feat: phase 4 of AI pipeline hardening — quiz & content quality

- src/lib/random.js: Fisher–Yates shuffle/sample/pickInt; replace every
  biased .sort(() => 0.5 - Math.random()) site in testService.
- testService: debias correctIndex via prompt + runtime re-roll (up to 2x
  when one position holds >50%); quality gate rejecting <4 distinct
  options, banned filler ("all of the above" etc) and explanations
  shorter than 20 chars; dedup new questions against the existing bank
  via normalised question text.
- Quiz schema/tool/prompt require difficulty ('easy'|'medium'|'hard');
  db.getQuizBank defaults legacy records to 'medium' on read.
- learningService.generateCustomTopic: kebab-case slug ID from the
  polished label with collision suffixes; default learning_relevance
  'standard' when the model omits it.
- Tests for random helpers, dedup/quality-gate behaviour and the
  extended quiz schema.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/admin/TestManager.jsx

@@ -34,7 +34,7 @@ const TestManager = () => {
     loadData();
   }, [selectedTopic]);
 
-  const handleGenerate = async (topic, count = 10) => {
+  const handleGenerate = async (topic, count = 5) => {
     setLoadingTopicId(topic.id);
     setError(null);
     try {
@@ -97,8 +97,8 @@ const TestManager = () => {
           {questions.length === 0 ? (
             <Card className="text-center py-12 text-fg-muted border-dashed border-2">
               <p>No questions generated for this topic yet.</p>
-              <Button className="mt-4" onClick={() => handleGenerate(selectedTopic, 10)} disabled={loadingTopicId === selectedTopic.id}>
-                Generate 10 Questions
+              <Button className="mt-4" onClick={() => handleGenerate(selectedTopic, 5)} disabled={loadingTopicId === selectedTopic.id}>
+                Generate 5 Questions
               </Button>
             </Card>
           ) : (

src/lib/__tests__/llmSchemas.test.js

@@ -108,7 +108,7 @@ describe('learning schemas', () => {
 });
 
 describe('quizQuestionsSchema', () => {
-  it('accepts a quiz with four options and a valid correctIndex', () => {
+  it('accepts a quiz with four options, a valid correctIndex and a difficulty', () => {
     const parsed = quizQuestionsSchema.parse({
       questions: [
         {
@@ -118,10 +118,12 @@ describe('quizQuestionsSchema', () => {
           options: ['A', 'B', 'C', 'D'],
           correctIndex: 2,
           explanation: 'C describes the buddy system best.',
+          difficulty: 'easy',
         },
       ],
     });
     expect(parsed.questions[0].options).toHaveLength(4);
+    expect(parsed.questions[0].difficulty).toBe('easy');
   });
 
   it('rejects three options or an out-of-range correctIndex', () => {
@@ -135,6 +137,7 @@ describe('quizQuestionsSchema', () => {
             options: ['A', 'B', 'C'],
             correctIndex: 0,
             explanation: 'e',
+            difficulty: 'medium',
           },
         ],
       }),
@@ -149,11 +152,27 @@ describe('quizQuestionsSchema', () => {
             options: ['A', 'B', 'C', 'D'],
             correctIndex: 4,
             explanation: 'e',
+            difficulty: 'medium',
           },
         ],
       }),
     ).toThrow();
   });
+
+  it('rejects a missing or unknown difficulty', () => {
+    const base = {
+      id: 'q',
+      question: 'q',
+      topicLabel: 't',
+      options: ['A', 'B', 'C', 'D'],
+      correctIndex: 0,
+      explanation: 'because',
+    };
+    expect(() => quizQuestionsSchema.parse({ questions: [base] })).toThrow();
+    expect(() =>
+      quizQuestionsSchema.parse({ questions: [{ ...base, difficulty: 'trivial' }] }),
+    ).toThrow();
+  });
 });
 
