ERC-7540: Asynchronous ERC-4626 Tokenized Vaults

Extension of ERC-4626 with asynchronous deposit and redemption support

Abstract

The following standard extends ERC-4626 by adding support for asynchronous deposit and redemption flows. The async flows are called Requests.

New methods are added to asynchronously Request a deposit or redemption, and view the status of the Request. The existing deposit, mint, withdraw, and redeem ERC-4626 methods are used for executing Claimable Requests.

Implementations can choose whether to add asynchronous flows for deposits, redemptions, or both.

Motivation

The ERC-4626 Tokenized Vaults standard has helped to make yield-bearing tokens more composable across decentralized finance. The standard is optimized for atomic deposits and redemptions up to a limit. If the limit is reached, no new deposits or redemptions can be submitted.

This limitation does not work well for any smart contract system with asynchronous actions or delays as a prerequisite for interfacing with the Vault (e.g. real-world asset protocols, undercollateralized lending protocols, cross-chain lending protocols, liquid staking tokens, or insurance safety modules).

This standard expands the utility of ERC-4626 Vaults for asynchronous use cases. The existing Vault interface (deposit/withdraw/mint/redeem) is fully utilized to claim asynchronous Requests.

Specification

Definitions:

The existing definitions from ERC-4626 apply. In addition, this spec defines:

• Request: a request to enter (requestDeposit) or exit (requestRedeem) the Vault
• Pending: the state where a Request has been made but is not yet Claimable
• Claimable: the state where a Request is processed by the Vault enabling the user to claim corresponding shares (for async deposit) or assets (for async redeem)
• Claimed: the state where a Request is finalized by the user and the user receives the output token (e.g. shares for a deposit Request)
• Claim function: the corresponding Vault method to bring a Request to Claimed state (e.g. deposit or mint claims shares from requestDeposit). Lowercase claim always describes the verb action of calling a Claim function.
• asynchronous deposit Vault: a Vault that implements asynchronous Requests for deposit flows
• asynchronous redemption Vault: a Vault that implements asynchronous Requests for redemption flows
• fully asynchronous Vault: a Vault that implements asynchronous Requests for both deposit and redemption flows
• controller: owner of the Request, who can manage any actions related to the Request including claiming the assets or shares
• operator: an account that can manage Requests on behalf of another account.

Request Flows

ERC-7540 Vaults MUST implement one or both of asynchronous deposit and redemption Request flows. If either flow is not implemented in a Request pattern, it MUST use the ERC-4626 standard synchronous interaction pattern.

All ERC-7540 asynchronous tokenized Vaults MUST implement ERC-4626 with overrides for certain behavior described below.

Asynchronous deposit Vaults MUST override the ERC-4626 specification as follows:

1. The deposit and mint methods do not transfer assets to the Vault, because this already happened on requestDeposit.
2. previewDeposit and previewMint MUST revert for all callers and inputs.

Asynchronous redeem Vaults MUST override the ERC-4626 specification as follows:

1. The redeem and withdraw methods do not transfer shares to the Vault, because this already happened on requestRedeem.
2. The owner field of redeem and withdraw SHOULD be renamed to controller, and the controller MUST be msg.sender unless the controller has approved the msg.sender as an operator.
3. previewRedeem and previewWithdraw MUST revert for all callers and inputs.

Request Lifecycle

After submission, Requests go through Pending, Claimable, and Claimed stages. An example lifecycle for a deposit Request is visualized in the table below.

Note that maxDeposit increases and decreases in sync with claimableDepositRequest.

Requests MUST NOT skip or otherwise short-circuit the Claim state. In other words, to initiate and claim a Request, a user MUST call both request* and the corresponding Claim function separately, even in the same block. Vaults MUST NOT “push” tokens onto the user after a Request, users MUST “pull” the tokens via the Claim function.

For asynchronous Vaults, the exchange rate between shares and assets including fees and yield is up to the Vault implementation. In other words, pending redemption Requests MAY NOT be yield-bearing and MAY NOT have a fixed exchange rate.

Request Ids

The request ID (requestId) of a request is returned by the corresponding requestDeposit and requestRedeem functions.

Multiple requests may have the same requestId, so a given Request is discriminated by both the requestId and the controller.

