Guide

Gas & payers

An EIP-8130 transaction can name a payer: a second account that co-signs and pays gas. The sender authorizes the calls (sender_auth); the payer authorizes paying (payer_auth). This is the native mechanism behind sponsorship and pay-with-token, no bundler or EntryPoint.

Co-sign locally

When you hold the payer key (self-sponsor or your own backend), pass a payer signer to sendCalls8130. It signs payer_auth bound to the sender and sets the payer wire field. Set payer.address when the onchain payer account differs from the signing key.

Payer co-signts

import { privateKeyToAccount } from 'viem/accounts'
import { sendCalls8130 } from 'viem/experimental/eip8130'

const sponsor = privateKeyToAccount(process.env.SPONSOR_KEY as `0x${string}`)

const hash = await sendCalls8130(client, {
  account,                     // signs sender_auth
  calls: [{ to: recipient, data }],
  gas: 250_000n,
  payer: { account: sponsor }, // signs payer_auth, pays the fee
})

Whether a key may pay depends on its scope: SELF_PAYER funds the account's own transactions, while SPONSOR_PAYER funds others. The distinction and its risk model live in Gas & payers (protocol).

Estimating a sponsored transaction

Pass the payer address (and payerAuthVerifier if the payer uses a non-K1 key) so the node prices the payer authentication too.

estimateGas8130 with payerts

import { canonicalAuthenticators, estimateGas8130 } from 'viem/experimental/eip8130'

const gas = await estimateGas8130(client, {
  sender: account.address,
  calls: [[{ to: recipient, data }]],
  payer: sponsor.address,
  // Only needed if the payer uses a non-K1 key.
  payerAuthVerifier: canonicalAuthenticators.k1,
})

Pay with a token

To charge the sender in an ERC-20 (e.g. USDV) while the payer fronts native gas, run a two-phase transaction: phase 0 transfers the token to the payer, phase 1 runs the user's calls. The payer co-signs because it can see its fee is paid in phase 0.

Payer services call phases: batch 0 commits a token transfer to the payer; batch 1 can revert without undoing that payment. — Token payment lands in phase 0 and stays committed even if a later phase fails. ERC-8168 covers ERC-20 gas payment and other action-for-sponsorship flows.

Two-phase token paymentts

import { encodeTokenTransfer } from 'viem/experimental/eip8168'
import { sendCalls8130 } from 'viem/experimental/eip8130'

const hash = await sendCalls8130(client, {
  account,
  calls: [
    // phase 0: pay the payer in the token
    [encodeTokenTransfer({ token, to: sponsor.address, amount: fee })],
    // phase 1: the actual calls
    [{ to: recipient, data }],
  ],
  gas: 300_000n,
  payer: { account: sponsor },
})

Payer services (ERC-8168)

When a service holds the payer key, ERC-8168 standardizes a payer_* JSON-RPC endpoint the wallet queries to sponsor a transaction or accept a token as gas. Viem exposes the client and helpers under viem/experimental/eip8168. sendSponsoredCalls runs the whole flow: fetch terms, select an offer, build the phases (including any phase-0 token transfer), sign, and hand off to the payer.

sendSponsoredCallsts

import { createPayerClient, sendSponsoredCalls } from 'viem/experimental/eip8168'

const payerClient = createPayerClient({ url: 'https://payer.example.com/v1' })

const { transactionHash, tokenCharged } = await sendSponsoredCalls(client, {
  account,      // signs sender_auth; payer_auth is filled by the service
  payerClient,
  calls: [{ to: recipient, data }],
  token,        // omit to prefer a fully-sponsored offer
})

If the payer rejects with a recoverable reason (a higher token charge or gas limit), sendSponsoredCalls re-signs only when you approve each attempt via confirmRetry.

Confirming re-signsts

import { formatUnits } from 'viem'
import { sendSponsoredCalls } from 'viem/experimental/eip8168'

const result = await sendSponsoredCalls(client, {
  account,
  payerClient,
  calls: [{ to: recipient, data }],
  token,
  retries: 2,
  // Each retry mints a NEW sender_auth (and a fresh passkey gesture),
  // so nothing is re-signed without explicit approval.
  confirmRetry: async (request) => {
    if (request.kind === 'requote')
      return confirm(`Pay ${formatUnits(request.paymentAmount, 6)} in token?`)
    return confirm(`Raise gas limit to ${request.gasLimit}?`)
  },
})

For full control, drive the primitives directly: payerClient.getTerms, selectPaymentOption, buildSponsoredCalls, then prepare, sign, and payerClient.sendTransaction. Decode rejections with parsePayerError.

Session keys and gas

A POLICY-only session key cannot touch account ETH; sponsor it, or route it through a payer service that accepts spend-limit gas.
Adding SELF_PAYER lets a session key self-fund gas, but a griefing key could then burn the balance on fees. Treat it as a deliberate opt-in. See Session keys.