diff --git a/scripts/test-hints.ts b/scripts/test-hints.ts
index f7c4c33..ef5acf3 100644
--- a/scripts/test-hints.ts
+++ b/scripts/test-hints.ts
@@ -20,7 +20,7 @@
 
 import * as fs from 'fs';
 import * as path from 'path';
-import sanitizeHtml from 'sanitize-html';
+import { formatHintOutput } from '../src/lib/formatHintOutput';
 
 // --- ANSI colors ---
 const RED = '\x1b[0;31m';
@@ -71,6 +71,7 @@ interface ContractCase {
   payload: Record<string, unknown>;
   expectedStatus: number;
   apiKey?: string | null;
+  requiresAuthEnforcement?: boolean;
 }
 
 // --- Helpers ---
@@ -108,12 +109,14 @@ const contractCases: ContractCase[] = [
     payload: validContractRequest,
     expectedStatus: 401,
     apiKey: null,
+    requiresAuthEnforcement: true,
   },
   {
     name: 'invalid API key is rejected before processing',
     payload: validContractRequest,
     expectedStatus: 403,
     apiKey: `${API_KEY}-invalid`,
+    requiresAuthEnforcement: true,
   },
   {
     name: 'unknown request fields are rejected',
@@ -179,11 +182,38 @@ async function checkHealth(): Promise<void> {
   }
 }
 
+async function detectAuthEnforcement(): Promise<boolean> {
+  try {
+    const response = await fetch(`${BASE_URL}/hint`, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ probe: 'auth-enforcement' }),
+    });
+    return response.status === 401;
+  } catch {
+    return false;
+  }
+}
+
 async function runContractTests(): Promise<boolean> {
   console.log(`${YELLOW}Running local API contract checks...${NC}`);
   let failed = 0;
+  let skipped = 0;
+
+  const authEnforced = await detectAuthEnforcement();
+  if (!authEnforced) {
+    console.log(
+      `  ${YELLOW}!${NC} API key auth is not enforced on this server; auth cases will be skipped`,
+    );
+  }
 
   for (const testCase of contractCases) {
+    if (testCase.requiresAuthEnforcement && !authEnforced) {
+      skipped++;
+      console.log(`  ${YELLOW}-${NC} ${testCase.name} (skipped)`);
+      continue;
+    }
+
     const headers: Record<string, string> = { 'Content-Type': 'application/json' };
     const apiKey = testCase.apiKey === undefined ? API_KEY : testCase.apiKey;
     if (apiKey !== null) headers['X-API-Key'] = apiKey;
@@ -210,18 +240,18 @@ async function runContractTests(): Promise<boolean> {
     }
   }
 
+  if (skipped > 0) {
+    console.log(
+      `  ${YELLOW}${skipped} auth case(s) skipped${NC} — run against staging to exercise them`,
+    );
+  }
+
   console.log('');
   return failed === 0;
 }
 
 function followsHintOutputContract(hint: string): boolean {
-  return (
-    sanitizeHtml(hint, {
-      allowedTags: ['code'],
-      allowedAttributes: {},
-      disallowedTagsMode: 'escape',
-    }) === hint
-  );
+  return formatHintOutput(hint) === hint;
 }
 
 // --- Run a single test ---
diff --git a/src/config/swagger.ts b/src/config/swagger.ts
index 667cc9f..4f3224e 100644
--- a/src/config/swagger.ts
+++ b/src/config/swagger.ts
@@ -1,4 +1,5 @@
 import { SERVER_URL } from './env';
+import { MAX_HINT_RESPONSE_CHARS } from '../lib/formatHintOutput';
 
 const swaggerDefinition: Record<string, unknown> = {
   openapi: '3.0.0',
@@ -160,8 +161,9 @@ export const sharedSchemas = [
     properties: {
       hint: {
         type: 'string',
+        maxLength: MAX_HINT_RESPONSE_CHARS,
         description:
-          'The AI-generated hint. Only <code> elements without attributes are active HTML; all other tags are encoded as text.',
+          'The AI-generated hint. Only <code> elements without attributes are active HTML; all other tags are encoded as text. Safe for element-context insertion only: double and single quotes are not escaped, so do not interpolate this value into an HTML attribute.',
         example: 'Check whether your <code>sum</code> function returns a value.',
       },
       model_used: {
diff --git a/src/lib/__tests__/formatHintOutput.test.ts b/src/lib/__tests__/formatHintOutput.test.ts
index 6f72347..95a54cb 100644
--- a/src/lib/__tests__/formatHintOutput.test.ts
+++ b/src/lib/__tests__/formatHintOutput.test.ts
@@ -1,5 +1,9 @@
 import { describe, expect, it } from 'vitest';
-import { formatHintOutput, MAX_HINT_CODE_POINTS } from '../formatHintOutput';
+import {
+  formatHintOutput,
+  MAX_HINT_CODE_POINTS,
+  MAX_HINT_RESPONSE_CHARS,
+} from '../formatHintOutput';
 
