diff --git a/frontend/src/spa/lobby-i18n.ts b/frontend/src/spa/lobby-i18n.ts
index f68e411..b3e31de 100644
--- a/frontend/src/spa/lobby-i18n.ts
+++ b/frontend/src/spa/lobby-i18n.ts
@@ -2,6 +2,7 @@ import lobbyCatalog from '../../../shared/i18n/lobby.json';
 
 const frontendErrors = lobbyCatalog.frontend.errors;
 const localeConfig = lobbyCatalog.locales;
+const backendToFrontendErrorKeys = lobbyCatalog.contract.backend_to_frontend_error_keys as Record<string, keyof typeof frontendErrors>;
 
 type FrontendErrorKey = keyof typeof frontendErrors;
 type SupportedLocale = (typeof localeConfig.supported)[number];
@@ -40,9 +41,15 @@ export function lobbyMessageFromApiPayload(payload: unknown, fallbackKey: Fronte
   const record = payload as Record<string, unknown>;
   const code = typeof record.error_code === 'string' ? record.error_code : '';
   const payloadLocale = typeof record.locale === 'string' ? record.locale : locale;
-  if (!isFrontendErrorKey(code)) {
-    return lobbyMessage(fallbackKey, payloadLocale);
+  const mappedKey = code ? backendToFrontendErrorKeys[code] : undefined;
+
+  if (mappedKey && isFrontendErrorKey(mappedKey)) {
+    return lobbyMessage(mappedKey, payloadLocale);
   }
 
-  return lobbyMessage(code, payloadLocale);
+  if (isFrontendErrorKey(code)) {
+    return lobbyMessage(code, payloadLocale);
+  }
+
+  return lobbyMessage(fallbackKey, payloadLocale);
 }
diff --git a/frontend/tests/lobby-i18n.contract.test.ts b/frontend/tests/lobby-i18n.contract.test.ts
index c75c6e8..0a20522 100644
--- a/frontend/tests/lobby-i18n.contract.test.ts
+++ b/frontend/tests/lobby-i18n.contract.test.ts
@@ -48,11 +48,11 @@ describe('lobbyMessage locale handling', () => {
     ).toBe('Sessionskoden er ugyldig, eller sessionen findes ikke længere.');
   });
 
-  it('uses shared backend→frontend key-map when backend key differs from frontend copy key', () => {
+  it('uses shared backend→frontend key-map at runtime even when fallback key differs', () => {
     expect(
       lobbyMessageFromApiPayload(
         { error_code: 'session_not_joinable', locale: 'da' },
-        'join_failed',
+        'start_round_failed',
       ),
     ).toBe('Kunne ikke joine. Tjek kode eller kaldenavn og prøv igen.');
   });