Requests of the same requestId MUST be fungible with each other (except in the special case requestId == 0 described below). I.e. all Requests with the same requestId MUST transition from Pending to Claimable at the same time and receive the same exchange rate between assets and shares. If a Request with requestId != 0 becomes partially claimable, all requests of the same requestId MUST become claimable at the same pro-rata rate.

There are no assumptions or requirements of requests with different requestId. I.e. they MAY transition to Claimable at different times and exchange rates with no ordering or correlation enforced in any way.

When requestId==0, the Vault MUST use purely the controller to discriminate the request state. The Pending and Claimable state of multiple requests from the same controller would be aggregated. If a Vault returns 0 for the requestId of any request, it MUST return 0 for all requests.

Methods

requestDeposit

Transfers assets from owner into the Vault and submits a Request for asynchronous deposit. This places the Request in Pending state, with a corresponding increase in pendingDepositRequest for the amount assets.

The output requestId is used to partially discriminate the request along with the controller. See Request Ids section for more info.

When the Request is Claimable, claimableDepositRequest will be increased for the controller. deposit or mint can subsequently be called by controller to receive shares. A Request MAY transition straight to Claimable state but MUST NOT skip the Claimable state.

The shares that will be received on deposit or mint MAY NOT be equivalent to the value of convertToShares(assets) at the time of Request, as the price can change between Request and Claim.

MUST support ERC-20 approve / transferFrom on asset as a deposit Request flow.

owner MUST equal msg.sender unless the owner has approved the msg.sender as an operator.

MUST revert if all of assets cannot be requested for deposit/mint (due to deposit limit being reached, slippage, the user not approving enough underlying tokens to the Vault contract, etc).

Note that most implementations will require pre-approval of the Vault with the Vault’s underlying asset token.

MUST emit the DepositRequest event.

- name: requestDeposit
  type: function
  stateMutability: nonpayable

  inputs:
    - name: assets
      type: uint256
    - name: controller
      type: address
    - name: owner
      type: address
  outputs:
    - name: requestId
      type: uint256

pendingDepositRequest

The amount of requested assets in Pending state for the controller with the given requestId to deposit or mint.

MUST NOT include any assets in Claimable state for deposit or mint.

MUST NOT show any variations depending on the caller.

MUST NOT revert unless due to integer overflow caused by an unreasonably large input.

- name: pendingDepositRequest
  type: function
  stateMutability: view

  inputs:
    - name: requestId
      type: uint256
    - name: controller
      type: address

  outputs:
    - name: assets
      type: uint256

claimableDepositRequest

The amount of requested assets in Claimable state for the controller with the given requestId to deposit or mint.

MUST NOT include any assets in Pending state for deposit or mint.

MUST NOT show any variations depending on the caller.

MUST NOT revert unless due to integer overflow caused by an unreasonably large input.

- name: claimableDepositRequest
  type: function
  stateMutability: view

  inputs:
    - name: requestId
      type: uint256
    - name: controller
      type: address

  outputs:
    - name: assets
      type: uint256

requestRedeem

Assumes control of shares from owner and submits a Request for asynchronous redeem. This places the Request in Pending state, with a corresponding increase in pendingRedeemRequest for the amount shares.

The output requestId is used to discriminate the request along with the controller. See Request Ids section for more info.

shares MAY be temporarily locked in the Vault until the Claimable or Claimed state for accounting purposes, or they MAY be burned immediately upon requestRedeem.

In either case, the shares MUST be removed from the custody of owner upon requestRedeem and burned by the time the request is Claimed.

Redeem Request approval of shares for a msg.sender NOT equal to owner may come either from ERC-20 approval over the shares of owner or if the owner has approved the msg.sender as an operator. This MUST be consistent with similar behaviour pointed out in ERC-6909, within “Approvals and Operators” section: “In accordance with the transferFrom method, spenders with operator permission are not subject to allowance restrictions, spenders with infinite approvals SHOULD NOT have their allowance deducted on delegated transfers, but spenders with non-infinite approvals MUST have their balance deducted on delegated transfers.”

When the Request is Claimable, claimableRedeemRequest will be increased for the controller. redeem or withdraw can subsequently be called by controller to receive assets. A Request MAY transition straight to Claimable state but MUST NOT skip the Claimable state.

The assets that will be received on redeem or withdraw MAY NOT be equivalent to the value of convertToAssets(shares) at the time of Request, as the price can change between Pending and Claimed.

