Version 48.5.0 of the .NET library is pinned to API version 2025-08-27.basil[0]. This means that all requests initiated by the SDK use that API version by setting the version header[1], which overrides your account’s default API version. Effectively, this means that all requests made with that SDK version use that API version.

API version 2025-03-31.basil introduced a breaking change[2], which removed the payment_intent field from the Invoice object (due to the introduction of support for multiple partial Invoice Payments).

For API versions 2025-03-31.basil, and later, the first PaymentIntent's client_secret can now be accessed from the Invoice object in invoice.confirmation_secret.client_secret [3]. You should use this instead of invoice.payment_intent.client_secret.

[0] https://github.com/stripe/stripe-dotnet/blob/master/CHANGELOG.md#4850---2025-08-27

[1] https://docs.stripe.com/api/versioning

[2] https://docs.stripe.com/changelog/basil/2025-03-31/add-support-for-multiple-partial-payments-on-invoices

[3]https://docs.stripe.com/changelog/basil/2025-03-31/add-support-for-multiple-partial-payments-on-invoices#use-for-payment-element-integrations