UUPS Proxy

The Universal Upgradeable Proxy Standard (UUPS) is a minimal and gas-efficient pattern for upgradeable contracts. Defined in the ERC-1822 specification, UUPS delegates upgrade logic to the implementation contract itself — reducing proxy complexity and deployment costs.

The OpenZeppelin Stylus Contracts provide a full implementation of the UUPS pattern via UUPSUpgradeable and Erc1967Proxy.

Overview

UUPS uses the ERC-1967 proxy architecture to separate upgrade logic from proxy behavior. Instead of maintaining upgradeability in the proxy, all upgrade control is implemented within the logic contract.

Key components:

Proxy Contract (Erc1967Proxy) — delegates calls via delegate_call.
Implementation Contract — contains application logic and upgrade control.
Upgrade Functions — reside in the implementation, not the proxy.

In Solidity, upgrade safety often relies on an immutable self-address and context checks. Stylus, however, uses a small adaptation that is covered below.

Context Detection in Stylus

Stylus does not currently support the immutable keyword. Instead of storing __self = address(this), the implementation uses a dedicated boolean flag in a unique storage slot to distinguish direct vs delegated (proxy) execution contexts.

// boolean flag stored in a unique slot
logic_flag: bool;

The implementation’s constructor sets logic_flag = true in the implementation’s own storage. When code runs via a proxy (delegatecall), the proxy’s storage does not contain this flag, so reads as false. This enables only_proxy() to check for delegated execution without relying on an address sentinel.

Initialization

Stylus requires explicit initialization for both the implementation and the proxy:

Constructor – called exactly once on deployment of the logic (implementation) contract. This sets the implementation-only logic_flag used for context checks.
Version setup (set_version) – called via the proxy (delegatecall) to write the logic’s VERSION_NUMBER into the proxy’s storage. This aligns the proxy’s version with the logic and enables upgrade paths guarded by only_proxy().

Call set_version() is triggered automatically via upgrade_to_and_call(). Devs are only required to manually call set_version() when deploying the proxy.

Trade-offs:

Storage Cost: Requires one additional storage slot.
Runtime Safety: Maintains the same guarantees as the Solidity version.
Gas Impact: One-time cost during initialization; negligible runtime overhead.

Why UUPS?

Gas Efficient — Upgrades are handled within the logic contract.
Secure — Authorization and validation are managed in one place.
Standardized — Conforms to ERC-1822 and ERC-1967.
Flexible — Upgrade logic can include custom access control and validation.
Safe by Design — Uses dedicated ERC-1967 slots to prevent storage collisions.

How It Works

Deploy Erc1967Proxy with an initial implementation and encoded set_version data.
Proxy delegates all calls to the implementation contract via delegate_call.
Implementation exposes upgrade_to_and_call, guarded by access control (e.g. Ownable).
Upgrades validate the new implementation using proxiable_uuid().
Stylus uses a two-step pattern: constructor on logic deployment, then set_version during proxy setup.

Implementing a UUPS Contract

Minimal example with Ownable, UUPSUpgradeable, and Erc20 logic:

#[entrypoint]
#[storage]
struct MyUUPSContract {
    erc20: Erc20,
    ownable: Ownable,
    uups: UUPSUpgradeable,
}

#[public]
#[implements(IErc20<Error = erc20::Error>, IUUPSUpgradeable, IErc1822Proxiable, IOwnable)]
impl MyUUPSContract {
    // Accepting owner here only to enable invoking functions directly on the
    // UUPS
    #[constructor]
    fn constructor(&mut self, owner: Address) -> Result<(), Error> {
        self.uups.constructor();
        self.ownable.constructor(owner)?;
        Ok(())
    }

    fn mint(&mut self, to: Address, value: U256) -> Result<(), erc20::Error> {
        self.erc20._mint(to, value)
    }

