# Custom \[Build your own payment method] The `mppx` SDK supports dynamic extensibility for new payment methods. You can implement custom payment methods to integrate any payment rails such as other blockchains, card processors, or proprietary systems. ## Overview A custom payment method requires: 1. **Method definition:** Define the method name, intent, and schemas for request parameters and Credential payloads 2. **Client logic:** Create Credentials when the client gets a `402` response 3. **Server logic:** Verify Credentials and return Receipts ## Define a method Start by defining your payment method with `Method.from`. The definition includes the method name, intent type, and schemas for request parameters and Credential payloads. ```ts twoslash import { Method, z } from 'mppx' const lightning = Method.from({ intent: 'charge', name: 'lightning', schema: { credential: { payload: z.object({ preimage: z.string(), }), }, request: z.object({ amount: z.string(), currency: z.string(), invoice: z.string(), paymentHash: z.string(), recipient: z.string(), }), }, }) ``` ## Client implementation Extend the method with Credential creation logic using `Method.toClient`. The `createCredential` function runs when the client gets a `402` response: ```ts twoslash import { Credential, Method, z } from 'mppx' const lightning = Method.from({ intent: 'charge', name: 'lightning', schema: { credential: { payload: z.object({ preimage: z.string(), }), }, request: z.object({ amount: z.string(), currency: z.string(), invoice: z.string(), paymentHash: z.string(), recipient: z.string(), }), }, }) declare function payInvoice(invoice: string): Promise<{ preimage: string }> // ---cut--- const clientMethod = Method.toClient(lightning, { async createCredential({ challenge }) { const result = await payInvoice(challenge.request.invoice) return Credential.serialize({ challenge, payload: { preimage: result.preimage, }, }) }, }) ``` ## Server implementation Extend the method with verification logic using `Method.toServer`. For Lightning Network, verify that the preimage hashes to the payment hash: ```ts twoslash import { Method, Receipt, z } from 'mppx' const lightning = Method.from({ intent: 'charge', name: 'lightning', schema: { credential: { payload: z.object({ preimage: z.string(), }), }, request: z.object({ amount: z.string(), currency: z.string(), invoice: z.string(), paymentHash: z.string(), recipient: z.string(), }), }, }) declare function bytesToHex(bytes: Uint8Array): string declare function hexToBytes(hex: string): Uint8Array declare function sha256(data: Uint8Array): Uint8Array // ---cut--- const serverMethod = Method.toServer(lightning, { async verify({ credential }) { const preimage = credential.payload.preimage const expectedHash = credential.challenge.request.paymentHash const actualHash = bytesToHex(sha256(hexToBytes(preimage))) const isValid = actualHash === expectedHash return Receipt.from({ method: 'lightning', reference: preimage, status: 'success', timestamp: new Date().toISOString(), }) }, }) ``` ## Use in your app ### Client Pass the client method to `Mppx.create`: ```ts twoslash import { Credential, Method, z } from 'mppx' import { Mppx } from 'mppx/client' const lightning = Method.from({ intent: 'charge', name: 'lightning', schema: { credential: { payload: z.object({ preimage: z.string(), }), }, request: z.object({ amount: z.string(), currency: z.string(), invoice: z.string(), paymentHash: z.string(), recipient: z.string(), }), }, }) declare function payInvoice(invoice: string): Promise<{ preimage: string }> const clientMethod = Method.toClient(lightning, { async createCredential({ challenge }) { const result = await payInvoice(challenge.request.invoice) return Credential.serialize({ challenge, payload: { preimage: result.preimage, }, }) }, }) // ---cut--- const { fetch } = Mppx.create({ methods: [clientMethod], polyfill: false, }) const response = await fetch('https://api.example.com/premium') ``` ### Server Pass the server method to `Mppx.create`: ```ts twoslash import { Method, Receipt, z } from 'mppx' import { Mppx } from 'mppx/server' const lightning = Method.from({ intent: 'charge', name: 'lightning', schema: { credential: { payload: z.object({ preimage: z.string(), }), }, request: z.object({ amount: z.string(), currency: z.string(), invoice: z.string(), paymentHash: z.string(), recipient: z.string(), }), }, }) declare function bytesToHex(bytes: Uint8Array): string declare function hexToBytes(hex: string): Uint8Array declare function sha256(data: Uint8Array): Uint8Array const serverMethod = Method.toServer(lightning, { async verify({ credential }) { const preimage = credential.payload.preimage const expectedHash = credential.challenge.request.paymentHash const actualHash = bytesToHex(sha256(hexToBytes(preimage))) const isValid = actualHash === expectedHash return Receipt.from({ method: 'lightning', reference: preimage, status: 'success', timestamp: new Date().toISOString(), }) }, }) // ---cut--- const mppx = Mppx.create({ methods: [serverMethod], }) ``` ## SDK references * [`Method.from`](/sdk/typescript/Method.from)—Define a payment method with schemas * [`Method.toClient`](/sdk/typescript/core/Method.toClient)—Extend a method with client-side Credential creation logic * [`Method.toServer`](/sdk/typescript/core/Method.toServer)—Extend a method with server-side verification logic