feat: support feePayer as URL on Tempo server methods

.changeset/fee-payer-url.md

@@ -0,0 +1,5 @@
+---
+"mppx": patch
+---
+
+Added support for `feePayer` as a URL string on `tempo` method.

package.json

@@ -37,13 +37,14 @@
     "@vitest/coverage-v8": "^4.0.17",
     "browserslist": "^4.28.1",
     "elysia": "^1.4.27",
-    "file-type": "^21.3.2",
     "eslint": "^9.28.0",
     "eslint-plugin-compat": "^7.0.1",
     "express": "^5.2.1",
+    "file-type": "^21.3.2",
     "hono": "^4.11.9",
     "playwright": "^1.58.2",
     "prool": "^0.2.2",
+    "tempo.ts": "^0.14.2",
     "testcontainers": "^11.11.0",
     "tsx": "^4.21.0",
     "typescript": "~5.9.3",

src/tempo/internal/account.ts

@@ -6,9 +6,10 @@ import type { Account, Address } from 'viem'
  * Accepts either `account` or `recipient` as the parameter name. When the value
  * is an `Account`, its address is extracted. If `feePayer` is `true`, the
  * account also acts as the fee payer. Alternatively, a separate `Account`
- * can be provided as the fee payer.
+ * can be provided as the fee payer, or a URL string pointing to a fee payer
+ * relay service (used with `withFeePayer` transport wrapping).
  *
- * @returns An object with `account`, `feePayer`, and `recipient`.
+ * @returns An object with `account`, `feePayer`, `feePayerUrl`, and `recipient`.
  */
 export function resolve(parameters: resolve.Parameters) {
   const account = (() => {
@@ -20,13 +21,15 @@ export function resolve(parameters: resolve.Parameters) {
     if (typeof parameters.account === 'object') return parameters.account.address
     return parameters.account
   })()
+  const feePayerUrl = typeof parameters.feePayer === 'string' ? parameters.feePayer : undefined
   const feePayer = (() => {
+    if (typeof parameters.feePayer === 'string') return undefined
     if (typeof parameters.account === 'object' && parameters.feePayer === true)
       return parameters.account
     if (typeof parameters.feePayer === 'object') return parameters.feePayer
     return undefined
   })()
-  return { account, feePayer, recipient: recipient as Address | undefined }
+  return { account, feePayer, feePayerUrl, recipient: recipient as Address | undefined }
 }
 
 export declare namespace resolve {
@@ -40,8 +43,8 @@ export declare namespace resolve {
     | {
         /** Address that receives payment. */
         account?: Address | undefined
-        /** Optional fee payer account for covering transaction fees. */
-        feePayer?: Account | undefined
+        /** Optional fee payer account or fee payer URL for covering transaction fees. */
+        feePayer?: Account | string | undefined
       }
   )
 }

src/tempo/server/Charge.test.ts

@@ -2,8 +2,9 @@ import { Challenge, Credential, Receipt } from 'mppx'
 import { Mppx as Mppx_client, tempo as tempo_client } from 'mppx/client'
 import { Mppx as Mppx_server, tempo as tempo_server } from 'mppx/server'
 import type { Hex } from 'ox'
+import { Handler } from 'tempo.ts/server'
 import { encodeFunctionData, parseUnits } from 'viem'
-import { prepareTransactionRequest, signTransaction } from 'viem/actions'
+import { getTransactionReceipt, prepareTransactionRequest, signTransaction } from 'viem/actions'
 import { Abis, Actions, Addresses, Tick } from 'viem/tempo'
 import { beforeAll, describe, expect, test } from 'vitest'
 import * as Http from '~test/Http.js'
@@ -487,6 +488,62 @@ describe('tempo', () => {
       httpServer.close()
     })
 
