diff --git a/open-sse/translator/index.ts b/open-sse/translator/index.ts
index dec3b92574..49fb8add0e 100644
--- a/open-sse/translator/index.ts
+++ b/open-sse/translator/index.ts
@@ -279,7 +279,7 @@ export function translateRequest(
 
       const cacheKey = hasToolCalls
         ? msg.tool_calls[0]?.id
-        : getAssistantMessageCacheKey(result, messageIndex);
+        : getAssistantMessageCacheKey(result, 0);
       if (cacheKey) {
         const cached = lookupReasoning(cacheKey);
         if (cached) {