diff --git a/src/backend/services/cms_service.ts b/src/backend/services/cms_service.ts
index 0d2bdd86..c7c02181 100644
--- a/src/backend/services/cms_service.ts
+++ b/src/backend/services/cms_service.ts
@@ -1,5 +1,4 @@
 import { errors, HttpContext } from '@adonisjs/core/http';
-import config from '@adonisjs/core/services/config';
 import type {
   CmsConfig,
   Version,
@@ -9,6 +8,7 @@ import type {
   AppUserInterface,
   UiConfig,
 } from '../../types';
+import { isValidLanguageTag } from '../../shared/language_helpers.js';
 import { defineConfig } from '../define_config.js';
 import { PreferenceService } from './preference_service.js';
 import Config from '../models/config.js';
@@ -49,7 +49,8 @@ export class CmsService {
     active.data = newConfig;
     await active.save();
 
-    const trackedConfig = config.get<CmsConfig>('cms') || {};
+    const { default: adonisConfig } = await import('@adonisjs/core/services/config');
+    const trackedConfig = adonisConfig.get<CmsConfig>('cms') || {};
 
     // make sure we do not clobber the tracked config
     this.#config = { ...newConfig, ...trackedConfig };
@@ -76,7 +77,7 @@ export class CmsService {
   }
 
   public localeFromQuery(ctx: HttpContext): string {
-    return ctx.request.qs()['locale'] || this.sourceLocale;
+    return this.localeFromQueryParam(ctx.request.qs()['locale']);
   }
 
   public versionFromPath(ctx: HttpContext): Version {
@@ -93,10 +94,22 @@ export class CmsService {
   public versionFromQuery(ctx: HttpContext): Version {
     return {
       apiVersion: 1,
-      locale: ctx.request.qs()['locale'] || this.sourceLocale,
+      locale: this.localeFromQueryParam(ctx.request.qs()['locale']),
     };
   }
 
+  private localeFromQueryParam(queryLocale: string | undefined): string {
+    if (!queryLocale) return this.sourceLocale;
+
+    // Query-param locales are not auth-gated like path locales (localeFromPath), but
+    // must still be valid BCP 47 tags. Underscore variants (e.g. zh_Hans) pass i18n
+    // string lookup yet crash IntlMessageFormat during formatting → 500 on RSS/API.
+    // Reject malformed tags here so downstream i18n/ICU never sees them.
+    if (!isValidLanguageTag(queryLocale)) throw errors.E_ROUTE_NOT_FOUND;
+
+    return queryLocale;
+  }
+
   protected getLanguage(locale: string | null): LanguageSpecification {
     const found =
       this.#config.languages.find(
diff --git a/src/shared/language_helpers.ts b/src/shared/language_helpers.ts
index 1f48f99d..d2343f2a 100644
--- a/src/shared/language_helpers.ts
+++ b/src/shared/language_helpers.ts
@@ -2,6 +2,16 @@ import type { LanguageSpecification, LocaleItem } from '../types.js';
 
 export const LANGUAGE_LABEL_SEPARATOR = /\s*-\s*|\s*\|\s*/;
 
+/** Whether tag is a well-formed IETF BCP 47 language tag (hyphen-separated). */
+export function isValidLanguageTag(tag: string): boolean {
+  try {
+    new Intl.Locale(tag);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
 /** Name, native name, and locale from a language specification. */
 export function parseLanguageSpecification(spec: LanguageSpecification): LocaleItem {
   const { language, locale, languageDirection } = spec;
diff --git a/tests/unit/audioRule.spec.ts b/tests/unit/audioRule.spec.ts
index 8e7efe85..018f6208 100644
--- a/tests/unit/audioRule.spec.ts
+++ b/tests/unit/audioRule.spec.ts
@@ -4,6 +4,10 @@ import audioRule from '../../src/backend/validators/audio_rule.js';
 import { Audio } from '../../src/types';
 
 test.describe('Audio Rule Validator', () => {
+  test.beforeEach(() => {
+    vine.convertEmptyStringsToNull = false;
+  });
+
   test('should validate valid audio objects with .mp3 extension', async () => {
     const validAudios: Audio[] = [
       { url: 'https://example.com/audio.mp3', length: 120 },
diff --git a/tests/unit/bundleService.spec.ts b/tests/unit/bundleService.spec.ts
index 2fd72fa8..a7fca5c1 100644
--- a/tests/unit/bundleService.spec.ts
+++ b/tests/unit/bundleService.spec.ts
@@ -4,6 +4,10 @@ import { complexFields, nestedFields, simpleFields } from '../mocks.js';
 import vine from '@vinejs/vine';
 
