diff --git a/open-sse/utils/diagnostics.ts b/open-sse/utils/diagnostics.ts
index 39161e4137..4402656281 100644
--- a/open-sse/utils/diagnostics.ts
+++ b/open-sse/utils/diagnostics.ts
@@ -122,6 +122,7 @@ export function synthOpenAIErrorChunk(opts: {
     error: {
       message: safeMessage,
       type: "upstream_empty_response",
+      code: "upstream_empty_response",
     },
   };
   return `data: ${JSON.stringify(body)}\n\n`;
diff --git a/tests/unit/diagnostics.test.ts b/tests/unit/diagnostics.test.ts
index 901feebff7..93a85380b5 100644
--- a/tests/unit/diagnostics.test.ts
+++ b/tests/unit/diagnostics.test.ts
@@ -39,6 +39,7 @@ test("synthOpenAIErrorChunk payload has expected OpenAI chunk shape", () => {
   assert.equal(payload.choices.length, 1);
   assert.ok(payload.error, "must have error field");
   assert.equal(payload.error.type, "upstream_empty_response");
+  assert.equal(payload.error.code, "upstream_empty_response");
   assert.ok(typeof payload.error.message === "string" && payload.error.message.length > 0);
 });