custom payment methods

04.md

@@ -101,6 +101,35 @@ The mint then responds with:
 
 where `signatures` is an array of blind signatures on the outputs.
 
+## Custom Payment Methods
+
+Payment methods not specified in a dedicated NUT can be supported as custom payment methods. A custom payment method is identified by a lowercase string `{method}` (e.g., `paypal`, `stripe`, `onchain-payment-processor`). The `{method}` string **MUST** contain only ASCII alphanumeric characters, hyphens (`-`), and underscores (`_`), and **MUST** be non-empty.
+
+### Mint Quote
+
+For a custom `{method}`, the wallet sends a request following the common mint quote request format (see [General Flow](#general-flow)). Method-specific fields (e.g., an `amount` of tokens to mint, a `description`, or a `pubkey` for [NUT-20][20] locks) are defined by the method-specific NUT.
+
+The mint responds with the common mint quote response format. The `request` field contains the method-specific payment request (e.g., a payment URL, an on-chain address, an account identifier). Additional method-specific fields (such as `amount` or `expiry`) are defined by the method-specific NUT.
+
+
+### Method-Specific Fields
+
+Custom payment methods **MAY** include additional fields in requests and responses. Implementations **MUST** ignore unrecognized fields to preserve forward compatibility. When a custom method gains widespread adoption, its fields **MAY** be formalized in a dedicated NUT.
+
+Custom payment methods **MAY** include extra fields that the mint forwards to the payment processor without validation.
+
+### Websocket Notifications
+
+Custom methods **MAY** support websocket subscriptions via [NUT-17][17] with kind `"{method}_mint_quote"`.
+
+### Cached Responses
+
+Custom method endpoints **MAY** support caching via [NUT-19][19] (e.g., `POST /v1/mint/{method}`).
+
+### Amount Precision
+
+If an `amount` is provided, fiat-denominated units (e.g., `usd`, `eur`) **SHOULD** represent the value in the smallest currency unit (e.g., cents). For example, `"amount": 100` with `"unit": "usd"` represents $1.00 USD.
+
 ## Adding New Payment Methods
 
 To add a new payment method (e.g., BOLT12), implement the following:

05.md

@@ -203,6 +203,24 @@ GET https://mint.host:3338/v1/melt/quote/bolt11/TRmjduhIsPxd...
 }
 ```
 
+## Custom Payment Methods
+
+Payment methods not specified in a dedicated NUT can be supported as custom payment methods. See [NUT-04][04] for the custom method identifier definition and general conventions.
+
+### Melt Quote
+
+For a custom `{method}`, the wallet sends a request following the common melt quote request format (see [General Flow](#general-flow)). The `request` field is the method-specific payment target (e.g., a bank account identifier, an on-chain address, a payment processor reference). `unit` is the unit the wallet would like to pay with.
+
+The mint responds with the common melt quote response format. Method-specific fields are defined by the method-specific NUT.
+
+### Websocket Notifications
+
+Custom methods **MAY** support websocket subscriptions via [NUT-17][17] with kind `"{method}_melt_quote"`.
+
+### Cached Responses
+
+Custom method endpoints **MAY** support caching via [NUT-19][19] (e.g., `POST /v1/melt/{method}`).
+
 ## Adding New Payment Methods
 
 To add a new payment method (e.g., BOLT12), implement the following:
@@ -257,6 +275,8 @@ The mint's settings for this NUT indicate the supported method-unit pairs for me
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