diff --git a/open-sse/services/accountFallback.js b/open-sse/services/accountFallback.js
index 573a5d64..957655c3 100644
--- a/open-sse/services/accountFallback.js
+++ b/open-sse/services/accountFallback.js
@@ -32,6 +32,12 @@ export function checkFallbackError(status, errorText, backoffLevel = 0) {
       return { shouldFallback: true, cooldownMs: COOLDOWN_MS.requestNotAllowed };
     }
 
+    // Kiro: "improperly formed request" = model not available on this account tier
+    // Treat as paymentRequired (long cooldown) so the model is locked and fallback occurs
+    if (lowerError.includes("improperly formed request")) {
+      return { shouldFallback: true, cooldownMs: COOLDOWN_MS.paymentRequired };
+    }
+
     // Rate limit keywords - exponential backoff
     if (
       lowerError.includes("rate limit") ||