Python: Fix DevUI streaming memory growth regression (#6038)

* Fix DevUI streaming memory growth regression

Bounds retained streaming/debug state in DevUI and strengthens browser regression coverage for long streamed responses.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address DevUI memory review feedback

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix DevUI bundle trailing whitespace

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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

@@ -519,6 +519,7 @@ class ApiClient {
           lastMessageId,
           lastSequenceNumber,
           accumulatedText: storedState?.accumulatedText,
+          accumulatedTextIsPreview: storedState?.accumulatedTextIsPreview,
         }),
         event,
         currentResponseId,

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

@@ -15,11 +15,13 @@ export interface StreamingState {
   lastSequenceNumber: number;
   timestamp: number; // When this state was last updated
   completed: boolean; // Whether the stream completed successfully
-  accumulatedText?: string; // Accumulated text content for quick restoration
+  accumulatedText?: string; // Bounded tail preview for refresh restoration
+  accumulatedTextIsPreview?: boolean;
 }
 
 const STORAGE_KEY_PREFIX = "devui_streaming_state_";
 const STATE_EXPIRY_MS = 24 * 60 * 60 * 1000; // 24 hours
+const MAX_ACCUMULATED_TEXT_PREVIEW_CHARS = 16 * 1024;
 
 interface CreateStreamingStateOptions {
   conversationId: string;
@@ -27,6 +29,7 @@ interface CreateStreamingStateOptions {
   lastMessageId?: string;
   lastSequenceNumber?: number;
   accumulatedText?: string;
+  accumulatedTextIsPreview?: boolean;
 }
 
 /**
@@ -36,6 +39,21 @@ function getStorageKey(conversationId: string): string {
   return `${STORAGE_KEY_PREFIX}${conversationId}`;
 }
 
+function normalizeAccumulatedTextPreview(state: StreamingState): StreamingState {
+  if (
+    state.accumulatedText === undefined ||
+    state.accumulatedText.length <= MAX_ACCUMULATED_TEXT_PREVIEW_CHARS
+  ) {
+    return state;
+  }
+
+  return {
+    ...state,
+    accumulatedText: state.accumulatedText.slice(-MAX_ACCUMULATED_TEXT_PREVIEW_CHARS),
+    accumulatedTextIsPreview: true,
+  };
+}
+
 /**
  * Read raw streaming state from storage, including completed entries.
  */
@@ -56,7 +74,7 @@ function readStreamingState(conversationId: string): StreamingState | null {
     return null;
   }
 
-  return state;
+  return normalizeAccumulatedTextPreview(state);
 }
 
 /**
@@ -68,8 +86,9 @@ export function createStreamingState({
   lastMessageId,
   lastSequenceNumber = -1,
   accumulatedText,
+  accumulatedTextIsPreview = false,
 }: CreateStreamingStateOptions): StreamingState {
-  return {
+  return normalizeAccumulatedTextPreview({
     conversationId,
     responseId,
     lastMessageId,
@@ -77,7 +96,8 @@ export function createStreamingState({
     timestamp: Date.now(),
     completed: false,
     accumulatedText,
-  };
+    accumulatedTextIsPreview,
+  });
 }
 
 /**
@@ -108,7 +128,15 @@ export function applyStreamingEventToState(
     typeof event.delta === "string" &&
     event.delta.length > 0
   ) {
-    nextState.accumulatedText = `${state.accumulatedText ?? ""}${event.delta}`;
+    const accumulatedText = `${state.accumulatedText ?? ""}${event.delta}`;
+    const isPreview =
+      state.accumulatedTextIsPreview ||
+      accumulatedText.length > MAX_ACCUMULATED_TEXT_PREVIEW_CHARS;
+
+    nextState.accumulatedText = isPreview
+      ? accumulatedText.slice(-MAX_ACCUMULATED_TEXT_PREVIEW_CHARS)
+      : accumulatedText;
+    nextState.accumulatedTextIsPreview = isPreview;
   }
 
   return nextState;
@@ -120,7 +148,7 @@ export function applyStreamingEventToState(
 export function saveStreamingState(state: StreamingState): void {
   try {
     const key = getStorageKey(state.conversationId);
-    const data = JSON.stringify(state);
+    const data = JSON.stringify(normalizeAccumulatedTextPreview(state));
     localStorage.setItem(key, data);
   } catch (error) {
     console.error("Failed to save streaming state:", error);
@@ -129,7 +157,7 @@ export function saveStreamingState(state: StreamingState): void {
       clearExpiredStreamingStates();
       // Try again
       const key = getStorageKey(state.conversationId);
-      const data = JSON.stringify(state);
+      const data = JSON.stringify(normalizeAccumulatedTextPreview(state));
       localStorage.setItem(key, data);
     } catch {
       console.error("Failed to save streaming state even after cleanup");