diff --git a/tests/unit/chatcore-translation-paths.test.ts b/tests/unit/chatcore-translation-paths.test.ts
index 3b3274a467..6eb72e4b0c 100644
--- a/tests/unit/chatcore-translation-paths.test.ts
+++ b/tests/unit/chatcore-translation-paths.test.ts
@@ -324,7 +324,7 @@ async function invokeChatCore({
       return responseFactory(captured, calls);
     }
 
-    const upstreamStream = String(headers.accept || "")
+    const upstreamStream = String(headers.Accept || headers.accept || "")
       .toLowerCase()
       .includes("text/event-stream");
     if (responseFormat === "claude") return buildClaudeResponse(upstreamStream);