MUST revert if all of shares cannot be requested for redeem / withdraw (due to withdrawal limit being reached, slippage, the owner not having enough shares, etc).

MUST emit the RedeemRequest event.

- name: requestRedeem
  type: function
  stateMutability: nonpayable

  inputs:
    - name: shares
      type: uint256
    - name: controller
      type: address
    - name: owner
      type: address
  outputs:
    - name: requestId
    - type: uint256

pendingRedeemRequest

The amount of requested shares in Pending state for the controller with the given requestId to redeem or withdraw.

MUST NOT include any shares in Claimable state for redeem or withdraw.

MUST NOT show any variations depending on the caller.

MUST NOT revert unless due to integer overflow caused by an unreasonably large input.

- name: pendingRedeemRequest
  type: function
  stateMutability: view

  inputs:
    - name: requestId
      type: uint256
    - name: controller
      type: address

  outputs:
    - name: shares
      type: uint256

claimableRedeemRequest

The amount of requested shares in Claimable state for the controller with the given requestId to redeem or withdraw.

MUST NOT include any shares in Pending state for redeem or withdraw.

MUST NOT show any variations depending on the caller.

MUST NOT revert unless due to integer overflow caused by an unreasonably large input.

- name: claimableRedeemRequest
  type: function
  stateMutability: view

  inputs:
    - name: requestId
      type: uint256
    - name: controller
      type: address

  outputs:
    - name: shares
      type: uint256

isOperator

Returns true if the operator is approved as an operator for a controller.

- name: isOperator
  type: function
  stateMutability: view

  inputs:
    - name: controller
      type: address
    - name: operator
      type: address

  outputs:
    - name: status
      type: bool

setOperator

Grants or revokes permissions for operator to manage Requests on behalf of the msg.sender.

MUST set the operator status to the approved value.

MUST log the OperatorSet event.

MUST return True.

- name: setOperator
  type: function
  stateMutability: nonpayable

  inputs:
    - name: operator
      type: address
    - name: approved
      type: bool

  outputs:
    - name: success
      type: bool

deposit and mint overloaded methods

Implementations MUST support an additional overloaded deposit and mint method on the specification from ERC-4626, with an additional controller input of type address:

• deposit(uint256 assets, address receiver, address controller)
• mint(uint256 shares, address receiver, address controller)

Calls MUST revert unless msg.sender is either equal to controller or an operator approved by controller.

The controller field is used to discriminate the Request for which the assets should be claimed in the case where msg.sender is NOT controller.

When the Deposit event is emitted, the first parameter MUST be the controller, and the second parameter MUST be the receiver.

Events

DepositRequest

owner has locked assets in the Vault to Request a deposit with request ID requestId. controller controls this Request. sender is the caller of the requestDeposit which may not be equal to the owner.

MUST be emitted when a deposit Request is submitted using the requestDeposit method.

- name: DepositRequest
  type: event

  inputs:
    - name: controller
      indexed: true
      type: address
    - name: owner
      indexed: true
      type: address
    - name: requestId
      indexed: true
      type: uint256
    - name: sender
      indexed: false
      type: address
    - name: assets
      indexed: false
      type: uint256

RedeemRequest

sender has locked shares, owned by owner, in the Vault to Request a redemption. controller controls this Request, but is not necessarily the owner.

MUST be emitted when a redemption Request is submitted using the requestRedeem method.

- name: RedeemRequest
  type: event

  inputs:
    - name: controller
      indexed: true
      type: address
    - name: owner
      indexed: true
      type: address
    - name: requestId
      indexed: true
      type: uint256
    - name: sender
      indexed: false
      type: address
    - name: shares
      indexed: false
      type: uint256

OperatorSet

The controller has set the approved status to an operator.

MUST be logged when the operator status is set.

MAY be logged when the operator status is set to the same status it was before the current call.

- name: OperatorSet
  type: event

  inputs:
    - name: controller
      indexed: true
      type: address
    - name: operator
      indexed: true
      type: address
    - name: approved
      indexed: false
      type: bool

ERC-165 support

Smart contracts implementing this Vault standard MUST implement the ERC-165 supportsInterface function.

All asynchronous Vaults MUST return the constant value true if either 0xe3bc4e65 (representing the operator methods that all ERC-7540 Vaults implement) or 0x2f0a18c5 (representing the ERC-7575 interface) is passed through the interfaceID argument.