 describe('customTopicSchema', () => {

src/lib/__tests__/random.test.js

@@ -0,0 +1,48 @@
+import { describe, expect, it } from 'vitest';
+import { shuffle, sample, pickInt } from '../random';
+
+describe('shuffle', () => {
+  it('returns a new array containing the same elements', () => {
+    const input = [1, 2, 3, 4, 5];
+    const out = shuffle(input);
+    expect(out).not.toBe(input);
+    expect([...out].sort()).toEqual([...input].sort());
+    expect(input).toEqual([1, 2, 3, 4, 5]);
+  });
+
+  it('handles empty and single-element arrays', () => {
+    expect(shuffle([])).toEqual([]);
+    expect(shuffle([42])).toEqual([42]);
+  });
+});
+
+describe('sample', () => {
+  it('returns up to n unique elements from the source array', () => {
+    const out = sample([1, 2, 3, 4, 5], 3);
+    expect(out).toHaveLength(3);
+    expect(new Set(out).size).toBe(3);
+    for (const v of out) expect([1, 2, 3, 4, 5]).toContain(v);
+  });
+
+  it('returns the full shuffled array when n exceeds length', () => {
+    const out = sample([1, 2, 3], 10);
+    expect(out).toHaveLength(3);
+    expect([...out].sort()).toEqual([1, 2, 3]);
+  });
+
+  it('returns an empty array when n is zero or negative', () => {
+    expect(sample([1, 2, 3], 0)).toEqual([]);
+    expect(sample([1, 2, 3], -2)).toEqual([]);
+  });
+});
+
+describe('pickInt', () => {
+  it('returns an integer in the inclusive range', () => {
+    for (let i = 0; i < 100; i++) {
+      const v = pickInt(2, 5);
+      expect(Number.isInteger(v)).toBe(true);
+      expect(v).toBeGreaterThanOrEqual(2);
+      expect(v).toBeLessThanOrEqual(5);
+    }
+  });
+});

