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Important

The code in this repository is still under audit. It is not yet recommended for production use.

Magic Spend

Magic Spend is a contract that allows onchain accounts to present valid Withdraw Requests and receive funds. A Withdraw Request is defined as

struct WithdrawRequest {
    bytes signature;
    address asset;
    uint256 amount;
    uint256 nonce;
    uint48 expiry;
 }

Where signature is an EIP-191 compliant signature of the message

abi.encode(
  <Magic Spend contract address>,
  <UserOperation.sender and/or msg.sender of the withdraw call>,
  <chain ID>,
  withdrawRequest.asset,
  withdrawRequest.amount,
  withdrawRequest.nonce,
  withdrawRequest.expiry
)

Magic Spend is an ERC-4337 compliant paymaster (EntryPoint v0.6) and also enables withdraw requests with asset ETH (address(0)) to be used to pay transaction gas.

This contract is part of a broader Magic Spend product from Coinbase, which as a whole allows Coinbase users to seamlessly use their assets onchain.

Diagram of Coinbase user making use of Magic Spend

We have started a discussion around a possible new wallet RPC to enable apps to have better awareness of this "just in time" funding.

Detailed Flows

When the withdrawing account is an ERC-4337 compliant smart contract (like Smart Wallet), there are three different ways the Magic Spend smart contract can be used

  1. Pay gas only
  2. Transfer funds during execution only
  3. Pay gas and transfer funds during execution

Pay gas only

Pay gas only flow diagram
  1. A ERC-4337 UserOperation is submitted to the bundler. The paymasterAndData field includes the Magic Spend address and the withdrawal request.
  2. Bundler (EOA) calls EntryPoint smart contract.
  3. Entrypoint first calls to UserOperation.sender, a smart contract wallet (SCW), to validate the user operation.
  4. Entrypoint decrements the paymaster’s deposit in the Entrypoint. If the paymaster’s deposit is less than the gas cost, the transaction will revert.
  5. EntryPoint calls the Magic Spend contract to run validations on the withdrawal, including checking the signature and ensuring withdraw.value is greater than transaction max gas cost.
  6. Entrypoint calls to SCW with UserOperation.calldata
  7. SCW does arbitrary operation, invoked by UserOperation.calldata.
  8. Entrypoint makes post-op call to Magic Spend, with actual gas used in transaction.
  9. Magic Spend sends the SCW any withdraw.value minus actual gas used.
  10. Entrypoint refunds the paymaster if actual gas < estimated gas from (4.)
  11. Entrypoint pays bundler for tx gas

Transfer funds during execution only

Diagram of 'Transfer funds during execution only' flow

This is the simplest flow. The Magic Spend account is agnostic to any details of this transaction, even whether or not the caller is a SCW. It simply validates the withdraw and transfers funds if valid.

Pay gas and transfer funds during execution

Pay gas and transfer funds during execution

This flow is like "Pay gas only” with the addition of (7.) and (8.). Here, the SCW also requests funds during execution. In this flow, a user might be, for example, trying to mint an NFT and needs funds for the mint.

Deployments

Network Contract Address
Base Sepolia 0x619CcD22eF045De3b63d3D03224BFF5491cd5D11

Developing

After cloning the repo, run the tests using Forge, from Foundry

forge test

You can run the echinda tests with this make command

make echidna-test