diff --git a/packages/ai/src/providers/openai-codex-responses.ts b/packages/ai/src/providers/openai-codex-responses.ts
index 80eaf496..dc6b6463 100644
--- a/packages/ai/src/providers/openai-codex-responses.ts
+++ b/packages/ai/src/providers/openai-codex-responses.ts
@@ -352,7 +352,7 @@ function buildRequestBody(
 		model: model.id,
 		store: false,
 		stream: true,
-		instructions: context.systemPrompt,
+		instructions: context.systemPrompt || "You are a helpful assistant.",
 		input: messages,
 		text: { verbosity: options?.textVerbosity || "low" },
 		include: ["reasoning.encrypted_content"],