src/lib/__tests__/testService.test.js

@@ -0,0 +1,112 @@
+import { describe, expect, it, vi, beforeEach, afterEach } from 'vitest';
+
+const bankStore = new Map();
+const callLLMMock = vi.fn();
+
+vi.mock('../pb', () => ({ pb: { collection: () => ({}) } }));
+
+vi.mock('../db', () => ({
+  getQuizBank: vi.fn(async (topicId) => bankStore.get(topicId) || []),
+  setQuizBank: vi.fn(async (topicId, qs) => { bankStore.set(topicId, qs); }),
+  getTopics: vi.fn(async () => []),
+  deleteQuestionFromBank: vi.fn(),
+  getCachedQuiz: vi.fn(),
+  setCachedQuiz: vi.fn(),
+  getQuizResult: vi.fn(),
+  saveQuizResult: vi.fn(),
+  getTeamMembers: vi.fn(async () => []),
+  upsertLeaderboardEntry: vi.fn(),
+  getCurriculum: vi.fn(),
+}));
+
+vi.mock('../llm', () => ({ callLLM: (...args) => callLLMMock(...args) }));
+vi.mock('../curriculumService', () => ({
+  getCurriculumTopic: vi.fn(async () => ({ topic: null })),
+  getQuarterForWeek: vi.fn(() => 1),
+}));
+
+import { forceGenerateTopicQuestions } from '../testService';
+
+const topic = { id: 'onboarding', label: 'Onboarding', type: 'concept', description: 'Onboarding for new joiners.' };
+
+function makeQuestion(i, overrides = {}) {
+  return {
+    id: `q-${i}`,
+    question: `Sample question ${i}?`,
+    topicLabel: 'Onboarding',
+    options: ['A) one', 'B) two', 'C) three', 'D) four'],
+    correctIndex: i % 4,
+    explanation: 'This is a substantive explanation for the correct answer.',
+    difficulty: 'medium',
+    ...overrides,
+  };
+}
+
+function llmEmits(questions) {
+  callLLMMock.mockResolvedValueOnce({ toolUses: [{ name: 'emit_quiz_questions', input: { questions } }] });
+}
+
+describe('forceGenerateTopicQuestions', () => {
+  let debugSpy, warnSpy;
+  beforeEach(() => {
+    bankStore.clear();
+    callLLMMock.mockReset();
+    debugSpy = vi.spyOn(console, 'debug').mockImplementation(() => {});
+    warnSpy = vi.spyOn(console, 'warn').mockImplementation(() => {});
+  });
+  afterEach(() => { debugSpy.mockRestore(); warnSpy.mockRestore(); });
+
+  it('persists a well-formed batch and assigns topic-scoped ids', async () => {
+    llmEmits([0, 1, 2, 3, 4].map((i) => makeQuestion(i)));
+    const out = await forceGenerateTopicQuestions(topic, 5);
+    expect(out).toHaveLength(5);
+    for (const q of out) expect(q.id.startsWith('onboarding-')).toBe(true);
+    expect(bankStore.get('onboarding')).toHaveLength(5);
+  });
+
+  it('re-rolls when one correctIndex dominates the batch, then accepts on the third try', async () => {
+    const allZero = [0, 1, 2, 3, 4].map((i) => makeQuestion(i, { correctIndex: 0 }));
+    llmEmits(allZero);
+    llmEmits(allZero);
+    llmEmits(allZero);
+    const out = await forceGenerateTopicQuestions(topic, 5);
+    expect(callLLMMock).toHaveBeenCalledTimes(3);
+    expect(out).toHaveLength(5);
+    expect(warnSpy).toHaveBeenCalledWith(expect.stringContaining('correctIndex dominated'));
+  });
+
+  it('rejects a batch containing a banned "all of the above" option', async () => {
+    const bad = [0, 1, 2, 3, 4].map((i) =>
+      makeQuestion(i, { options: ['A) x', 'B) y', 'C) z', 'D) All of the above'] }),
+    );
+    llmEmits(bad);
+    llmEmits(bad);
+    llmEmits(bad);
+    await expect(forceGenerateTopicQuestions(topic, 5)).rejects.toThrow(/banned filler|rejected/i);
+  });
+
+  it('rejects a batch where an explanation is too short', async () => {
+    const bad = [0, 1, 2, 3, 4].map((i) => makeQuestion(i, { explanation: 'Because.' }));
+    llmEmits(bad);
+    llmEmits(bad);
+    llmEmits(bad);
+    await expect(forceGenerateTopicQuestions(topic, 5)).rejects.toThrow(/too short|rejected/i);
+  });
+
+  it('drops duplicates whose normalized text matches an existing bank entry', async () => {
+    bankStore.set('onboarding', [
+      { ...makeQuestion(99), id: 'old-1', question: 'What is the BUDDY system???' },
+    ]);
+    llmEmits([
+      makeQuestion(0, { question: 'what is the buddy system!' }),
+      makeQuestion(1, { question: 'Brand new question one?' }),
+      makeQuestion(2, { question: 'Brand new question two?' }),
+      makeQuestion(3, { question: 'Brand new question three?' }),
+      makeQuestion(4, { question: 'Brand new question four?' }),
+    ]);
+    const out = await forceGenerateTopicQuestions(topic, 5);
+    expect(out).toHaveLength(4);
+    expect(out.find((q) => q.question.toLowerCase().includes('buddy'))).toBeUndefined();
+    expect(debugSpy).toHaveBeenCalledWith(expect.stringContaining('dropped duplicate'), expect.any(String));
+  });
+});

src/lib/db.js

@@ -90,10 +90,15 @@ export async function deleteContent(topicId) {
 
 // ── Quiz Banks ───────────────────────────────────────────────────────────────
 
+function normalizeQuizQuestion(q) {
+  if (!q || typeof q !== 'object') return q;
+  return q.difficulty ? q : { ...q, difficulty: 'medium' };
+}
+
 export async function getQuizBank(topicId) {
   try {
     const r = await pb.collection('quiz_banks').getFirstListItem(`topic_id="${topicId}"`);
-    return r.questions || [];
+    return (r.questions || []).map(normalizeQuizQuestion);
   } catch { return []; }
 }
 

src/lib/learningService.js

@@ -154,6 +154,28 @@ export async function deleteCachedContent(topicId) {
   return db.deleteContent(topicId);
 }
 
