diff --git a/packages/kosong/src/providers/openai-common.ts b/packages/kosong/src/providers/openai-common.ts
index b83377e4e..ae829077a 100644
--- a/packages/kosong/src/providers/openai-common.ts
+++ b/packages/kosong/src/providers/openai-common.ts
@@ -151,7 +151,7 @@ export function isFunctionToolCall<T extends { type: string }>(
 export function thinkingEffortToReasoningEffort(effort: ThinkingEffort): string | undefined {
   switch (effort) {
     case 'off':
-      return undefined;
+      return 'none';
     case 'low':
       return 'low';
     case 'medium':