Asynchronous deposit Vaults MUST return the constant value true if 0xce3bbe50 is passed through the interfaceID argument.

Asynchronous redemption Vaults MUST return the constant value true if 0x620ee8e4 is passed through the interfaceID argument.

ERC-7575 support

Smart contracts implementing this Vault standard MUST implement the ERC-7575 standard (in particular the share method).

Rationale

Including Request IDs but not including a Claim by ID method

Requests in an Asynchronous Vault have properties of NFTs or Semi-Fungible tokens due to their asynchronicity. However, trying to pigeonhole all ERC-7540 Vaults into supporting ERC-721 or ERC-1155 for Requests would create too much interface bloat.

Using both an id and address to discriminate Requests allows for any of these use cases to be developed at an external layer without adding too much complexity to the core interface.

Certain Vaults, especially requestId==0 cases, benefit from using the underlying ERC-4626 methods for claiming because there is no discrimination at the requestId level. This standard is written primarily with those use cases in mind. A future standard can optimize for nonzero request ID with support for claiming and transferring requests discriminated also with a requestId.

Symmetry and Non-inclusion of requestWithdraw and requestMint

In ERC-4626, the spec was written to be fully symmetrical with respect to converting assets and shares by including deposit/withdraw and mint/redeem.

Due to the nature of Requests, asynchronous Vaults can only operate with certainty on the quantity that is fully known at the time of the Request (assets for deposit and shares for redeem). Therefore the deposit Request flow cannot work with a mint call, because the amount of assets for the requested shares amount may fluctuate before the fulfillment of the Request. Likewise, the redemption Request flow cannot work with a withdraw call.

Optionality of Flows

Certain use cases are only asynchronous on one side of the deposit or redeem Request flow. A good example of an asynchronous redemption Vault is a liquid staking token. The unstaking period necessitates support for asynchronous withdrawals, however, deposits can be fully synchronous.

Non-inclusion of a Request Cancelation Flow

In many cases, canceling a Request may not be straightforward or even technically feasible. The state transition of cancelations could be synchronous or asynchronous, and the way to claim a cancelation interfaces with the remaining Vault functionality in complex ways.

A separate EIP should be developed to standardize the behavior of cancelling a pending Request. Defining the cancel flow is still important for certain classes of use cases for which the fulfillment of a Request can take a considerable amount of time.

Request Implementation Flexibility

The standard is flexible enough to support a wide range of interaction patterns for Request flows. Pending Requests can be handled via internal accounting, globally or on per-user levels, use ERC-20 or ERC-721, etc.

Likewise yield on redemption Requests can accrue or not, and the exchange rate of any Request may be fixed or variable depending on the implementation.

Not Allowing Short-circuiting for Claims

If claims can short-circuit, this creates ambiguity for integrators and complicates the interface with overloaded behavior on Request functions.

An example of a short-circuiting Request flow could be as follows: user triggers a Request which enters Pending state. When the Vault fulfills the Request, the corresponding assets/shares are pushed straight to the user. This requires only 1 step on the user’s behalf.

This approach has a few issues:

• cost/lack of scalability: as the number of vault users grows it can become intractably expensive to offload the Claim costs to the Vault operator
• hinders integration potential: Vault integrators would need to handle both the 2-step and 1-step cases, with the 1-step pushing arbitrary tokens in from an unknown Request at an unknown time. This pushes complexity out onto integrators and reduces the standard’s utility.

The 2-step approach used in the standard may be abstracted into a 1-step approach from the user perspective through the use of routers, relayers, message signing, or account abstraction.

In the case where a Request may become Claimable immediately in the same block, there can be router contracts that atomically check for Claimable amounts immediately upon Request. Frontends can dynamically route Requests in this way depending on the state and implementation of the Vault to handle this edge case.

No Outputs for Request Functions

requestDeposit and requestRedeem may not have a known exchange rate that will happen when the Request becomes Claimed. Returning the corresponding assets or shares could not work in this case.

The Requests could also output a timestamp representing the minimum amount of time expected for the Request to become Claimable, however, not all Vaults will be able to return a reliable timestamp.

No Event for Claimable State

The state transition of a Request from Pending to Claimable happens at the Vault implementation level and is not specified in the standard. Requests may be batched into the Claimable state, or the state may transition automatically after a timestamp has passed. It is impractical to require an event to emit after a Request becomes Claimable at the user or batch level.

