diff --git a/app/api/parse-pdf/route.ts b/app/api/parse-pdf/route.ts
index 94feff54..422a64cc 100644
--- a/app/api/parse-pdf/route.ts
+++ b/app/api/parse-pdf/route.ts
@@ -62,8 +62,8 @@ export async function POST(req: NextRequest) {
     const resultWithMetadata: ParsedPdfContent = {
       ...result,
       metadata: {
-        pageCount: result.metadata?.pageCount || 0, // Ensure pageCount is always a number
         ...result.metadata,
+        pageCount: result.metadata?.pageCount ?? 0, // Ensure pageCount is always a number
         fileName: pdfFile.name,
         fileSize: pdfFile.size,
       },
diff --git a/app/api/quiz-grade/route.ts b/app/api/quiz-grade/route.ts
index d0aab62e..f49a9366 100644
--- a/app/api/quiz-grade/route.ts
+++ b/app/api/quiz-grade/route.ts
@@ -34,6 +34,11 @@ export async function POST(req: NextRequest) {
       return apiError('MISSING_REQUIRED_FIELD', 400, 'question and userAnswer are required');
     }
 
+    // Validate points is a positive finite number
+    if (!points || !Number.isFinite(points) || points <= 0) {
+      return apiError('INVALID_REQUEST', 400, 'points must be a positive number');
+    }
+
     // Resolve model from request headers
     const { model: languageModel } = resolveModelFromHeaders(req);
 
diff --git a/lib/action/engine.ts b/lib/action/engine.ts
index 22b80c87..cb0d49e6 100644
--- a/lib/action/engine.ts
+++ b/lib/action/engine.ts
@@ -214,15 +214,25 @@ export class ActionEngine {
 
     useCanvasStore.getState().playVideo(action.elementId);
 
-    // Wait until the video finishes playing
+    // Wait until the video finishes playing, with a safety timeout to prevent
+    // the playback engine from hanging indefinitely if the video element is
+    // invalid or the state change is missed.
     return new Promise<void>((resolve) => {
+      const MAX_VIDEO_WAIT_MS = 5 * 60 * 1000; // 5 minutes
+      const timeout = setTimeout(() => {
+        unsubscribe();
+        log.warn(`[playVideo] Timeout waiting for video ${action.elementId} to finish`);
+        resolve();
+      }, MAX_VIDEO_WAIT_MS);
       const unsubscribe = useCanvasStore.subscribe((state) => {
         if (state.playingVideoElementId !== action.elementId) {
+          clearTimeout(timeout);
           unsubscribe();
           resolve();
         }
       });
       if (useCanvasStore.getState().playingVideoElementId !== action.elementId) {
+        clearTimeout(timeout);
         unsubscribe();
         resolve();
       }
diff --git a/lib/generation/action-parser.ts b/lib/generation/action-parser.ts
index 7f934ba2..a471b688 100644
--- a/lib/generation/action-parser.ts
+++ b/lib/generation/action-parser.ts
@@ -127,28 +127,27 @@ export function parseActionsFromStructuredOutput(
   }
 
   // Step 6: Filter out slide-only actions for non-slide scenes (defense in depth)
+  let result = actions;
   if (sceneType && sceneType !== 'slide') {
-    const before = actions.length;
-    const filtered = actions.filter((a) => !SLIDE_ONLY_ACTIONS.includes(a.type as ActionType));
-    if (filtered.length < before) {
-      log.info(`Stripped ${before - filtered.length} slide-only action(s) from ${sceneType} scene`);
+    const before = result.length;
+    result = result.filter((a) => !SLIDE_ONLY_ACTIONS.includes(a.type as ActionType));
+    if (result.length < before) {
+      log.info(`Stripped ${before - result.length} slide-only action(s) from ${sceneType} scene`);
     }
-    return filtered;
   }
 
   // Step 7: Filter by allowedActions whitelist (defense in depth for role-based isolation)
   // Catches hallucinated actions not in the agent's permitted set, e.g. a student agent
   // mimicking spotlight/laser after seeing teacher actions in chat history.
   if (allowedActions && allowedActions.length > 0) {
-    const before = actions.length;
-    const filtered = actions.filter((a) => a.type === 'speech' || allowedActions.includes(a.type));
-    if (filtered.length < before) {
+    const before = result.length;
+    result = result.filter((a) => a.type === 'speech' || allowedActions.includes(a.type));
+    if (result.length < before) {
       log.info(
-        `Stripped ${before - filtered.length} disallowed action(s) by allowedActions whitelist`,
+        `Stripped ${before - result.length} disallowed action(s) by allowedActions whitelist`,
       );
     }
-    return filtered;
   }
 
-  return actions;
+  return result;
 }
diff --git a/lib/generation/json-repair.ts b/lib/generation/json-repair.ts
index 0487714b..89f7fa0b 100644
--- a/lib/generation/json-repair.ts
+++ b/lib/generation/json-repair.ts
@@ -111,10 +111,15 @@ export function tryParseJson<T>(jsonStr: string): T | null {
 