 test.describe('Bundle builder', () => {
+  test.beforeEach(() => {
+    vine.convertEmptyStringsToNull = false;
+  });
+
   test('Identify junk', () => {
     // arrange
     const service = new BundleService(simpleFields);
@@ -66,6 +70,10 @@ test.describe('Bundle builder', () => {
 });
 
 test.describe('Bundle updater', () => {
+  test.beforeEach(() => {
+    vine.convertEmptyStringsToNull = false;
+  });
+
   test('updates bundle with new changes', async () => {
     // arrange
     const service = new BundleService(simpleFields);
@@ -228,6 +236,10 @@ test.describe('Bundle updater', () => {
 });
 
 test.describe('Bundle validator', () => {
+  test.beforeEach(() => {
+    vine.convertEmptyStringsToNull = false;
+  });
+
   test('it can validate a complex object', async () => {
     // arrange
     const objectSpec = [
diff --git a/tests/unit/cms_service.spec.ts b/tests/unit/cms_service.spec.ts
new file mode 100644
index 00000000..cb3a65d9
--- /dev/null
+++ b/tests/unit/cms_service.spec.ts
@@ -0,0 +1,54 @@
+import { test, expect } from '@playwright/test';
+import type { HttpContext } from '@adonisjs/core/http';
+
+import { CmsService } from '../../src/backend/services/cms_service';
+
+function mockContext(query: Record<string, string | undefined>): HttpContext {
+  return {
+    request: {
+      qs: () => query,
+    },
+  } as HttpContext;
+}
+
+test.describe('CmsService query locale', () => {
+  const cms = CmsService.default();
+
+  test('localeFromQuery returns sourceLocale when param is absent', () => {
+    expect(cms.localeFromQuery(mockContext({}))).toBe('en');
+    expect(cms.localeFromQuery(mockContext({ locale: undefined }))).toBe('en');
+  });
+
+  test('localeFromQuery returns valid tags as-is', () => {
+    expect(cms.localeFromQuery(mockContext({ locale: 'en' }))).toBe('en');
+    expect(cms.localeFromQuery(mockContext({ locale: 'zh' }))).toBe('zh');
+    expect(cms.localeFromQuery(mockContext({ locale: 'zh-Hans' }))).toBe('zh-Hans');
+    expect(cms.localeFromQuery(mockContext({ locale: 'en-US' }))).toBe('en-US');
+  });
+
+  test('localeFromQuery throws E_ROUTE_NOT_FOUND for invalid tags', () => {
+    for (const locale of ['zh_Hans', 'zh_CN']) {
+      expect(() => cms.localeFromQuery(mockContext({ locale }))).toThrow();
+      try {
+        cms.localeFromQuery(mockContext({ locale }));
+      } catch (error) {
+        expect((error as { code: string; status: number }).code).toBe('E_ROUTE_NOT_FOUND');
+        expect((error as { code: string; status: number }).status).toBe(404);
+      }
+    }
+  });
+
+  test('versionFromQuery mirrors localeFromQuery behavior', () => {
+    expect(cms.versionFromQuery(mockContext({}))).toEqual({
+      apiVersion: 1,
+      locale: 'en',
+    });
+
+    expect(cms.versionFromQuery(mockContext({ locale: 'zh' }))).toEqual({
+      apiVersion: 1,
+      locale: 'zh',
+    });
+
+    expect(() => cms.versionFromQuery(mockContext({ locale: 'zh_Hans' }))).toThrow();
+  });
+});
diff --git a/tests/unit/course.spec.ts b/tests/unit/course.spec.ts
index bdce219a..3cb20ed6 100644
--- a/tests/unit/course.spec.ts
+++ b/tests/unit/course.spec.ts
@@ -1,4 +1,5 @@
 import { test, expect } from '@playwright/test';
+import vine from '@vinejs/vine';
 import { CourseValidator } from '../../src/backend/validators/course.js';
 
 const validBundle = () => ({
@@ -13,6 +14,10 @@ const validBundle = () => ({
 });
 
 test.describe('CourseValidator', () => {
+  test.beforeEach(() => {
+    vine.convertEmptyStringsToNull = false;
+  });
+
   test('accepts bundle without imageUrl', async () => {
     const validator = new CourseValidator();
     const result = await validator.validate(validBundle());
diff --git a/tests/unit/dateRangeRule.spec.ts b/tests/unit/dateRangeRule.spec.ts
index b6e01a5c..5a039795 100644
--- a/tests/unit/dateRangeRule.spec.ts
+++ b/tests/unit/dateRangeRule.spec.ts
@@ -3,6 +3,10 @@ import vine from '@vinejs/vine';
 import dateRangeRule from '../../src/backend/validators/date_range_rule.js';
 
