diff --git a/src/agents/openai-transport-stream.ts b/src/agents/openai-transport-stream.ts
index c341720e00b..357305504b9 100644
--- a/src/agents/openai-transport-stream.ts
+++ b/src/agents/openai-transport-stream.ts
@@ -1520,7 +1520,8 @@ async function processOpenAICompletionsStream(
       }
     }
     const choiceDelta =
-      choice.delta ?? (choice as unknown as { message?: ChatCompletionChunk["delta"] }).message;
+      choice.delta ??
+      (choice as unknown as { message?: ChatCompletionChunk["choices"][number]["delta"] }).message;
     if (!choiceDelta) {
       continue;
     }