diff --git a/src/agents/pi-embedded-helpers.isbillingerrormessage.test.ts b/src/agents/pi-embedded-helpers.isbillingerrormessage.test.ts
index 41a10d8c533..2f478af3b6d 100644
--- a/src/agents/pi-embedded-helpers.isbillingerrormessage.test.ts
+++ b/src/agents/pi-embedded-helpers.isbillingerrormessage.test.ts
@@ -684,6 +684,12 @@ describe("classifyFailoverReason", () => {
     ).toBeNull();
   });
 
+  it("classifies OpenAI Responses unknown-no-details message as unknown", () => {
+    const message = "Unknown error (no error details in response)";
+    expect(classifyFailoverReason(message)).toBe("unknown");
+    expect(isFailoverErrorMessage(message)).toBe(true);
+  });
+
   it("classifies provider-scoped generic upstream messages", () => {
     expect(classifyFailoverReason("An unknown error occurred", { provider: "anthropic" })).toBe(
       "timeout",
diff --git a/src/agents/pi-embedded-helpers/errors.ts b/src/agents/pi-embedded-helpers/errors.ts
index 560981b09f8..e81407f6e9c 100644
--- a/src/agents/pi-embedded-helpers/errors.ts
+++ b/src/agents/pi-embedded-helpers/errors.ts
@@ -668,6 +668,10 @@ function isOpenRouterKeyLimitExceededError(raw: string, provider?: string): bool
   );
 }
 
+function isExactUnknownNoDetailsError(raw: string): boolean {
+  return normalizeOptionalLowercaseString(raw)?.trim() === "unknown error (no error details in response)";
+}
+
 function classifyFailoverClassificationFromMessage(
   raw: string,
   provider?: string,
@@ -737,6 +741,9 @@ function classifyFailoverClassificationFromMessage(
   if (isCloudCodeAssistFormatError(raw)) {
     return toReasonClassification("format");
   }
+  if (isExactUnknownNoDetailsError(raw)) {
+    return toReasonClassification("unknown");
+  }
   if (isTimeoutErrorMessage(raw)) {
     return toReasonClassification("timeout");
   }