diff --git a/src/agents/failover-error.test.ts b/src/agents/failover-error.test.ts
index a43ae289f..d81781a90 100644
--- a/src/agents/failover-error.test.ts
+++ b/src/agents/failover-error.test.ts
@@ -11,6 +11,7 @@ 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: 400 })).toBe("format");
   });
 
   it("infers format errors from error messages", () => {
diff --git a/src/agents/failover-error.ts b/src/agents/failover-error.ts
index 5026394f3..6f6e33948 100644
--- a/src/agents/failover-error.ts
+++ b/src/agents/failover-error.ts
@@ -120,6 +120,7 @@ export function resolveFailoverReasonFromError(err: unknown): FailoverReason | n
   if (status === 429) return "rate_limit";
   if (status === 401 || status === 403) return "auth";
   if (status === 408) return "timeout";
+  if (status === 400) return "format";
 
   const code = (getErrorCode(err) ?? "").toUpperCase();
   if (["ETIMEDOUT", "ESOCKETTIMEDOUT", "ECONNRESET", "ECONNABORTED"].includes(code)) {