fix(payment): suppress checkout trial_period_days when promotion code is applied (#23300)

backend/routers/payment.py

@@ -1,4 +1,4 @@
 from config.plan_catalog import configured_ladder_prices, validate_ladder_price
 from database.fsr_read_cache import fresh_reader, fresh_reads
 import os
 from datetime import datetime, timezone
@@ -447,7 +447,7 @@
                     current_price_id = stripe_sub['items']['data'][0]['price']['id']
 
                     # Check for pending subscription schedules
                     customer_id = stripe_sub.get('customer')
                     if customer_id:
                         try:
                             # Get all subscription schedules for this customer
@@ -655,7 +655,7 @@
     # subscriber promised something this checkout cannot give, so refuse rather than charge at once.
     if request.trial_days and existing_customer_id:
         raise HTTPException(status_code=400, detail="trial_unavailable")
-    trial_period_days = request.trial_days or None
+    trial_period_days = None if resolved_checkout_promo_id else (request.trial_days or None)
     try:
         session = stripe_utils.create_subscription_checkout_session(
             uid,
@@ -809,7 +809,7 @@
                         from utils.processing_backlog_worker import upgrade
 
                         if allowance_increased(current_subscription, new_subscription):
                             upgrade(
                                 uid,
                                 f'plan-change:{new_subscription.stripe_subscription_id}:{request.price_id}:{new_subscription.current_period_start}',
                                 intent_at=time.time(),
@@ -1124,7 +1124,7 @@
                                     f"Ignoring checkout session for subscription with unknown price_id: {price_id}"
                                 )
                 except Exception as e:
                     logger.error(f"Error retrieving subscription for notification: {sanitize(str(e))}")
 
     if event['type'] in [
         'customer.subscription.updated',

backend/tests/unit/test_payment_checkout_trial.py

@@ -71,3 +71,64 @@ def test_no_trial_requested_keeps_the_old_checkout():
     assert result['session_id'] == 'cs_test'
     assert captured['trial_period_days'] is None
     assert captured['customer_id'] == 'cus_existing'
+
+
+def test_promo_code_suppresses_trial_period_days_on_checkout_endpoint():
+    """A promotion code on a trial checkout suppresses trial_period_days so the
+    initial invoice is not $0 and Stripe's minimum_amount check passes (#23300)."""
+    captured = {}
+
+    def create_session(
+        uid, price_id, idempotency_key, customer_id=None, promotion_code_id=None, trial_period_days=None
+    ):
+        captured['promotion_code_id'] = promotion_code_id
+        captured['trial_period_days'] = trial_period_days
+        return SimpleNamespace(url='https://checkout.stripe.com/c/pay/cs_test', id='cs_test')
+
+    promo_obj = SimpleNamespace(id='promo_unlimited_year')
+    with (
+        patch.object(payment, 'is_purchasable_price_id', return_value=True),
+        patch.object(payment.subscription_utils, 'can_user_make_payment', return_value=(True, '')),
+        patch.object(payment.subscription_utils, 'new_buyer_single_plan_violation', return_value=None, create=True),
+        patch.object(payment, '_try_reactivate_subscription', return_value=None),
+        patch.object(payment.users_db, 'get_stripe_customer_id', return_value=None),
+        patch.object(payment.stripe.PromotionCode, 'list', return_value=SimpleNamespace(data=[promo_obj])),
+        patch.object(payment.stripe_utils, 'create_subscription_checkout_session', side_effect=create_session),
+    ):
+        result = payment.create_checkout_session_endpoint(
+            CreateCheckoutRequest(price_id='price_1', trial_days=7, promotion_code='OMI-4FHY-W6SR'),
+            uid='uid_1',
+        )
+
+    assert result['session_id'] == 'cs_test'
+    assert captured['promotion_code_id'] == 'promo_unlimited_year'
+    assert captured['trial_period_days'] is None
+
+
+def test_stripe_helper_omits_trial_period_days_when_promotion_code_present():
+    """create_subscription_checkout_session omits subscription_data.trial_period_days
+    when promotion_code_id is provided so minimum_amount codes are not rejected (#23300)."""
+    import importlib
+    from unittest.mock import MagicMock
+    import utils.stripe as stripe_utils
+
+    if isinstance(getattr(stripe_utils, 'create_subscription_checkout_session', None), MagicMock):
+        stripe_utils = importlib.reload(stripe_utils)
+
+    captured = {}
+
+    def fake_session_create(**params):
+        captured.update(params)
+        return SimpleNamespace(url='https://checkout.stripe.com/c/pay/cs_test', id='cs_test')
+
+    with patch('utils.stripe.stripe.checkout.Session.create', side_effect=fake_session_create):
+        stripe_utils.create_subscription_checkout_session(
+            uid='uid_1',
+            price_id='price_unlimited_yearly',
+            idempotency_key='idem_1',
+            promotion_code_id='promo_unlimited_year',
+            trial_period_days=7,
+        )
+
+    assert captured['discounts'] == [{'promotion_code': 'promo_unlimited_year'}]
+    assert 'trial_period_days' not in captured['subscription_data']

backend/utils/stripe.py

@@ -80,7 +80,10 @@ def create_subscription_checkout_session(
         }
 
         if trial_period_days:
-            session_params['subscription_data']['trial_period_days'] = trial_period_days
+            # A free trial makes the initial Checkout invoice $0, so Stripe rejects
+            # promotion codes with a minimum_amount restriction (issue #23300).
+            if not promotion_code_id:
+                session_params['subscription_data']['trial_period_days'] = trial_period_days
 
         if promotion_code_id:
             session_params['discounts'] = [{'promotion_code': promotion_code_id}]