Read reserved field names. Custom fields names that collide with any of these are invalid.
curl --request GET \
--url https://api.sendgrid.com/v3/contactdb/reserved_fields \
--header 'authorization: Bearer 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' \
--data '{}'
{
"reserved_fields": [