browser_process.terminate()
+        browser_process.wait(timeout=5)
+    _terminate_browser_processes(browser_process.pid, profile_dir)
+
+
+def test_parse_posix_process_rows() -> None:
+    output = """
+      101 1 2048 /usr/bin/google-chrome --user-data-dir=/tmp/devui-memory
+      202 101 4096 /usr/bin/google-chrome --type=renderer --lang=en-US
+    """.strip()
+
+    assert _parse_posix_process_rows(output) == [
+        _BrowserProcessRow(
+            pid=101,
+            parent_pid=1,
+            rss_kb=2048,
+            command="/usr/bin/google-chrome --user-data-dir=/tmp/devui-memory",
+        ),
+        _BrowserProcessRow(
+            pid=202,
+            parent_pid=101,
+            rss_kb=4096,
+            command="/usr/bin/google-chrome --type=renderer --lang=en-US",
+        ),
+    ]
+
+
+def test_parse_windows_process_rows() -> None:
+    output = json.dumps([
+        {
+            "pid": 301,
+            "parent_pid": 1,
+            "rss_kb": 2048,
+            "command": r"C:\Program Files\Google\Chrome\Application\chrome.exe",
+        },
+        {
+            "pid": 302,
+            "parent_pid": 301,
+            "rss_kb": 6144,
+            "command": r"C:\Program Files\Google\Chrome\Application\chrome.exe --type=renderer",
+        },
+    ])
+
+    assert _parse_windows_process_rows(output) == [
+        _BrowserProcessRow(
+            pid=301,
+            parent_pid=1,
+            rss_kb=2048,
+            command=r"C:\Program Files\Google\Chrome\Application\chrome.exe",
+        ),
+        _BrowserProcessRow(
+            pid=302,
+            parent_pid=301,
+            rss_kb=6144,
+            command=r"C:\Program Files\Google\Chrome\Application\chrome.exe --type=renderer",
+        ),
+    ]
+
+
+def test_sample_peak_renderer_rss_mb_uses_browser_process_tree(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    profile_dir = "/tmp/devui-memory-browser"
+    process_rows = [
+        _BrowserProcessRow(
+            pid=101,
+            parent_pid=1,
+            rss_kb=1024,
+            command="/usr/bin/google-chrome",
+        ),
+        _BrowserProcessRow(
+            pid=102,
+            parent_pid=101,
+            rss_kb=4096,
+            command="/usr/bin/google-chrome --type=renderer",
+        ),
+        _BrowserProcessRow(
+            pid=201,
+            parent_pid=1,
+            rss_kb=2048,
+            command=f"/usr/bin/google-chrome --user-data-dir={profile_dir}",
+        ),
+        _BrowserProcessRow(
+            pid=202,
+            parent_pid=201,
+            rss_kb=8192,
+            command="/usr/bin/google-chrome --type=renderer",
+        ),
+        _BrowserProcessRow(
+            pid=999,
+            parent_pid=1,
+            rss_kb=32768,
+            command="/usr/bin/google-chrome --type=renderer",
+        ),
+    ]
+
+    monkeypatch.setattr(sys.modules[__name__], "_read_process_rows", lambda: process_rows)
+
+    assert _sample_peak_renderer_rss_mb(101, profile_dir) == 8.0
+
+
+@pytest.fixture
+def memory_regression_server() -> Generator[tuple[str, str]]:
+    """Start DevUI with a synthetic streaming agent and yield the base URL plus entity ID."""
+
+    server = DevServer(host="127.0.0.1", port=0)
+    server.register_entities([
+        MemoryStressAgent(
+            id="memory-stream-agent",
+            name="MemoryStreamAgent",
+            description="Streams many small chunks for UI memory profiling.",
+            chunk_count=_STREAM_CHUNK_COUNT,
+            chunk_size=_STREAM_CHUNK_SIZE,
+            delay_ms=1.0,
+        )
+    ])
+
+    app = server.get_app()
+    server_config = uvicorn.Config(
+        app=app,
+        host="127.0.0.1",
+        port=0,
+        log_level="error",
+        ws="none",
+    )
+    server_instance = uvicorn.Server(server_config)
+
+    def run_server() -> None:
+        asyncio.run(server_instance.serve())
+
+    server_thread = threading.Thread(target=run_server, daemon=True)
+    server_thread.start()
+
+    actual_port, entity_id = _wait_for_server_details(server_instance)
+    yield f"http://127.0.0.1:{actual_port}", entity_id
+
+    with contextlib.suppress(Exception):
+        server_instance.should_exit = True
+    server_thread.join(timeout=5)
+
+
+async def _wait_for_expression(
+    client: _CDPClient,
+    *,
+    session_id: str,
+    expression: str,
+    timeout_s: float,
+) -> Any:
+    deadline = time.monotonic() + timeout_s
+    while time.monotonic() < deadline:
+        value = await client.evaluate(expression, session_id=session_id)
+        if value:
+            return value
+        await asyncio.sleep(0.1)
+
+    raise AssertionError(f"Timed out waiting for expression: {expression}")
+
+
+async def test_devui_streaming_renderer_memory_is_bounded(
+    memory_regression_server: tuple[str, str],
+) -> None:
+    """Fail when frontend renderer memory grows unbounded during streaming."""
