diff --git a/src/app/api/tts/route.ts b/src/app/api/tts/route.ts
index 4e97091..3c8cc2e 100644
--- a/src/app/api/tts/route.ts
+++ b/src/app/api/tts/route.ts
@@ -252,12 +252,14 @@ export async function POST(req: NextRequest) {
         : voice
     ) as SpeechCreateParams['voice'];
 
+    const resolvedFormat = format || 'mp3';
+
     const createParams: ExtendedSpeechParams = {
       model: model,
       voice: normalizedVoice,
       input: text,
       speed: speed,
-      response_format: format,
+      response_format: resolvedFormat,
     };
     // Only add instructions if model is gpt-4o-mini-tts and instructions are provided
     if ((model as string) === 'gpt-4o-mini-tts' && instructions) {
diff --git a/src/contexts/TTSContext.tsx b/src/contexts/TTSContext.tsx
index b62c402..c2592f8 100644
--- a/src/contexts/TTSContext.tsx
+++ b/src/contexts/TTSContext.tsx
@@ -1036,6 +1036,7 @@ export function TTSProvider({ children }: { children: ReactNode }): ReactElement
         text: sentence,
         voice,
         speed,
+        format: 'mp3',
         model: ttsModel,
         instructions: ttsModel === 'gpt-4o-mini-tts' ? ttsInstructions : undefined,
       };