+function slugify(label) {
+  const base = String(label || '')
+    .toLowerCase()
+    .normalize('NFKD')
+    .replace(/\p{Diacritic}/gu, '')
+    .replace(/[^a-z0-9]+/g, '-')
+    .replace(/^-+|-+$/g, '');
+  return base || 'topic';
+}
+
+async function pickUniqueTopicId(label) {
+  const existing = await db.getTopics();
+  const used = new Set(existing.map((t) => t.id));
+  const base = slugify(label);
+  if (!used.has(base)) return base;
+  for (let i = 2; i < 1000; i++) {
+    const candidate = `${base}-${i}`;
+    if (!used.has(candidate)) return candidate;
+  }
+  return `${base}-${Date.now().toString(36)}`;
+}
+
 export async function generateCustomTopic(label) {
   const result = await callLLM({
     task: 'topic.custom',
@@ -168,7 +190,12 @@ export async function generateCustomTopic(label) {
   const emitted = result.toolUses[0]?.input;
   if (!emitted) throw new Error('Could not process custom topic. Please try again.');
 
-  const newTopic = { ...emitted, id: 'custom_' + Date.now().toString(36) };
+  const id = await pickUniqueTopicId(emitted.label);
+  const newTopic = {
+    ...emitted,
+    id,
+    learning_relevance: emitted.learning_relevance || 'standard',
+  };
   await db.upsertTopic(newTopic);
   return newTopic;
 }

src/lib/llmSchemas.js

@@ -122,6 +122,8 @@ export const learningAllSchema = z.object({
   infographic: infographicBodySchema,
 });
 
+const quizDifficultyEnum = z.enum(['easy', 'medium', 'hard']);
+
 const quizQuestionSchema = z.object({
   id: z.string().min(1),
   question: z.string().min(1),
@@ -129,6 +131,7 @@ const quizQuestionSchema = z.object({
   options: z.array(z.string().min(1)).length(4),
   correctIndex: z.number().int().min(0).max(3),
   explanation: z.string().min(1),
+  difficulty: quizDifficultyEnum,
 });
 
 export const quizQuestionsSchema = z.object({

src/lib/llmTools.js

@@ -205,6 +205,8 @@ export const EMIT_CUSTOM_TOPIC_TOOL = {
   },
 };
 
+const QUIZ_DIFFICULTIES = ['easy', 'medium', 'hard'];
+
 const quizQuestionSchema = {
   type: 'object',
   properties: {
@@ -214,8 +216,9 @@ const quizQuestionSchema = {
     options: { type: 'array', items: { type: 'string' }, minItems: 4, maxItems: 4 },
     correctIndex: { type: 'integer', minimum: 0, maximum: 3 },
     explanation: { type: 'string', description: 'Why the correct answer is correct (1–2 sentences).' },
+    difficulty: { type: 'string', enum: QUIZ_DIFFICULTIES, description: 'Per-question difficulty tag.' },
   },
-  required: ['id', 'question', 'topicLabel', 'options', 'correctIndex', 'explanation'],
+  required: ['id', 'question', 'topicLabel', 'options', 'correctIndex', 'explanation', 'difficulty'],
 };
 
 export const EMIT_QUIZ_QUESTIONS_TOOL = {

src/lib/random.js

@@ -0,0 +1,29 @@
+/**
+ * Shared randomness helpers.
+ *
+ * `Array.prototype.sort(() => 0.5 - Math.random())` is biased — modern V8
+ * sorts use Timsort, which compares each element more than once and skews
+ * the resulting permutation. Use `shuffle` for anything user-visible
+ * (quiz options, review topic selection, leaderboards).
+ */
+
+export function shuffle(arr) {
+  const out = [...arr];
+  for (let i = out.length - 1; i > 0; i--) {
+    const j = Math.floor(Math.random() * (i + 1));
+    [out[i], out[j]] = [out[j], out[i]];
+  }
+  return out;
+}
+
+export function sample(arr, n) {
+  if (n <= 0) return [];
+  if (n >= arr.length) return shuffle(arr);
+  return shuffle(arr).slice(0, n);
+}
+
+export function pickInt(min, maxInclusive) {
+  const lo = Math.ceil(min);
+  const hi = Math.floor(maxInclusive);
+  return lo + Math.floor(Math.random() * (hi - lo + 1));
+}

