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
  */