Reversion of Preview Functions in Async Request Flows

The preview functions do not take an address parameter, therefore the only way to discriminate discrepancies in the exchange rate is via the msg.sender. However, this could lead to integration/implementation complexities where support contracts cannot determine the output of a claim on behalf of a controller.

In addition, there is no on-chain benefit to previewing the Claim step as the only valid state transition is to Claim anyway. If the output of a Claim is undesirable for any reason, the calling contract can revert on the output of that function call.

It reduces code and implementation complexity at little to no cost to simply mandate reversion for the preview functions of an async flow.

Mandated Support for ERC-165

Implementing support for ERC-165 is mandated because of the optionality of flows. Integrations can use the supportsInterface method to check whether a vault is fully asynchronous, partially asynchronous, or fully synchronous (for which it is just following the ERC-4626), and use a single contract to support all cases.

Not Allowing Pending Claims to be Fungible

The async pending claims represent a sort of semi-fungible intermediate share class. Vaults can elect to wrap these claims in any token standard they like, for example, ERC-20, ERC-1155, or ERC-721 depending on the use case. This is intentionally left out of the spec to provide flexibility to implementers.

Backwards Compatibility

The interface is fully backward compatible with ERC-4626. The specification of the deposit, mint, redeem, and withdraw methods is different as described in Specification.

Reference Implementation

    // This code snippet is incomplete pseudocode used for example only and is no way intended to be used in production or guaranteed to be secure

    mapping(address => uint256) public pendingDepositRequest;
    
    mapping(address => uint256) public claimableDepositRequest;

    mapping(address controller => mapping(address operator => bool)) public isOperator;

    function requestDeposit(uint256 assets, address controller, address owner) external returns (uint256 requestId) {
        require(assets != 0);
        require(owner == msg.sender || isOperator[owner][msg.sender]);

        requestId = 0; // no requestId associated with this request

        asset.safeTransferFrom(owner, address(this), assets); // asset here is the Vault underlying asset

        pendingDepositRequest[controller] += assets;

        emit DepositRequest(controller, owner, requestId, msg.sender, assets);
        return requestId;
    }

    /**
     * Include some arbitrary transition logic here from Pending to Claimable
     */

    function deposit(uint256 assets, address receiver, address controller) external returns (uint256 shares) {
        require(assets != 0);
        require(controller == msg.sender || isOperator[controller][msg.sender]);

        claimableDepositRequest[controller] -= assets; // underflow would revert if not enough claimable assets

        shares = convertToShares(assets); // this naive example uses the instantaneous exchange rate. It may be more common to use the rate locked in upon Claimable stage.

        balanceOf[receiver] += shares;

        emit Deposit(controller, receiver, assets, shares);
    }

    function setOperator(address operator, bool approved) public returns (bool) {
        isOperator[msg.sender][operator] = approved;
        emit OperatorSet(msg.sender, operator, approved);
        return true;
    }

Security Considerations

In general, asynchronicity concerns make state transitions in the Vault much more complex and vulnerable to security risks. Access control on Vault operations, clear documentation of state transitions, and invariant checks should all be performed to mitigate these risks. For example:

• The view methods for viewing Pending and Claimable request states (e.g. pendingDepositRequest) are estimates useful for display purposes but can be outdated. The inability to know the final exchange rate on any Request requires users to trust the implementation of the asynchronous Vault in the computation of the exchange rate and fulfillment of their Request.
• Shares or assets locked for Requests can be stuck in the Pending state. Vaults may elect to allow for the fungibility of pending claims or implement some cancellation functionality to protect users.

Operators

An operator has the ability to transfer the asset of the vault from the approver to any address, and simultaneously grants control over the share of the vault.

Any user approving an operator must trust that operator with both the asset and share of the Vault.

Copyright

Copyright and related rights waived via CC0.

Citation

Please cite this document as:

Jeroen Offerijns (@hieronx), Alina Sinelnikova (@ilinzweilin), Vikram Arun (@vikramarun), Joey Santoro (@joeysantoro), Farhaan Ali (@0xfarhaan), João Martins (@0xTimepunk), "ERC-7540: Asynchronous ERC-4626 Tokenized Vaults," Ethereum Improvement Proposals, no. 7540, October 2023. [Online serial]. Available: https://eips.ethereum.org/EIPS/eip-7540.