src/lib/testService.js

@@ -2,81 +2,73 @@ import * as db from './db';
 import { callLLM } from './llm';
 import { EMIT_QUIZ_QUESTIONS_TOOL } from './llmTools';
 import { getCurriculumTopic, getQuarterForWeek } from './curriculumService';
+import { shuffle, sample } from './random';
 
 const QUIZ_SYSTEM = `You are a quiz generator for Respellion, an internal IT company learning platform.
 You generate multiple-choice questions to test employee knowledge on specific topics.
 Always write in clear, professional English.
 
-Emit questions through the emit_quiz_questions tool. Each question has exactly four options; correctIndex is 0-based; mix difficulty roughly 4 easy / 4 medium / 2 hard.`;
+Emit questions through the emit_quiz_questions tool. Each question has exactly four options; correctIndex is 0-based. Tag every question with difficulty ('easy', 'medium' or 'hard'). For a typical 5-question batch, mix difficulty roughly 2 easy / 2 medium / 1 hard; scale the ratio proportionally for larger batches.
+
+Distribute correctIndex roughly evenly across 0, 1, 2, and 3. Do not place the correct answer at the same position more than 4 out of 10 times.
+
+Never use filler options such as "all of the above", "none of the above", or "both A and B". Every explanation must be a substantive sentence (≥ 20 characters) describing why the correct answer is correct.`;
+
+const BANNED_OPTION_PATTERNS = [
+  /all of the above/i,
+  /none of the above/i,
+  /both a and b/i,
+  /both b and c/i,
+  /both c and d/i,
+  /both a and c/i,
+  /both b and d/i,
+  /both a and d/i,
+];
 
 const cachedSystem = (text) => [{ type: 'text', text, cache_control: { type: 'ephemeral' } }];
 
-async function selectTestTopics(userId, weekNumber) {
-  const allTopics = await db.getTopics();
-  const topics = allTopics.filter(t => t.type !== 'fact' && t.learning_relevance !== 'exclude');
-  if (!topics || topics.length === 0) return { primaryTopic: null, reviewTopics: [], isReviewWeek: false };
-
-  // Try curriculum-based selection first
-  try {
-    const { topic, curriculumEntry } = await getCurriculumTopic(weekNumber);
-
-    if (curriculumEntry?.is_review_week) {
-      // Review week: pull topics from the whole quarter
-      const quarter = getQuarterForWeek(weekNumber);
-      const curriculum = await db.getCurriculum(new Date().getFullYear());
-      const quarterTopicIds = curriculum
-        .filter(w => w.quarter === quarter && w.topic_id && !w.is_review_week)
-        .map(w => w.topic_id);
-      const quarterTopics = topics.filter(t => quarterTopicIds.includes(t.id));
-      // Use all quarter topics as review topics (no single primary)
-      return {
-        primaryTopic: quarterTopics[0] || topics[0],
-        reviewTopics: quarterTopics.slice(1),
-        isReviewWeek: true,
-      };
-    }
-
-    if (topic) {
-      const others = topics.filter(t => t.id !== topic.id);
-      const shuffled = others.sort(() => 0.5 - Math.random());
-      const reviewTopics = shuffled.slice(0, Math.min(5, shuffled.length));
-      return { primaryTopic: topic, reviewTopics, isReviewWeek: false };
-    }
-  } catch (e) {
-    console.warn('[Test] Curriculum lookup failed, falling back to hash:', e.message);
-  }
-
-  // Fallback: hash-based selection
-  const str = `${userId}:${weekNumber}`;
-  let hash = 0;
-  for (let i = 0; i < str.length; i++) {
-    hash = (hash << 5) - hash + str.charCodeAt(i);
-    hash |= 0;
-  }
-  const primaryIndex = Math.abs(hash) % topics.length;
-  const primaryTopic = topics[primaryIndex];
-
-  const others = topics.filter((_, i) => i !== primaryIndex);
-  const shuffled = others.sort(() => 0.5 - Math.random());
-  const reviewTopics = shuffled.slice(0, Math.min(5, shuffled.length));
-
-  return { primaryTopic, reviewTopics, isReviewWeek: false };
+function normalizeQuestionText(text) {
+  return String(text || '')
+    .toLowerCase()
+    .replace(/[\p{P}\p{S}]/gu, ' ')
+    .replace(/\s+/g, ' ')
+    .trim();
 }
 
