ui_mode can now return "form"

What changed

Producer-side tools call this additive. It breaks exhaustive switches, strict parsers and generated client enums.

Affects ui_mode on checkout.session reachable from 15 operations.

New value: form.

can now return "form"

Who is affected

Code that reads Stripe's checkout.session objects, used by 15 Stripe operations.

Before and after

Code that breaks

switch (response.ui_mode) {
  // existing branches …
  // no branch for "form"
  default:
    throw new Error("unexpected ui_mode");
}

Based on Stripe’s published API specification. How severity is decided.