diff --git a/packages/core/src/agent/content-utils.ts b/packages/core/src/agent/content-utils.ts
index 42b0b7fec7..667714d778 100644
--- a/packages/core/src/agent/content-utils.ts
+++ b/packages/core/src/agent/content-utils.ts
@@ -93,13 +93,16 @@ export function contentPartsToGeminiParts(content: ContentPart[]): Part[] {
         // References are converted to text for the model
         result.push({ text: part.text });
         break;
-      default:
+      default: {
+        const _exhaustiveCheck: never = part;
+        void _exhaustiveCheck;
         debugLogger.warn(
           `Unhandled ContentPart type: ${JSON.stringify(part)} fallback to serialization`,
         );
         // Serialize unknown ContentPart variants instead of dropping them
         result.push({ text: JSON.stringify(part) });
         break;
+      }
     }
   }
   return result;