     // Fix 1: Handle LaTeX-style escapes that break JSON (e.g., \frac, \left, \right, \times, etc.)
     // These are common in math content and need to be double-escaped
-    // Match backslash followed by letters (LaTeX commands) inside strings
-    fixed = fixed.replace(/"([^"]*?)"/g, (_match, content) => {
-      // Double-escape any backslash followed by a letter (except valid JSON escapes)
-      const fixedContent = content.replace(/\\([a-zA-Z])/g, '\\\\$1');
+    // Match backslash followed by letters (LaTeX commands) inside strings,
+    // but skip valid JSON escape sequences (\b, \f, \n, \r, \t, \u)
+    fixed = fixed.replace(/"([^"\\]*(?:\\.[^"\\]*)*)"/g, (_match, content) => {
+      // Double-escape backslash+letter ONLY for non-JSON-escape letters
+      const fixedContent = content.replace(/\\([a-zA-Z])/g, (_m: string, ch: string) => {
+        // Preserve valid JSON escape sequences
+        if ('bfnrtu'.includes(ch)) return `\\${ch}`;
+        return `\\\\${ch}`;
+      });
       return `"${fixedContent}"`;
     });
 
diff --git a/lib/generation/scene-generator.ts b/lib/generation/scene-generator.ts
index 1dc22937..ff81a840 100644
--- a/lib/generation/scene-generator.ts
+++ b/lib/generation/scene-generator.ts
@@ -1079,7 +1079,9 @@ function formatElementsForPrompt(elements: PPTElement[]): string {
 function formatQuestionsForPrompt(questions: QuizQuestion[]): string {
   return questions
     .map((q, i) => {
-      const optionsText = q.options ? `Options: ${q.options.join(', ')}` : '';
+      const optionsText = q.options
+        ? `Options: ${q.options.map((o) => `${o.value}. ${o.label}`).join(', ')}`
+        : '';
       return `Q${i + 1} (${q.type}): ${q.question}\n${optionsText}`;
     })
     .join('\n\n');
diff --git a/lib/hooks/use-canvas-operations.ts b/lib/hooks/use-canvas-operations.ts
index 92c6d0ca..bfcbfd3e 100644
--- a/lib/hooks/use-canvas-operations.ts
+++ b/lib/hooks/use-canvas-operations.ts
@@ -370,9 +370,7 @@ export function useCanvasOperations() {
     const firstGroupId = activeElementList[0].groupId;
     if (!firstGroupId) return true;
 
-    const inSameGroup = activeElementList.every(
-      (el) => (el.groupId && el.groupId) === firstGroupId,
-    );
+    const inSameGroup = activeElementList.every((el) => el.groupId && el.groupId === firstGroupId);
     return !inSameGroup;
   }, [activeElementList]);
 
diff --git a/lib/hooks/use-order-element.ts b/lib/hooks/use-order-element.ts
index 5fef1e5f..5defc12a 100644
--- a/lib/hooks/use-order-element.ts
+++ b/lib/hooks/use-order-element.ts
@@ -35,6 +35,8 @@ export function useOrderElement() {
       const { minLevel, maxLevel } = getCombineElementLevelRange(elementList, combineElementList);
 
       // Already at the top level, cannot move further
+      if (maxLevel >= elementList.length - 1) return;
+
       const nextElement = copyOfElementList[maxLevel + 1];
       const movedElementList = copyOfElementList.splice(minLevel, combineElementList.length);
 
diff --git a/lib/media/video-providers.ts b/lib/media/video-providers.ts
index bdad4dd0..8c9c4d69 100644
--- a/lib/media/video-providers.ts
+++ b/lib/media/video-providers.ts
@@ -134,10 +134,8 @@ export function normalizeVideoOptions(
       !normalized.aspectRatio ||
       !provider.supportedAspectRatios.includes(normalized.aspectRatio)
     ) {
-      normalized.aspectRatio =
-        normalized.aspectRatio && provider.supportedAspectRatios.includes(normalized.aspectRatio)
-          ? normalized.aspectRatio
-          : (provider.supportedAspectRatios[0] as VideoGenerationOptions['aspectRatio']);
+      normalized.aspectRatio = provider
+        .supportedAspectRatios[0] as VideoGenerationOptions['aspectRatio'];
     }
   }
 
diff --git a/lib/orchestration/stateless-generate.ts b/lib/orchestration/stateless-generate.ts
index c307f091..56c69e04 100644
--- a/lib/orchestration/stateless-generate.ts
+++ b/lib/orchestration/stateless-generate.ts
@@ -213,12 +213,12 @@ export function parseStructuredChunk(chunk: string, state: ParserState): ParseRe
       const remaining = content.slice(state.lastPartialTextLength);
       if (remaining) {
         result.textChunks.push(remaining);
+        // Only push ordered entry when there is actual content to emit
+        result.ordered.push({
+          type: 'text',
+          index: result.textChunks.length - 1,
+        });
       }
-      // Use per-call array index for consistency with emitItem fix
-      result.ordered.push({
-        type: 'text',
-        index: result.textChunks.length - 1,
-      });
       textSegmentIndex++;
       state.lastPartialTextLength = 0;
       continue;
diff --git a/lib/playback/engine.ts b/lib/playback/engine.ts
index c9c5c8bf..cd55d36f 100644
--- a/lib/playback/engine.ts
+++ b/lib/playback/engine.ts
@@ -500,8 +500,10 @@ export class PlaybackEngine {
             ? { dimOpacity: action.dimOpacity }
             : { color: action.color }),
         } as Effect);
-        // Don't block — continue immediately
-        this.processNext();
+        // Don't block — continue immediately (use queueMicrotask to avoid
+        // stack overflow from deep synchronous recursion when many consecutive
+        // spotlight/laser actions appear in sequence)
+        queueMicrotask(() => this.processNext());
         break;
       }
 
@@ -633,7 +635,9 @@ export class PlaybackEngine {
       // No usable voice configured — detect text language so the browser
       // auto-selects an appropriate voice.
       const cjkRatio =
-        (chunkText.match(/[\u4e00-\u9fff\u3400-\u4dbf]/g) || []).length / chunkText.length;
+        chunkText.length > 0
+          ? (chunkText.match(/[\u4e00-\u9fff\u3400-\u4dbf]/g) || []).length / chunkText.length
+          : 0;
       utterance.lang = cjkRatio > CJK_LANG_THRESHOLD ? 'zh-CN' : 'en-US';
     }