+    test('behavior: fee payer URL (withFeePayer transport)', async () => {
+      const feePayerHandler = Handler.feePayer({
+        account: accounts[0],
+        client,
+      })
+      const feePayerServer = await Http.createServer(feePayerHandler.listener)
+
+      const serverWithFeePayer = Mppx_server.create({
+        methods: [
+          tempo_server.charge({
+            feePayer: feePayerServer.url,
+            getClient: () => client,
+            currency: asset,
+          }),
+        ],
+        realm,
+        secretKey,
+      })
+
+      const mppx = Mppx_client.create({
+        polyfill: false,
+        methods: [
+          tempo_client({
+            account: accounts[1],
+            getClient: () => client,
+          }),
+        ],
+      })
+
+      const httpServer = await Http.createServer(async (req, res) => {
+        const result = await Mppx_server.toNodeListener(
+          serverWithFeePayer.charge({
+            amount: '1',
+            currency: asset,
+            recipient: accounts[0].address,
+          }),
+        )(req, res)
+        if (result.status === 402) return
+        res.end('OK')
+      })
+
+      const response = await mppx.fetch(httpServer.url)
+      expect(response.status).toBe(200)
+
+      const receipt = Receipt.fromResponse(response)
+      expect(receipt.status).toBe('success')
+
+      const txReceipt = await getTransactionReceipt(client, {
+        hash: receipt.reference as Hex.Hex,
+      })
+      expect((txReceipt as any).feePayer).toBe(accounts[0].address.toLowerCase())
+
+      httpServer.close()
+      feePayerServer.close()
+    })
+
     test('error: rejects fee-payer transaction with unauthorized calls', async () => {
       const httpServer = await Http.createServer(async (req, res) => {
         const result = await Mppx_server.toNodeListener(

src/tempo/server/Charge.ts

@@ -42,10 +42,11 @@ export function charge<const parameters extends charge.Parameters>(
     waitForConfirmation = true,
   } = parameters
 
-  const { recipient, feePayer } = Account.resolve(parameters)
+  const { recipient, feePayer, feePayerUrl } = Account.resolve(parameters)
 
   const getClient = Client.getResolver({
     chain: { ...tempo_chain, experimental_preconfirmationTime: 500 },
+    feePayerUrl,
     getClient: parameters.getClient,
     rpcUrl: defaults.rpcUrl,
   })
@@ -82,7 +83,7 @@ export function charge<const parameters extends charge.Parameters>(
 
       const resolvedFeePayer = (() => {
         const account = typeof request.feePayer === 'object' ? request.feePayer : feePayer
-        const requested = request.feePayer !== false && (account ?? feePayer)
+        const requested = request.feePayer !== false && (account ?? feePayer ?? feePayerUrl)
         if (credential) return account
         if (requested) return true
         return undefined
@@ -235,7 +236,7 @@ export function charge<const parameters extends charge.Parameters>(
               recipient,
             })
 
-          if (feePayer && methodDetails?.feePayer !== false)
+          if ((feePayer || feePayerUrl) && methodDetails?.feePayer !== false)
             FeePayer.validateCalls(calls, { amount, currency, recipient })
 
           const resolvedFeeToken =

src/tempo/server/Session.ts

@@ -98,12 +98,14 @@ export function session<const parameters extends session.Parameters>(p?: paramet
 
   const store = ChannelStore.fromStore(rawStore)
 
+  const { account, recipient, feePayer, feePayerUrl } = Account.resolve(parameters)
+
   const getClient = Client.getResolver({
     chain: tempo_chain,
+    feePayerUrl,
     getClient: parameters.getClient,
     rpcUrl: defaults.rpcUrl,
   })
-  const { account, recipient, feePayer } = Account.resolve(parameters)
 
   type Transport = parameters['sse'] extends false | undefined ? undefined : Transport.Sse
   const transport = parameters.sse
@@ -154,7 +156,7 @@ export function session<const parameters extends session.Parameters>(p?: paramet
       // Extract feePayer.
       const resolvedFeePayer = (() => {
         const account = typeof request.feePayer === 'object' ? request.feePayer : feePayer
-        const requested = request.feePayer !== false && (account ?? feePayer)
+        const requested = request.feePayer !== false && (account ?? feePayer ?? feePayerUrl)
         if (credential) return account
         if (requested) return true
         return undefined

