Align Stripe plan changes to first-of-month proration anchors

src/Exceptionless.Web/Controllers/OrganizationController.cs

@@ -505,7 +505,9 @@ public async Task<ActionResult<ChangePlanResult>> ChangePlanAsync(
                 var subscriptionOptions = new SubscriptionCreateOptions
                 {
                     Customer = customer.Id,
-                    Items = [new SubscriptionItemOptions { Price = model.PlanId }]
+                    Items = [new SubscriptionItemOptions { Price = model.PlanId }],
+                    BillingCycleAnchorConfig = CreateMonthlyBillingCycleAnchorConfig(),
+                    ProrationBehavior = "create_prorations"
                 };
 
                 if (isPaymentMethod)
@@ -522,8 +524,18 @@ public async Task<ActionResult<ChangePlanResult>> ChangePlanAsync(
             // Existing customer: update (or create) their Stripe subscription and optionally swap payment method.
             else
             {
-                var update = new SubscriptionUpdateOptions { Items = [] };
-                var create = new SubscriptionCreateOptions { Customer = organization.StripeCustomerId, Items = [] };
+                var update = new SubscriptionUpdateOptions
+                {
+                    Items = [],
+                    ProrationBehavior = "create_prorations"
+                };
+                var create = new SubscriptionCreateOptions
+                {
+                    Customer = organization.StripeCustomerId,
+                    Items = [],
+                    BillingCycleAnchorConfig = CreateMonthlyBillingCycleAnchorConfig(),
+                    ProrationBehavior = "create_prorations"
+                };
                 bool cardUpdated = false;
 
                 var customerUpdateOptions = new CustomerUpdateOptions { Description = organization.Name };
@@ -610,6 +622,17 @@ await Task.WhenAll(
         return Ok(new ChangePlanResult { Success = true });
     }
 
+    private static SubscriptionBillingCycleAnchorConfigOptions CreateMonthlyBillingCycleAnchorConfig()
+    {
+        return new SubscriptionBillingCycleAnchorConfigOptions
+        {
+            DayOfMonth = 1,
+            Hour = 0,
+            Minute = 0,
+            Second = 0
+        };
+    }
+
     /// <summary>
     /// Add user
     /// </summary>

tests/Exceptionless.Tests/Controllers/OrganizationControllerTests.cs

@@ -1059,6 +1059,12 @@ public async Task ChangePlanAsync_NewCustomerCreatesStripeCustomerAndSubscriptio
         var item = Assert.Single(subscription.Items);
         Assert.Equal(_plans.SmallPlan.Id, item.Price);
         Assert.Equal("coupon_10", Assert.Single(subscription.Discounts).Coupon);
+        Assert.Equal("create_prorations", subscription.ProrationBehavior);
+        Assert.NotNull(subscription.BillingCycleAnchorConfig);
+        Assert.Equal(1, subscription.BillingCycleAnchorConfig.DayOfMonth);
+        Assert.Equal(0, subscription.BillingCycleAnchorConfig.Hour);
+        Assert.Equal(0, subscription.BillingCycleAnchorConfig.Minute);
+        Assert.Equal(0, subscription.BillingCycleAnchorConfig.Second);
 
         var organization = await _organizationRepository.GetByIdAsync(SampleDataService.FREE_ORG_ID);
         Assert.NotNull(organization);
@@ -1112,6 +1118,7 @@ public async Task ChangePlanAsync_ExistingCustomerUpdatesPaymentMethodAndSubscri
         Assert.Equal("si_active", item.Id);
         Assert.Equal(_plans.SmallPlan.Id, item.Price);
         Assert.Equal("coupon_10", Assert.Single(updatedSubscription.Options.Discounts).Coupon);
+        Assert.Equal("create_prorations", updatedSubscription.Options.ProrationBehavior);
         Assert.Empty(StripeBillingClient.CreatedSubscriptionOptions);
 
         var organization = await _organizationRepository.GetByIdAsync(SampleDataService.FREE_ORG_ID);