Add HTTP 499 to transient error codes for model fallback (#41468)

Merged via squash.

Prepared head SHA: 0053bae140
Co-authored-by: zeroasterisk <23422+zeroasterisk@users.noreply.github.com>
Co-authored-by: altaywtf <9790196+altaywtf@users.noreply.github.com>
Reviewed-by: @altaywtf

src/agents/failover-error.test.ts

@@ -67,6 +67,7 @@ describe("failover-error", () => {
     expect(resolveFailoverReasonFromError({ statusCode: "429" })).toBe("rate_limit");
     expect(resolveFailoverReasonFromError({ status: 403 })).toBe("auth");
     expect(resolveFailoverReasonFromError({ status: 408 })).toBe("timeout");
+    expect(resolveFailoverReasonFromError({ status: 499 })).toBe("timeout");
     expect(resolveFailoverReasonFromError({ status: 400 })).toBe("format");
     // Keep the status-only path behavior-preserving and conservative.
     expect(resolveFailoverReasonFromError({ status: 500 })).toBeNull();
@@ -93,6 +94,12 @@ describe("failover-error", () => {
         message: ANTHROPIC_OVERLOADED_PAYLOAD,
       }),
     ).toBe("overloaded");
+    expect(
+      resolveFailoverReasonFromError({
+        status: 499,
+        message: ANTHROPIC_OVERLOADED_PAYLOAD,
+      }),
+    ).toBe("overloaded");
     expect(
       resolveFailoverReasonFromError({
         status: 429,

src/agents/pi-embedded-helpers.isbillingerrormessage.test.ts

@@ -443,6 +443,7 @@ describe("isLikelyContextOverflowError", () => {
 
 describe("isTransientHttpError", () => {
   it("returns true for retryable 5xx status codes", () => {
+    expect(isTransientHttpError("499 Client Closed Request")).toBe(true);
     expect(isTransientHttpError("500 Internal Server Error")).toBe(true);
     expect(isTransientHttpError("502 Bad Gateway")).toBe(true);
     expect(isTransientHttpError("503 Service Unavailable")).toBe(true);
@@ -457,6 +458,19 @@ describe("isTransientHttpError", () => {
   });
 });
 
+describe("classifyFailoverReasonFromHttpStatus", () => {
+  it("treats HTTP 499 as transient for structured errors", () => {
+    expect(classifyFailoverReasonFromHttpStatus(499)).toBe("timeout");
+    expect(classifyFailoverReasonFromHttpStatus(499, "499 Client Closed Request")).toBe("timeout");
+    expect(
+      classifyFailoverReasonFromHttpStatus(
+        499,
+        '{"type":"error","error":{"type":"overloaded_error","message":"Overloaded"}}',
+      ),
+    ).toBe("overloaded");
+  });
+});
+
 describe("isFailoverErrorMessage", () => {
   it("matches auth/rate/billing/timeout", () => {
     const samples = [

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

@@ -189,7 +189,7 @@ const HTTP_STATUS_PREFIX_RE = /^(?:http\s*)?(\d{3})\s+(.+)$/i;
 const HTTP_STATUS_CODE_PREFIX_RE = /^(?:http\s*)?(\d{3})(?:\s+([\s\S]+))?$/i;
 const HTML_ERROR_PREFIX_RE = /^\s*(?:<!doctype\s+html\b|<html\b)/i;
 const CLOUDFLARE_HTML_ERROR_CODES = new Set([521, 522, 523, 524, 525, 526, 530]);
-const TRANSIENT_HTTP_ERROR_CODES = new Set([500, 502, 503, 504, 521, 522, 523, 524, 529]);
+const TRANSIENT_HTTP_ERROR_CODES = new Set([499, 500, 502, 503, 504, 521, 522, 523, 524, 529]);
 const HTTP_ERROR_HINTS = [
   "error",
   "bad request",
@@ -375,6 +375,12 @@ export function classifyFailoverReasonFromHttpStatus(
     }
     return "timeout";
   }
+  if (status === 499) {
+    if (message && isOverloadedErrorMessage(message)) {
+      return "overloaded";
+    }
+    return "timeout";
+  }
   if (status === 502 || status === 504) {
     return "timeout";
   }