Custom Payment Method in Magento 2 — The Minimal Working Extension
Most custom payment method tutorials show one or two files and leave the integration broken on Hyvä, broken in admin, or broken on capture. Here is the complete minimal working extension for Magento 2.4.4 — 2.4.9: the 9 files you cannot avoid (module.xml, config.xml, di.xml, payment.xml, adminhtml system.xml, Model/Payment.php, Knockout renderer, .html template, checkout_index_index.xml), the Hyvä Magewire parallel, a verified webhook handler with signature checks, and the refund flow wired through onlineRefund. Copy-paste ready, tested against Magento 2.4.9, PHP 8.4, Hyvä Checkout 1.2.