 test.describe('Date Range Rule Validator', () => {
+  test.beforeEach(() => {
+    vine.convertEmptyStringsToNull = false;
+  });
+
   test('should validate valid date ranges', async () => {
     const validDateRanges = [
       '2027-01-08T07:30:00.000Z|2027-01-15T07:30:00.000Z',
diff --git a/tests/unit/drop.spec.ts b/tests/unit/drop.spec.ts
index a0ff217e..aef36423 100644
--- a/tests/unit/drop.spec.ts
+++ b/tests/unit/drop.spec.ts
@@ -1,8 +1,13 @@
 import { test, expect } from '@playwright/test';
+import vine from '@vinejs/vine';
 import DropValidator from '../../src/backend/validators/drop.js';
 import { dropBundleFields } from '../mocks.js';
 
 test.describe('Drop Validator', () => {
+  test.beforeEach(() => {
+    vine.convertEmptyStringsToNull = false;
+  });
+
   test.describe('Draft mode (isPublished = false)', () => {
     test('should validate valid draft data with all fields', async () => {
       const validator = new DropValidator(false, dropBundleFields);
diff --git a/tests/unit/invitation.spec.ts b/tests/unit/invitation.spec.ts
index 5200304f..d5369a94 100644
--- a/tests/unit/invitation.spec.ts
+++ b/tests/unit/invitation.spec.ts
@@ -18,6 +18,10 @@ function createMockHttpContext(inputs: Record<string, any>): HttpContext {
 }
 
 test.describe('Invitation validator', () => {
+  test.beforeEach(() => {
+    vine.convertEmptyStringsToNull = false;
+  });
+
   test.describe('Draft Schema', () => {
     test('allows all fields to be optional', async () => {
       const data = {
diff --git a/tests/unit/language_helpers.spec.ts b/tests/unit/language_helpers.spec.ts
new file mode 100644
index 00000000..8060e203
--- /dev/null
+++ b/tests/unit/language_helpers.spec.ts
@@ -0,0 +1,31 @@
+import { test, expect } from '@playwright/test';
+
+import { languages } from '../../src/frontend/settings/languages/languages';
+import { isValidLanguageTag } from '../../src/shared/language_helpers';
+
+test.describe('isValidLanguageTag', () => {
+  test('accepts short and extended BCP 47 tags', () => {
+    expect(isValidLanguageTag('en')).toBe(true);
+    expect(isValidLanguageTag('zh')).toBe(true);
+    expect(isValidLanguageTag('zh-Hans')).toBe(true);
+    expect(isValidLanguageTag('en-US')).toBe(true);
+  });
+
+  test('accepts exotic, well-formed tags', () => {
+    expect(isValidLanguageTag('qqq')).toBe(true);
+    expect(isValidLanguageTag('xx-Sw')).toBe(true);
+  });
+
+  test('rejects underscore-style tags and empty string', () => {
+    expect(isValidLanguageTag('zh_Hans')).toBe(false);
+    expect(isValidLanguageTag('zh_CN')).toBe(false);
+    expect(isValidLanguageTag('')).toBe(false);
+  });
+
+  // Kit language picker defaults — syntax only, not per-client config.languages.
+  test('kit language catalog locales are valid BCP 47 tags', () => {
+    for (const { locale } of languages) {
+      expect(isValidLanguageTag(locale), `invalid locale: ${locale}`).toBe(true);
+    }
+  });
+});
diff --git a/tests/unit/page.spec.ts b/tests/unit/page.spec.ts
index 33d1f7c9..9a496179 100644
--- a/tests/unit/page.spec.ts
+++ b/tests/unit/page.spec.ts
@@ -1,4 +1,5 @@
 import { test, expect } from '@playwright/test';
+import vine from '@vinejs/vine';
 import PageValidator from '../../src/backend/validators/page.js';
 import type { HttpContext } from '@adonisjs/core/http';
 
@@ -12,6 +13,10 @@ function createMockHttpContext(inputs: Record<string, any>): HttpContext {
 }
 
 test.describe('Page Validator', () => {
+  test.beforeEach(() => {
+    vine.convertEmptyStringsToNull = false;
+  });
+
   test.describe('Draft Schema', () => {
     test('requires title', async () => {
       const data = {
diff --git a/tests/unit/videoRule.spec.ts b/tests/unit/videoRule.spec.ts
index fd06cbde..7c662404 100644
--- a/tests/unit/videoRule.spec.ts
+++ b/tests/unit/videoRule.spec.ts
@@ -4,6 +4,10 @@ import videoRule from '../../src/backend/validators/video_rule.js';
 import { Video } from '../../src/types';
 
 test.describe('Video Rule Validator', () => {
+  test.beforeEach(() => {
+    vine.convertEmptyStringsToNull = false;
+  });
+
   test('should validate valid video objects with .mp4 extension', async () => {
     const validVideos: Video[] = [
       { url: 'https://example.com/video.mp4' },