fix: require expiring nonce for fee payer

.changeset/veria-306-expiring-nonce-key.md

@@ -0,0 +1,5 @@
+---
+'mppx': patch
+---
+
+Required expiring nonce keys for fee-sponsored transactions.

src/tempo/internal/fee-payer.test.ts

@@ -1,4 +1,4 @@
-import { encodeFunctionData } from 'viem'
+import { encodeFunctionData, maxUint256 } from 'viem'
 import { Abis, Addresses } from 'viem/tempo'
 import { describe, expect, test } from 'vp/test'
 
@@ -443,7 +443,7 @@ describe('prepareSponsoredTransaction', () => {
     maxFeePerGas: 1_000_000_000n,
     maxPriorityFeePerGas: 1_000_000_000n,
     nonce: 1n,
-    nonceKey: 1n,
+    nonceKey: 'expiring',
     signature: { r: 1n, s: 1n, yParity: 0 } as any,
     validBefore: Math.floor(Date.now() / 1_000) + 300,
   } as const
@@ -460,6 +460,36 @@ describe('prepareSponsoredTransaction', () => {
     ).not.toThrow()
   })
 
+  test('accepts serialized expiring nonce key', () => {
+    expect(() =>
+      prepareSponsoredTransaction({
+        account: sponsor,
+        chainId: 42431,
+        details,
+        expectedFeeToken: bogus,
+        transaction: {
+          ...baseTransaction,
+          nonceKey: maxUint256,
+        } as any,
+      }),
+    ).not.toThrow()
+  })
+
+  test('error: rejects non-expiring nonce keys', () => {
+    expect(() =>
+      prepareSponsoredTransaction({
+        account: sponsor,
+        chainId: 42431,
+        details,
+        expectedFeeToken: bogus,
+        transaction: {
+          ...baseTransaction,
+          nonceKey: 1n,
+        } as any,
+      }),
+    ).toThrow('must use an expiring nonce')
+  })
+
   test('accepts higher Moderato priority fees by default', () => {
     expect(() =>
       prepareSponsoredTransaction({

src/tempo/internal/fee-payer.ts

@@ -2,7 +2,7 @@ import type { TempoAddress } from 'ox/tempo'
 import { TxEnvelopeTempo } from 'ox/tempo'
 import type { Hex } from 'viem'
 import type { Account } from 'viem'
-import { decodeFunctionData } from 'viem'
+import { decodeFunctionData, maxUint256 } from 'viem'
 import { Abis, Addresses, Transaction } from 'viem/tempo'
 
 import * as TempoAddress_internal from './address.js'
@@ -126,6 +126,10 @@ function getPolicy(chainId: number, overrides: Partial<Policy> | undefined): Pol
   }
 }
 
+function isExpiringNonceKey(nonceKey: SponsoredTransaction['nonceKey']): boolean {
+  return nonceKey === 'expiring' || nonceKey === maxUint256
+}
+
 /** Validates that a set of transaction calls matches an allowed fee-payer pattern. */
 export function validateCalls(
   calls: readonly { data?: `0x${string}` | undefined; to?: TempoAddress.Address | undefined }[],
@@ -358,7 +362,7 @@ export function prepareSponsoredTransaction(parameters: {
       maxPriorityFeePerGas: maxPriorityFeePerGas.toString(),
     })
 
-  if (nonceKey === undefined) fail('fee-sponsored transaction must use an expiring nonce')
+  if (!isExpiringNonceKey(nonceKey)) fail('fee-sponsored transaction must use an expiring nonce')
   if (validBefore === undefined)
     fail('fee-sponsored transaction must declare validBefore for the expiring nonce')
   const validBeforeValue = validBefore