 describe('formatHintOutput', () => {
   it('preserves attribute-free code elements', () => {
@@ -59,4 +63,44 @@ describe('formatHintOutput', () => {
   it('returns an empty string for empty input', () => {
     expect(formatHintOutput('   ')).toBe('');
   });
+
+  it('keeps attributes readable as text on escaped elements', () => {
+    expect(formatHintOutput('Your <img src="cat.jpg"> is missing alt text.')).toBe(
+      'Your &lt;img src="cat.jpg"&gt; is missing alt text.',
+    );
+  });
+
+  it('keeps attributes readable as text inside code elements', () => {
+    expect(formatHintOutput('Use <code><meta charset="utf-8"></code>.')).toBe(
+      'Use <code>&lt;meta charset="utf-8"&gt;</code>.',
+    );
+  });
+
+  it('does not let raw-text elements swallow the closing code tag', () => {
+    expect(formatHintOutput('Wrap it in <code><textarea></code> and try again.')).toBe(
+      'Wrap it in <code>&lt;textarea&gt;</code> and try again.',
+    );
+  });
+
+  it('escapes HTML comments instead of deleting them', () => {
+    expect(formatHintOutput('Comments look like <!-- this -->, and try again.')).toBe(
+      'Comments look like &lt;!-- this --&gt;, and try again.',
+    );
+  });
+
+  it('escapes doctype declarations instead of deleting them', () => {
+    expect(formatHintOutput('Start with <!doctype html> at the top.')).toBe(
+      'Start with &lt;!doctype html&gt; at the top.',
+    );
+  });
+
+  it('escapes a bare ampersand', () => {
+    expect(formatHintOutput('Tom & Jerry')).toBe('Tom &amp; Jerry');
+  });
+
+  it('stays within the documented response ceiling for maximal escaping', () => {
+    const worstCase = '&'.repeat(MAX_HINT_CODE_POINTS * 2);
+
+    expect(formatHintOutput(worstCase).length).toBeLessThanOrEqual(MAX_HINT_RESPONSE_CHARS);
+  });
 });
diff --git a/src/lib/__tests__/normalizeHintRequest.test.ts b/src/lib/__tests__/normalizeHintRequest.test.ts
index 706c170..a7723b8 100644
--- a/src/lib/__tests__/normalizeHintRequest.test.ts
+++ b/src/lib/__tests__/normalizeHintRequest.test.ts
@@ -71,6 +71,22 @@ describe('normalizeHintRequest', () => {
     expect(result.hints).toBe('failureinjected');
   });
 
+  it('removes prompt-frame tags nested inside themselves', () => {
+    const result = normalizeHintRequest(
+      validBody({
+        description: '<challenge_<challenge_description>description>',
+        userInput: '<stu<student_code>dent_code>',
+        seed: '</stu</student_code>dent_code>',
+        hints: [{ text: '<fail<failing_test>ing_test>', failed: true }],
+      }),
+    );
+
+    expect(result.description).toBe('');
+    expect(result.userInput).toBe('');
+    expect(result.seed).toBe('');
+    expect(result.hints).toBe('');
+  });
+
   it('preserves ordinary HTML in learner code', () => {
     const result = normalizeHintRequest(
       validBody({ userInput: '<main><output id="result"></output></main>' }),
@@ -79,6 +95,14 @@ describe('normalizeHintRequest', () => {
     expect(result.userInput).toBe('<main><output id="result"></output></main>');
   });
 
+  it('drops an unrecognised challengeType for direct callers', () => {
+    const result = normalizeHintRequest(
+      validBody({ challengeType: 'ruby' as HintRequestBody['challengeType'] }),
+    );
+
+    expect(result.challengeType).toBeUndefined();
+  });
+
   it('retains defensive guards for direct callers', () => {
     expect(() => normalizeHintRequest(validBody({ description: '' }))).toThrow(
       InputValidationError,
diff --git a/src/lib/formatHintOutput.ts b/src/lib/formatHintOutput.ts
index bb1d19a..b0e6325 100644
--- a/src/lib/formatHintOutput.ts
+++ b/src/lib/formatHintOutput.ts
@@ -1,30 +1,48 @@
-import sanitizeHtml from 'sanitize-html';
-
 export const MAX_HINT_CODE_POINTS = 1000;
 
