diff --git a/ui/src/pages/Alerts/RuleEditor/TriggerStep.tsx b/ui/src/pages/Alerts/RuleEditor/TriggerStep.tsx
index 61572e9a..9cb874d9 100644
--- a/ui/src/pages/Alerts/RuleEditor/TriggerStep.tsx
+++ b/ui/src/pages/Alerts/RuleEditor/TriggerStep.tsx
@@ -17,6 +17,13 @@ export function TriggerStep({
   const { toast } = useToast();
   const [lastResult, setLastResult] = useState<string | null>(null);
 
+  // PER_EXCHANGE fires exactly once per exchange (Task 3.3 server validator
+  // rejects non-zero reNotifyMinutes / forDurationSeconds for this mode).
+  // Gate the two inputs here so users can't type values the server will reject.
+  const isPerExchange =
+    form.conditionKind === 'EXCHANGE_MATCH' &&
+    (form.condition as Record<string, unknown>).fireMode === 'PER_EXCHANGE';
+
   const onTest = async () => {
     if (!ruleId) {
       toast({ title: 'Save rule first to run test evaluate', variant: 'error' });
@@ -40,20 +47,28 @@ export function TriggerStep({
           onChange={(e) => setForm({ ...form, evaluationIntervalSeconds: Number(e.target.value) })}
         />
       </FormField>
-      <FormField label="For-duration before firing (seconds, 0 = fire immediately)">
-        <Input
-          type="number"
-          min={0}
-          value={form.forDurationSeconds}
-          onChange={(e) => setForm({ ...form, forDurationSeconds: Number(e.target.value) })}
-        />
-      </FormField>
+      {!isPerExchange && (
+        <FormField label="For-duration before firing (seconds, 0 = fire immediately)">
+          <Input
+            type="number"
+            min={0}
+            value={form.forDurationSeconds}
+            onChange={(e) => setForm({ ...form, forDurationSeconds: Number(e.target.value) })}
+          />
+        </FormField>
+      )}
       <FormField label="Re-notify cadence (minutes, 0 = notify once)">
         <Input
           type="number"
           min={0}
           value={form.reNotifyMinutes}
           onChange={(e) => setForm({ ...form, reNotifyMinutes: Number(e.target.value) })}
+          disabled={isPerExchange}
+          title={
+            isPerExchange
+              ? 'Per-exchange rules fire exactly once per exchange — re-notify does not apply.'
+              : undefined
+          }
         />
       </FormField>
       <div>