src/viem/Client.ts

@@ -1,25 +1,41 @@
 import { type Chain, type Client, createClient, http } from 'viem'
+import { withFeePayer } from 'viem/tempo'
 import type { MaybePromise } from '../internal/types.js'
 
 export function getResolver(
   parameters: getResolver.Parameters & {
     /** Default chain to use if not provided. */
     chain?: Chain | undefined
+    /** Fee payer relay URL. When set, the transport is wrapped with `withFeePayer`. */
+    feePayerUrl?: string | undefined
     /** RPC URLs keyed by chain ID. */
     rpcUrl?: ({ [chainId: number]: string } & object) | undefined
   },
 ): (parameters: { chainId?: number | undefined }) => MaybePromise<Client> {
-  const { chain, getClient, rpcUrl } = parameters
+  const { chain, feePayerUrl, getClient, rpcUrl } = parameters
 
   if (getClient) {
     // When a default chain with serializers is provided (e.g. Tempo chain config),
     // ensure user-provided clients inherit those serializers. Without this, clients
     // created without the Tempo chain config will use the default viem serializer,
     // causing errors like "maxFeePerGas is not a valid Legacy Transaction attribute".
-    if (!chain?.serializers) return getClient
+    if (!chain?.serializers && !feePayerUrl) return getClient
     return async (params) => {
       const client = await getClient(params)
-      if (client.chain?.serializers?.transaction) return client
+
+      // Wrap the client's transport with `withFeePayer` when a fee payer URL is provided.
+      if (feePayerUrl && client.transport.key !== 'feePayer') {
+        const url = (client.transport as { url?: string }).url
+        if (url) {
+          const wrapped = createClient({
+            chain: client.chain,
+            transport: withFeePayer(http(url), http(feePayerUrl)),
+          })
+          Object.assign(client, { transport: wrapped.transport, request: wrapped.request })
+        }
+      }
+
+      if (!chain?.serializers || client.chain?.serializers?.transaction) return client
       return Object.assign({}, client, {
         chain: {
           ...chain,
@@ -40,9 +56,10 @@ export function getResolver(
     const resolvedChainId = chainId || Number(Object.keys(rpcUrl)[0])!
     const url = rpcUrl[resolvedChainId as keyof typeof rpcUrl]
     if (!url) throw new Error(`No \`rpcUrl\` configured for \`chainId\` (${resolvedChainId}).`)
+    const transport = feePayerUrl ? withFeePayer(http(url), http(feePayerUrl)) : http(url)
     return createClient({
       chain: { ...chain, id: resolvedChainId } as never,
-      transport: http(url),
+      transport,
     })
   }
 }

test/tempo/viem.ts

@@ -1,6 +1,6 @@
 import type * as Hex from 'ox/Hex'
 import { createClient, defineChain, type HttpTransportConfig, http as viem_http } from 'viem'
-import { type Account, english, generateMnemonic, mnemonicToAccount } from 'viem/accounts'
+import { english, generateMnemonic, type LocalAccount, mnemonicToAccount } from 'viem/accounts'
 import { tempo, tempoDevnet, tempoLocalnet, tempoModerato } from 'viem/chains'
 import { Actions } from 'viem/tempo'
 import { nodeEnv } from '../config.js'
@@ -17,7 +17,7 @@ export const accounts = Array.from({ length: 20 }, (_, i) =>
   mnemonicToAccount(accountsMnemonic, {
     accountIndex: i,
   }),
-) as unknown as FixedArray<Account, 20>
+) as unknown as FixedArray<LocalAccount, 20>
 
 export const chain = (() => {
   switch (nodeEnv) {