+
+    browser_path = _find_browser_executable()
+    if browser_path is None:
+        pytest.skip("No Chromium-based browser found for DevUI memory regression test")
+
+    base_url, entity_id = memory_regression_server
+    debug_port = _find_available_port()
+
+    with tempfile.TemporaryDirectory(prefix="devui-memory-browser-") as profile_dir:
+        browser_process = _launch_browser_process(
+            browser_path=browser_path,
+            debug_port=debug_port,
+            profile_dir=profile_dir,
+        )
+
+        try:
+            websocket_url = await _get_devtools_websocket_url(debug_port)
+
+            async with websocket_connect(websocket_url, max_size=None) as websocket:
+                client = _CDPClient(websocket)
+
+                target = await client.send("Target.createTarget", {"url": "about:blank"})
+                target_id = target["targetId"]
+                attached = await client.send(
+                    "Target.attachToTarget",
+                    {"targetId": target_id, "flatten": True},
+                )
+                session_id = attached["sessionId"]
+
+                await client.send("Page.enable", session_id=session_id)
+                await client.send("Runtime.enable", session_id=session_id)
+                await client.send(
+                    "Page.navigate",
+                    {"url": f"{base_url}/?entity_id={entity_id}"},
+                    session_id=session_id,
+                )
+
+                await _wait_for_expression(
+                    client,
+                    session_id=session_id,
+                    expression=(
+                        "Boolean("
+                        "document.querySelector('textarea') && "
+                        "document.querySelector('button[aria-label=\"Send message\"]')"
+                        ")"
+                    ),
+                    timeout_s=30.0,
+                )
+
+                start_renderer_rss_mb = _sample_peak_renderer_rss_mb(
+                    browser_process.pid,
+                    profile_dir,
+                )
+
+                await client.evaluate(
+                    """
+                    (() => {
+                      const textarea = document.querySelector("textarea");
+                      const valueSetter = Object.getOwnPropertyDescriptor(
+                        HTMLTextAreaElement.prototype,
+                        "value"
+                      ).set;
+                      valueSetter.call(textarea, "Stream a very long answer.");
+                      textarea.dispatchEvent(new Event("input", { bubbles: true }));
+                      document.querySelector('button[aria-label="Send message"]').click();
+                      return true;
+                    })()
+                    """,
+                    session_id=session_id,
+                )
+
+                await _wait_for_expression(
+                    client,
+                    session_id=session_id,
+                    expression="Boolean(document.querySelector('button[aria-label=\"Stop generating response\"]'))",
+                    timeout_s=10.0,
+                )
+
+                await asyncio.sleep(_POST_SEND_DELAY_S)
+
+                peak_renderer_rss_mb = start_renderer_rss_mb
+                samples: list[tuple[float, float]] = [(0.0, start_renderer_rss_mb)]
+                start_time = time.monotonic()
+
+                while time.monotonic() - start_time < _SAMPLE_WINDOW_S:
+                    current_sample = _sample_peak_renderer_rss_mb(
+                        browser_process.pid,
+                        profile_dir,
+                    )
+                    elapsed_s = round(time.monotonic() - start_time, 2)
+                    samples.append((elapsed_s, current_sample))
+                    peak_renderer_rss_mb = max(peak_renderer_rss_mb, current_sample)
+
+                    if peak_renderer_rss_mb - start_renderer_rss_mb > _MAX_RENDERER_GROWTH_MB:
+                        break
+
+                    await asyncio.sleep(_SAMPLE_INTERVAL_S)
+
+                renderer_growth_mb = round(peak_renderer_rss_mb - start_renderer_rss_mb, 2)
+                assert renderer_growth_mb <= _MAX_RENDERER_GROWTH_MB, (
+                    "DevUI renderer memory grew too much during a ~1.5 MB streaming response. "
+                    f"start={start_renderer_rss_mb:.2f}MB "
+                    f"peak={peak_renderer_rss_mb:.2f}MB "
+                    f"growth={renderer_growth_mb:.2f}MB "
+                    f"budget={_MAX_RENDERER_GROWTH_MB:.2f}MB "
+                    f"samples={samples}"
+                )
+        finally:
+            _shutdown_browser_process(browser_process, profile_dir=profile_dir)