diff --git a/frontend/src/__tests__/purchase-execution-toast.test.ts b/frontend/src/__tests__/purchase-execution-toast.test.ts
index 0f46c8aaa..c37ebbe80 100644
--- a/frontend/src/__tests__/purchase-execution-toast.test.ts
+++ b/frontend/src/__tests__/purchase-execution-toast.test.ts
@@ -1025,3 +1025,67 @@ describe('#1503 — payment-option coercion is disclosed in the purchase toast',
     expect(notice).toContain('Pay Monthly');
   });
 });
+
+describe('handleExecutePurchase - missing payment option (#431)', () => {
+  beforeEach(() => {
+    jest.clearAllMocks();
+    (recs.getFanOutBuckets as jest.Mock).mockReturnValue(null);
+    (plans.closePurchaseModal as jest.Mock).mockImplementation(() => undefined);
+  });
+
+  afterEach(() => {
+    document.body.textContent = '';
+  });
+
+  test.each([
+    ['undefined', undefined],
+    ['empty string', ''],
+  ])('refuses with an error toast and no POST when payment is %s', async (_label, payment) => {
+    const rec: Record<string, unknown> = { ...buildMinimalRec() };
+    if (payment === undefined) delete rec.payment;
+    else rec.payment = payment;
+    (recs.getPurchaseModalRecommendations as jest.Mock).mockReturnValue([rec]);
+
+    const btn = setup();
+    btn.click();
+    await new Promise((r) => setTimeout(r, 0));
+
+    expect(api.executePurchase).not.toHaveBeenCalled();
+    expect(confirmDialog).not.toHaveBeenCalled();
+    expect(lastToastKind()).toBe('error');
+    expect(lastToastMessage()).toContain('no payment option');
+    expect(btn.disabled).toBe(false);
+    expect(plans.closePurchaseModal).not.toHaveBeenCalled();
+  });
+
+  test('refuses the whole batch when only one of several recs lacks payment', async () => {
+    const missing: Record<string, unknown> = { ...buildMinimalRec(), id: 'rec-2' };
+    delete missing.payment;
+    (recs.getPurchaseModalRecommendations as jest.Mock).mockReturnValue([buildMinimalRec(), missing]);
+
+    const btn = setup();
+    btn.click();
+    await new Promise((r) => setTimeout(r, 0));
+
+    expect(api.executePurchase).not.toHaveBeenCalled();
+    expect(lastToastMessage()).toContain('1 of 2');
+  });
+
+  test('a rec with a payment option is sent unchanged', async () => {
+    (recs.getPurchaseModalRecommendations as jest.Mock).mockReturnValue([
+      { ...buildMinimalRec(), payment: 'no-upfront' },
+    ]);
+    (api.executePurchase as jest.Mock).mockResolvedValue({
+      execution_id: 'exec-1',
+      status: 'queued',
+      email_sent: true,
+    });
+
+    const btn = setup();
+    btn.click();
+    await new Promise((r) => setTimeout(r, 0));
+
+    const sent = (api.executePurchase as jest.Mock).mock.calls[0][0];
+    expect(sent[0].payment).toBe('no-upfront');
+  });
+});
diff --git a/frontend/src/app.ts b/frontend/src/app.ts
index 85400162b..764a0f339 100644
--- a/frontend/src/app.ts
+++ b/frontend/src/app.ts
@@ -364,6 +364,21 @@ export async function handleExecutePurchase(): Promise<void> {
     return;
   }
 
+  // Refuse before the confirm dialog and the POST when any rec has no payment
+  // option. Defaulting here would silently pick the most cash-intensive
+  // schedule on a money path (issue #431).
+  const recsWithPayment = localRecs.filter(
+    (r): r is typeof r & { payment: string } => !!r.payment,
+  );
+  const missingCount = localRecs.length - recsWithPayment.length;
+  if (missingCount > 0) {
+    showToast({
+      message: `Cannot submit: ${missingCount} of ${localRecs.length} selected recommendation${localRecs.length === 1 ? '' : 's'} ${missingCount === 1 ? 'has' : 'have'} no payment option. Choose a payment option for each row and try again.`,
+      kind: 'error',
+    });
+    return;
+  }
+
   // Read the execute mode set by the modal toggle (issue #289).
   // "direct" means the session has execute-any/execute-own and chose to
   // bypass approval; "" is the default approval-required path.
@@ -407,16 +422,15 @@ export async function handleExecutePurchase(): Promise<void> {
   // future additions) flow through unchanged. Only `payment`, `selected`,
   // and `purchased` are overridden: `payment` uses the user-edited value
   // (issue #111), and `selected`/`purchased` are forced to the canonical
-  // purchase-intent values. The `?? 'all-upfront'` on payment is defensive
-  // only — direct test-harness callers that bypass the modal may not set
-  // `payment`; the production path always does. Passing `details` ensures
+  // purchase-intent values. A missing `payment` was refused above, so it is
+  // never defaulted here. Passing `details` ensures
   // Windows EC2, dedicated-tenancy, AZ-scoped RIs, and non-default-engine
   // RDS/Cache recs reach the backend with the correct ServiceDetails payload
   // instead of falling back to Linux/regional/default (issue #597).
-  const apiRecs: api.Recommendation[] = localRecs.map((r) => ({
+  const apiRecs: api.Recommendation[] = recsWithPayment.map((r) => ({
     ...r,
     monthly_cost: r.monthly_cost ?? null,
-    payment: r.payment ?? 'all-upfront',
+    payment: r.payment,
     selected: true,
     purchased: false,
   }));