-export async function getCachedQuiz(userId, weekNumber) {
-  return db.getCachedQuiz(userId, weekNumber);
+function dominantCorrectIndex(questions) {
+  if (!questions.length) return null;
+  const counts = [0, 0, 0, 0];
+  for (const q of questions) counts[q.correctIndex] = (counts[q.correctIndex] || 0) + 1;
+  const max = Math.max(...counts);
+  return max / questions.length > 0.5 ? { index: counts.indexOf(max), ratio: max / questions.length } : null;
 }
 
-export async function forceGenerateTopicQuestions(topic, count = 10) {
-  let bank = await db.getQuizBank(topic.id);
+function validateBatchQuality(questions) {
+  for (const q of questions) {
+    const distinct = new Set(q.options.map((o) => o.trim().toLowerCase()));
+    if (distinct.size < 4) {
+      return `Question "${q.question}" has duplicate options.`;
+    }
+    for (const opt of q.options) {
+      if (BANNED_OPTION_PATTERNS.some((re) => re.test(opt))) {
+        return `Question "${q.question}" uses a banned filler option ("${opt}").`;
+      }
+    }
+    if (!q.explanation || q.explanation.trim().length < 20) {
+      return `Question "${q.question}" has an explanation that is too short.`;
+    }
+  }
+  return null;
+}
 
+async function callQuizModel(topic, count) {
   const prompt = `Generate exactly ${count} multiple-choice quiz questions for this knowledge topic and emit them via the emit_quiz_questions tool:
 
 Topic: ${topic.label}
 Type: ${topic.type}
 Description: ${topic.description}
 
-Options must be prefixed "A) ", "B) ", "C) ", "D) ". Make questions specific and practical, not trivial.`;
+Options must be prefixed "A) ", "B) ", "C) ", "D) ". Make questions specific and practical, not trivial. Example: a question whose correct answer is option C uses "correctIndex": 2.`;
 
   const result = await callLLM({
     task: 'quiz.generate',
@@ -89,28 +81,125 @@ Options must be prefixed "A) ", "B) ", "C) ", "D) ". Make questions specific and
   });
 
   const emitted = result.toolUses[0]?.input;
-  if (!emitted) throw new Error(`Could not generate questions for ${topic.label}`);
+  if (!emitted?.questions?.length) {
+    throw new Error(`Could not generate questions for ${topic.label}`);
+  }
+  return emitted.questions;
+}
 
