Transaction Fee Sponsorship

With ABC Paymaster, you can sponsor transaction fees for your users without changing their existing EOA wallet address.

Supported Networks

Currently, the supported networks for gas sponsorship are Ethereum, Kaia, and Base.
For testnets, Ethereum Sepolia, Kaia Kairos, and Base Sepolia are supported.

Using Avalanche C-Chain?

Avalanche C-Chain is not supported with this approach. Please refer to the SmartAccount guide.

API Guide

The overall flow consists of 3 steps.

Step	Method	Endpoint
1. Sponsor	POST	/core/evm/v2/eoa/paymaster/sponsor
2. Send	POST	/core/evm/v2/eoa/paymaster/send
3. Receipt	GET	/core/evm/v2/eoa/paymaster/receipt

1. Sponsor { #1-sponsor }

Send the gas sponsorship request and receive the data to sign.

Request

POST /core/evm/v2/eoa/paymaster/sponsor

{
  "network": "ethereum_sepolia",
  "from": "0xAbC...",
  "to": "0xDeF...",
  "value": "0x0",
  "data": "0x..."
}

Field	Description
network	Target network identifier (e.g., ethereum_sepolia, kaia_kairos, base_sepolia)
from	Sender EOA address
to	Recipient address (contract or EOA)
value	Amount of ETH to transfer (wei, hex)
data	Contract call data. Use "0x" for simple transfers

Response

sponsored_transaction is an ERC-4337 v0.7 UserOperation payload (split format). The client must pass it through to the send step without modifying any field.

{
  "sponsored_transaction": {
    "sender": "0xAbC...",
    "nonce": "0x1",
    "callData": "0x...",
    "callGasLimit": "0x...",
    "verificationGasLimit": "0x...",
    "preVerificationGas": "0x...",
    "maxFeePerGas": "0x...",
    "maxPriorityFeePerGas": "0x...",
    "paymaster": "0xabc7777777641b746429c25961fe99ea48797cbe",
    "paymasterVerificationGasLimit": "0x...",
    "paymasterPostOpGasLimit": "0x0",
    "paymasterData": "0x...",
    "signature": "0x"
  },
  "sign_hash": "0xabcdef...",
  "authorization": {
    "chainId": 11155111,
    "address": "0x...",
    "nonce": 0
  },
  "authorization_hash": "0x..."
}

Field	Description
sponsored_transaction	v0.7 UserOperation payload to forward as-is in Step 2 Send
sign_hash	UserOperation hash the EOA must sign
authorization	Included only on the first transaction. See below
authorization_hash	Included only on the first transaction. Hash to sign for authorization. See below

Delegation auto-detection

The paymaster reads the EOA's on-chain code and detects whether it has already been delegated to the SimpleAccount v0.7 implementation (EIP-7702 0xef0100… prefix).

• Not yet delegated: response includes authorization and authorization_hash. The client must sign the EIP-7702 authorization and pass the signed result to /send.
• Already delegated: authorization and authorization_hash are both omitted. The client signs only sign_hash and calls /send without the authorization body field.

2. Send { #2-send }

Submit the signed data.

Request

POST /core/evm/v2/eoa/paymaster/send

{
  "network": "ethereum_sepolia",
  "sponsored_transaction": { "..." },
  "signature": "0x...",
  "authorization": {
    "chainId": 11155111,
    "address": "0x...",
    "nonce": 0,
    "v": 0,
    "r": "0x...",
    "s": "0x..."
  }
}

Field	Description
sponsored_transaction	Pass the sponsored_transaction from Step 1 response as-is
signature	Signature over sign_hash. See below
authorization	Required only when the sponsor response included authorization. For an already-delegated EOA, omit this field — sending only sponsored_transaction and signature is sufficient. See below

warning

Modifying any field in sponsored_transaction will cause signature verification to fail. Pass it exactly as received.

Response

{
  "tx_id": "0x..."
}

3. Receipt { #3-receipt }

Query the transaction result using tx_id.

Request

GET /core/evm/v2/eoa/paymaster/receipt?tx_id={tx_id}&network={network}

Response

When complete:

{
  "success": true,
  "sender": "0xAbC...",
  "receipt": {
    "transactionHash": "0x...",
    "blockNumber": "0x...",
    "status": "0x1"
  }
}

While processing (not yet indexed):

null

If the Transaction is still in the mempool or receipt indexing is in progress, the response is null. Poll every 1–5 seconds until receipt.transactionHash is populated.

Signing { #signing }

Both sign_hash and authorization_hash are signed using the WaaS MPC Sign API.

POST /v3/wallet/sign

{
  "curve": "secp256k1",
  "encrypted_share": "Encrypted MPC share from wallet generation/recovery",
  "key_id": "Unique identifier of the signing key",
  "message": "Hash to sign (hex string)",
  "secret_store": "Key used to decrypt the share"
}

The two hashes differ in how they are processed before signing.

Hash	EIP-191 prefix	Value passed to MPC message
sign_hash	Applied (personal_sign)	The 32-byte hash with the EIP-191 prefix applied
authorization_hash	Not applied	authorization_hash as-is

Signing sign_hash

sign_hash must pass SimpleAccount v0.7's toEthSignedMessageHash().recover() verification, so compute the Ethereum personal_sign prefixed hash before passing it to /v3/wallet/sign.

import { keccak256, toBytes, concat } from 'viem';

// Apply personal_sign prefix
const prefix = toBytes(`\x19Ethereum Signed Message:\n32`);
const messageHash = keccak256(concat([prefix, toBytes(sponsorResponse.sign_hash)]));

const signRes = await fetch('/v3/wallet/sign', {
  method: 'POST',
  headers: { 'Content-Type': 'application/json', 'Authorization': `Bearer ${accessToken}` },
  body: JSON.stringify({
    curve: 'secp256k1',
    encrypted_share: encryptedShare,
    key_id: keyId,
    message: messageHash,
    secret_store: secretStore,
  }),
});
const { signature } = await signRes.json();

const sendSignature = `0x${signature}`;

Signing authorization_hash (first transaction only) { #authorization }

authorization and authorization_hash are included in the Sponsor response only when the EOA is sending a sponsored transaction for the first time. On subsequent transactions, both fields are omitted, and you only need to sign sign_hash and call /send.

Pass authorization_hash as message (no EIP-191 prefix), then split the 65-byte signature (130 hex chars) into r, s, v to populate the authorization object.

const authSignRes = await fetch('/v3/wallet/sign', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json', 'Authorization': `Bearer ${accessToken}` },
    body: JSON.stringify({
        curve: 'secp256k1',
        encrypted_share: encryptedShare,
        key_id: keyId,
        message: sponsorResponse.authorization_hash,
        secret_store: secretStore,
    }),
});
const { signature: authSig } = await authSignRes.json();

// Split 65-byte signature (130 hex chars) into r, s, v
const r = `0x${authSig.slice(0, 64)}`;
const s = `0x${authSig.slice(64, 128)}`;

// Normalize v to EIP-7702 yParity (0 or 1).
// The MPC response is in legacy ECDSA form (27/28), so subtract 27.
let v = parseInt(authSig.slice(128, 130), 16);
if (v >= 27) v -= 27;

// chainId / nonce are reused from the sponsor response (already decimal integers).
const authorization = {
    ...sponsorResponse.authorization,
    v,
    r,
    s,
};