One or more fields are not valid

The framework's field-validation refusal. The interesting part is not the code, it is errors[]: one entry per offending field, each with a field path, a machine code and a human message.

What to do

Render errors[] against your form rather than showing detail. field uses dotted paths into the body — amount.minor, customer.email — so a field-level message can be placed next to the input that caused it, which is the difference between a form a payer can fix and one they abandon.