diff --git a/ui/src/pages/Alerts/RuleEditor/form-state.test.ts b/ui/src/pages/Alerts/RuleEditor/form-state.test.ts
index 1563b1f1..c2d0b3fc 100644
--- a/ui/src/pages/Alerts/RuleEditor/form-state.test.ts
+++ b/ui/src/pages/Alerts/RuleEditor/form-state.test.ts
@@ -1,5 +1,9 @@
 import { describe, it, expect } from 'vitest';
-import { initialForm, toRequest, validateStep } from './form-state';
+// `applyFireModeChange` will be introduced by Task 4.3 as a pure helper in
+// `form-state.ts` so that the ExchangeMatchForm can reuse it on mode-toggle
+// and guarantee state hygiene (no stale COUNT_IN_WINDOW fields leaking into a
+// PER_EXCHANGE rule payload and vice-versa). These tests pin the contract.
+import { applyFireModeChange, initialForm, toRequest, validateStep } from './form-state';
 
 describe('initialForm', () => {
   it('defaults to env-wide ROUTE_METRIC with safe intervals', () => {
@@ -80,3 +84,79 @@ describe('validateStep', () => {
     expect(validateStep('condition', f)).toEqual([]);
   });
 });
+
+// ----------------------------------------------------------------------------
+// Task 4.2 RED: fire-mode toggle state hygiene
+//
+// `applyFireModeChange(form, newMode)` is the pure helper (Task 4.3) that the
+// ExchangeMatchForm's Fire-mode <Select> will call in place of the current
+// raw `patch({ fireMode })`. It must guarantee:
+//
+//   - Switching to PER_EXCHANGE clears COUNT_IN_WINDOW-only fields
+//     (condition.threshold, condition.windowSeconds) AND forces
+//     top-level reNotifyMinutes = 0, forDurationSeconds = 0. PER_EXCHANGE
+//     is exactly-once-per-exchange — re-notify cadence and hold-duration
+//     are semantically meaningless and must not leak into toRequest().
+//   - Switching back to COUNT_IN_WINDOW resets to defaults (zero), never
+//     restoring stale values from the previous COUNT_IN_WINDOW session.
+//
+// These tests will fail until Task 4.3 introduces the helper.
+// ----------------------------------------------------------------------------
+describe('applyFireModeChange (fire-mode toggle hygiene)', () => {
+  const exchangeMatchForm = () => {
+    const f = initialForm();
+    f.conditionKind = 'EXCHANGE_MATCH';
+    f.condition = {
+      kind: 'EXCHANGE_MATCH',
+      scope: {},
+      fireMode: 'COUNT_IN_WINDOW',
+    } as unknown as typeof f.condition;
+    return f;
+  };
+
+  it('clears COUNT_IN_WINDOW fields when switching to PER_EXCHANGE', () => {
+    let f = exchangeMatchForm();
+    // Simulate a user who filled in COUNT_IN_WINDOW fields first.
+    f.condition = {
+      ...(f.condition as Record<string, unknown>),
+      threshold: 5,
+      windowSeconds: 300,
+    } as typeof f.condition;
+
+    f = applyFireModeChange(f, 'PER_EXCHANGE');
+
+    const c = f.condition as Record<string, unknown>;
+    expect(c.fireMode).toBe('PER_EXCHANGE');
+    expect(c.threshold ?? 0).toBe(0);
+    expect(c.windowSeconds ?? 0).toBe(0);
+  });
+
+  it('resets to defaults (not stale values) when switching back to COUNT_IN_WINDOW', () => {
+    let f = exchangeMatchForm();
+    f.condition = {
+      ...(f.condition as Record<string, unknown>),
+      threshold: 5,
+      windowSeconds: 300,
+    } as typeof f.condition;
+
+    f = applyFireModeChange(f, 'PER_EXCHANGE');
+    f = applyFireModeChange(f, 'COUNT_IN_WINDOW');
+
+    const c = f.condition as Record<string, unknown>;
+    expect(c.fireMode).toBe('COUNT_IN_WINDOW');
+    // Must be fresh defaults, not the stale 5 / 300 the user typed before.
+    expect(c.threshold ?? 0).toBe(0);
+    expect(c.windowSeconds ?? 0).toBe(0);
+  });
+
+  it('forces reNotifyMinutes=0 and forDurationSeconds=0 when switching to PER_EXCHANGE', () => {
+    let f = exchangeMatchForm();
+    f.reNotifyMinutes = 60;
+    f.forDurationSeconds = 120;
+
+    f = applyFireModeChange(f, 'PER_EXCHANGE');
+
+    expect(f.reNotifyMinutes).toBe(0);
+    expect(f.forDurationSeconds).toBe(0);
+  });
+});