+const ELLIPSIS_CHARS = 3;
+const MAX_CHARS_PER_ESCAPED_CODE_POINT = 5;
+
+export const MAX_HINT_RESPONSE_CHARS =
+  MAX_HINT_CODE_POINTS * MAX_CHARS_PER_ESCAPED_CODE_POINT + ELLIPSIS_CHARS;
+
+const BARE_AMPERSAND = /&(?!(?:[a-zA-Z][a-zA-Z0-9]{1,31}|#\d{1,7}|#[xX][0-9a-fA-F]{1,6});)/g;
+const ESCAPED_CODE_TAG = /&lt;(\/?)code\b((?:(?!&gt;).)*)&gt;/gi;
+
 function truncateCodePoints(value: string): string {
   const codePoints = Array.from(value);
   if (codePoints.length <= MAX_HINT_CODE_POINTS) return value;
   return `${codePoints.slice(0, MAX_HINT_CODE_POINTS).join('').trim()}...`;
 }
 
-/**
- * Formats model output for the API's limited-HTML contract.
- * Only attribute-free <code> elements are preserved. All other raw tags are
- * escaped so code examples remain visible without becoming active HTML.
- */
+function escapeMarkup(value: string): string {
+  return value.replace(BARE_AMPERSAND, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;');
+}
+
+function restoreAttributeFreeCodeElements(escaped: string): string {
+  let unclosedCodeElements = 0;
+
+  const restored = escaped.replace(ESCAPED_CODE_TAG, (match: string, closingSlash: string) => {
+    if (closingSlash) {
+      if (unclosedCodeElements === 0) return match;
+      unclosedCodeElements -= 1;
+      return '</code>';
+    }
+    unclosedCodeElements += 1;
+    return '<code>';
+  });
+
+  return restored + '</code>'.repeat(unclosedCodeElements);
+}
+
 export function formatHintOutput(hint: string): string {
   const normalized = hint.trim().replace(/\s+/gu, ' ');
   if (!normalized) return '';
 
   const truncated = truncateCodePoints(normalized);
-  return sanitizeHtml(truncated, {
-    allowedTags: ['code'],
-    allowedAttributes: {},
-    disallowedTagsMode: 'escape',
-  })
-    .trim()
-    .replace(/\s+/gu, ' ');
+  const escaped = escapeMarkup(truncated);
+
+  return restoreAttributeFreeCodeElements(escaped).trim().replace(/\s+/gu, ' ');
 }
 
 export default formatHintOutput;
diff --git a/src/lib/normalizeHintRequest.ts b/src/lib/normalizeHintRequest.ts
index daeeb55..b103f32 100644
--- a/src/lib/normalizeHintRequest.ts
+++ b/src/lib/normalizeHintRequest.ts
@@ -1,14 +1,31 @@
 import { InputValidationError } from '../errors/inputValidationError';
-import type { HintRequestBody, NormalizedHintRequest } from '../types/hint';
+import {
+  CHALLENGE_TYPES,
+  type ChallengeType,
+  type HintRequestBody,
+  type NormalizedHintRequest,
+} from '../types/hint';
 
 function isNonEmptyString(value: unknown): value is string {
   return typeof value === 'string' && value.trim().length > 0;
 }
 
+function isValidChallengeType(value: unknown): value is ChallengeType {
+  return typeof value === 'string' && CHALLENGE_TYPES.includes(value as ChallengeType);
+}
+
 const PROMPT_FRAME_TAGS = /<\s*\/?\s*(?:challenge_description|student_code|failing_test)\s*>/gi;
 
 function stripPromptFrameTags(value: string): string {
-  return value.replace(PROMPT_FRAME_TAGS, '');
+  let previous: string;
+  let stripped = value;
+
+  do {
+    previous = stripped;
+    stripped = stripped.replace(PROMPT_FRAME_TAGS, '');
+  } while (stripped !== previous);
+
+  return stripped;
 }
 
 /**
@@ -47,7 +64,7 @@ export function normalizeHintRequest(raw: HintRequestBody): NormalizedHintReques
 
   return {
     userId: userId.trim(),
-    challengeType,
+    challengeType: isValidChallengeType(challengeType) ? challengeType : undefined,
     description: stripPromptFrameTags(description.trim()),
     userInput: stripPromptFrameTags(effectiveUserInput.trim()),
     seed: stripPromptFrameTags(typeof seed === 'string' ? seed.trim() : ''),