refactor(agents): share failover HTTP status classification (#36615)

* fix(agents): classify transient failover statuses consistently

* fix(agents): preserve legacy failover status mapping

src/agents/failover-error.test.ts

@@ -14,11 +14,15 @@ describe("failover-error", () => {
     expect(resolveFailoverReasonFromError({ status: 403 })).toBe("auth");
     expect(resolveFailoverReasonFromError({ status: 408 })).toBe("timeout");
     expect(resolveFailoverReasonFromError({ status: 400 })).toBe("format");
-    // Transient server errors (502/503/504) should trigger failover as timeout.
+    // Keep the status-only path behavior-preserving and conservative.
+    expect(resolveFailoverReasonFromError({ status: 500 })).toBeNull();
     expect(resolveFailoverReasonFromError({ status: 502 })).toBe("timeout");
     expect(resolveFailoverReasonFromError({ status: 503 })).toBe("timeout");
     expect(resolveFailoverReasonFromError({ status: 504 })).toBe("timeout");
-    // Anthropic 529 (overloaded) should trigger failover as rate_limit.
+    expect(resolveFailoverReasonFromError({ status: 521 })).toBeNull();
+    expect(resolveFailoverReasonFromError({ status: 522 })).toBeNull();
+    expect(resolveFailoverReasonFromError({ status: 523 })).toBeNull();
+    expect(resolveFailoverReasonFromError({ status: 524 })).toBeNull();
     expect(resolveFailoverReasonFromError({ status: 529 })).toBe("rate_limit");
   });
 

src/agents/failover-error.ts

@@ -1,7 +1,7 @@
 import { readErrorName } from "../infra/errors.js";
 import {
   classifyFailoverReason,
-  isAuthPermanentErrorMessage,
+  classifyFailoverReasonFromHttpStatus,
   isTimeoutErrorMessage,
   type FailoverReason,
 } from "./pi-embedded-helpers.js";
@@ -152,30 +152,10 @@ export function resolveFailoverReasonFromError(err: unknown): FailoverReason | n
   }
 
   const status = getStatusCode(err);
-  if (status === 402) {
-    return "billing";
-  }
-  if (status === 429) {
-    return "rate_limit";
-  }
-  if (status === 401 || status === 403) {
-    const msg = getErrorMessage(err);
-    if (msg && isAuthPermanentErrorMessage(msg)) {
-      return "auth_permanent";
-    }
-    return "auth";
-  }
-  if (status === 408) {
-    return "timeout";
-  }
-  if (status === 502 || status === 503 || status === 504) {
-    return "timeout";
-  }
-  if (status === 529) {
-    return "rate_limit";
-  }
-  if (status === 400) {
-    return "format";
+  const message = getErrorMessage(err);
+  const statusReason = classifyFailoverReasonFromHttpStatus(status, message);
+  if (statusReason) {
+    return statusReason;
   }
 
   const code = (getErrorCode(err) ?? "").toUpperCase();
@@ -197,8 +177,6 @@ export function resolveFailoverReasonFromError(err: unknown): FailoverReason | n
   if (isTimeoutError(err)) {
     return "timeout";
   }
-
-  const message = getErrorMessage(err);
   if (!message) {
     return null;
   }

src/agents/pi-embedded-helpers.ts

@@ -13,6 +13,7 @@ export {
   BILLING_ERROR_USER_MESSAGE,
   formatBillingErrorMessage,
   classifyFailoverReason,
+  classifyFailoverReasonFromHttpStatus,
   formatRawAssistantErrorForUi,
   formatAssistantErrorText,
   getApiErrorPayloadFingerprint,

src/agents/pi-embedded-helpers/errors.ts

@@ -251,6 +251,43 @@ export function isTransientHttpError(raw: string): boolean {
   return TRANSIENT_HTTP_ERROR_CODES.has(status.code);
 }
 
+export function classifyFailoverReasonFromHttpStatus(
+  status: number | undefined,
+  message?: string,
+): FailoverReason | null {
+  if (typeof status !== "number" || !Number.isFinite(status)) {
+    return null;
+  }
+
+  if (status === 402) {
+    return "billing";
+  }
+  if (status === 429) {
+    return "rate_limit";
+  }
+  if (status === 401 || status === 403) {
+    if (message && isAuthPermanentErrorMessage(message)) {
+      return "auth_permanent";
+    }
+    return "auth";
+  }
+  if (status === 408) {
+    return "timeout";
+  }
+  // Keep the status-only path conservative and behavior-preserving.
+  // Message-path HTTP heuristics are broader and should not leak in here.
+  if (status === 502 || status === 503 || status === 504) {
+    return "timeout";
+  }
+  if (status === 529) {
+    return "rate_limit";
+  }
+  if (status === 400) {
+    return "format";
+  }
+  return null;
+}
+
 function stripFinalTagsFromText(text: string): string {
   if (!text) {
     return text;