-  const newQuestions = (emitted.questions || []).map(q => ({
+async function selectTestTopics(userId, weekNumber) {
+  const allTopics = await db.getTopics();
+  const topics = allTopics.filter(t => t.type !== 'fact' && t.learning_relevance !== 'exclude');
+  if (!topics || topics.length === 0) return { primaryTopic: null, reviewTopics: [], isReviewWeek: false };
+
+  try {
+    const { topic, curriculumEntry } = await getCurriculumTopic(weekNumber);
+
+    if (curriculumEntry?.is_review_week) {
+      const quarter = getQuarterForWeek(weekNumber);
+      const curriculum = await db.getCurriculum(new Date().getFullYear());
+      const quarterTopicIds = curriculum
+        .filter(w => w.quarter === quarter && w.topic_id && !w.is_review_week)
+        .map(w => w.topic_id);
+      const quarterTopics = topics.filter(t => quarterTopicIds.includes(t.id));
+      return {
+        primaryTopic: quarterTopics[0] || topics[0],
+        reviewTopics: quarterTopics.slice(1),
+        isReviewWeek: true,
+      };
+    }
+
+    if (topic) {
+      const others = topics.filter(t => t.id !== topic.id);
+      const reviewTopics = sample(others, Math.min(5, others.length));
+      return { primaryTopic: topic, reviewTopics, isReviewWeek: false };
+    }
+  } catch (e) {
+    console.warn('[Test] Curriculum lookup failed, falling back to hash:', e.message);
+  }
+
+  const str = `${userId}:${weekNumber}`;
+  let hash = 0;
+  for (let i = 0; i < str.length; i++) {
+    hash = (hash << 5) - hash + str.charCodeAt(i);
+    hash |= 0;
+  }
+  const primaryIndex = Math.abs(hash) % topics.length;
+  const primaryTopic = topics[primaryIndex];
+
+  const others = topics.filter((_, i) => i !== primaryIndex);
+  const reviewTopics = sample(others, Math.min(5, others.length));
+
+  return { primaryTopic, reviewTopics, isReviewWeek: false };
+}
+
+export async function getCachedQuiz(userId, weekNumber) {
+  return db.getCachedQuiz(userId, weekNumber);
+}
+
+export async function forceGenerateTopicQuestions(topic, count = 5) {
+  const existingBank = await db.getQuizBank(topic.id);
+  const existingKeys = new Set(existingBank.map((q) => normalizeQuestionText(q.question)));
+
+  let lastQualityError = null;
+  let candidates = null;
+
+  for (let attempt = 0; attempt < 3; attempt++) {
+    const questions = await callQuizModel(topic, count);
+
+    const qualityError = validateBatchQuality(questions);
+    if (qualityError) {
+      lastQualityError = qualityError;
+      console.warn(`[quiz] batch rejected (attempt ${attempt + 1}): ${qualityError}`);
+      continue;
+    }
+
+    const dominant = dominantCorrectIndex(questions);
+    if (dominant && attempt < 2) {
+      console.warn(`[quiz] correctIndex dominated by ${dominant.index} (${Math.round(dominant.ratio * 100)}%) — re-rolling`);
+      continue;
+    }
+
+    candidates = questions;
+    break;
+  }
+
+  if (!candidates) {
+    throw new Error(`Quality gate rejected the generated batch for ${topic.label}: ${lastQualityError || 'unbalanced answer distribution'}. Click "Generate" to try again.`);
+  }
+
+  const accepted = [];
+  for (const q of candidates) {
+    const key = normalizeQuestionText(q.question);
+    if (existingKeys.has(key)) {
+      console.debug('[quiz] dropped duplicate:', q.question);
+      continue;
+    }
+    existingKeys.add(key);
+    accepted.push({
       ...q,
       id: `${topic.id}-${Math.random().toString(36).slice(2, 11)}`,
-  }));
+    });
+  }
 
-  bank = [...bank, ...newQuestions];
-  await db.setQuizBank(topic.id, bank);
-  return newQuestions;
+  if (!accepted.length) {
+    throw new Error(`All generated questions for ${topic.label} were duplicates of existing ones.`);
+  }
+
+  const merged = [...existingBank, ...accepted];
+  await db.setQuizBank(topic.id, merged);
+  return accepted;
 }
 
 async function getOrGenerateTopicQuestions(topic, count) {
   let bank = await db.getQuizBank(topic.id);
 
   if (bank.length < count) {
-    await forceGenerateTopicQuestions(topic, 10);
+    await forceGenerateTopicQuestions(topic, 5);
     bank = await db.getQuizBank(topic.id);
   }
 
-  const shuffled = [...bank].sort(() => 0.5 - Math.random());
-  return shuffled.slice(0, Math.min(count, shuffled.length));
+  return sample(bank, Math.min(count, bank.length));
 }
 
 export async function getTopicQuestionBank(topicId) {
@@ -151,10 +240,10 @@ export async function generateWeeklyQuiz(userId, weekNumber, force = false) {
     }
   }
 
-  questions.sort(() => 0.5 - Math.random());
+  const shuffled = shuffle(questions);
 
   const quiz = {
-    questions,
+    questions: shuffled,
     meta: {
       userId,
       weekNumber,