    /// Initializes the contract.
    fn initialize(&mut self, owner: Address) -> Result<(), Error> {
        self.uups.set_version()?;
        self.ownable.constructor(owner)?;
        Ok(())
    }

    fn set_version(&mut self) -> Result<(), Error> {
        Ok(self.uups.set_version()?)
    }

    fn get_version(&self) -> U32 {
        self.uups.get_version()
    }
}

#[public]
impl IUUPSUpgradeable for MyUUPSContract {
    #[selector(name = "UPGRADE_INTERFACE_VERSION")]
    fn upgrade_interface_version(&self) -> String {
        self.uups.upgrade_interface_version()
    }

    #[payable]
    fn upgrade_to_and_call(
        &mut self,
        new_implementation: Address,
        data: Bytes,
    ) -> Result<(), Vec<u8>> {
        // Make sure to provide upgrade authorization in your implementation
        // contract.
        self.ownable.only_owner()?;
        self.uups.upgrade_to_and_call(new_implementation, data)?;
        Ok(())
    }
}

#[public]
impl IErc1822Proxiable for MyUUPSContract {
    #[selector(name = "proxiableUUID")]
    fn proxiable_uuid(&self) -> Result<B256, Vec<u8>> {
        self.uups.proxiable_uuid()
    }
}

Implementing the Proxy

A simple UUPS-compatible proxy using ERC-1967:

#[entrypoint]
#[storage]
struct MyUUPSProxy {
    proxy: Erc1967Proxy,
}

#[public]
impl MyUUPSProxy {
    #[constructor]
    fn constructor(&mut self, implementation: Address, data: Bytes) -> Result<(), erc1967::utils::Error> {
        self.proxy.constructor(implementation, &data)
    }

    fn implementation(&self) -> Result<Address, Vec<u8>> {
        self.proxy.implementation()
    }

    #[fallback]
    fn fallback(&mut self, calldata: &[u8]) -> ArbResult {
        unsafe { self.proxy.do_fallback(calldata) }
    }
}

unsafe impl IProxy for MyUUPSProxy {
    fn implementation(&self) -> Result<Address, Vec<u8>> {
        self.proxy.implementation()
    }
}

Upgrade Safety

1. Access Control

Upgrades must be restricted to trusted accounts, e.g. via only_owner:

self.ownable.only_owner()?;

2. Proxy Context Enforcement

Ensures upgrade calls come from a delegate call:

self.uups.only_proxy()?; // Reverts if not called via proxy

Explanation: only_proxy() checks that execution is delegated (not direct), the caller is an ERC-1967 proxy (implementation slot is non-zero), and the proxy-stored version equals the logic’s VERSION_NUMBER.

3. Proxiable UUID Validation

Guarantees compatibility with UUPS:

self.uups.proxiable_uuid()? == IMPLEMENTATION_SLOT;

Initialization

The UUPS proxy supports initialization data that is delegated to the implementation on deployment. This is typically used to invoke set_version first, and optionally invoke your own initialization routines (e.g., ownership or token supply setup) if needed.

let data = IMyContract::setVersionCall {}.abi_encode();
MyUUPSProxy::deploy(implementation_addr, data.into());

⚠️ Initialization Must Be Explicit (Your Contract State)

If your contract needs additional initialization beyond set_version() (e.g., ownership, token supply), expose a properly designed initialization function and protect it appropriately (e.g., single-use guard or access control). Failing to do so can lead to: * Orphaned contracts with no owner. * Uninitialized token supply or core state. * Denial of future upgrades if your own guards are misused.

/// Optional contract initialization (example).
fn init_contract_state(&mut self, owner: Address) -> Result<(), Vec<u8>> {
    self.ownable.constructor(owner)?;

    /// other initialization logic.

    self.uups.set_version()?;

    Ok(())
}

If you expose additional initialization functions, ensure they are protected from re-execution after the proxy is live.

Initializing the Proxy

Initialization data is typically a call to the implementation’s set_version function:

let data = IMyContract::setVersionCall {}.abi_encode();
MyUUPSProxy::deploy(implementation_addr, data.into());

This setup call is run via delegate_call during proxy deployment.

Security Best Practices

Restrict upgrade access (e.g. only_owner).
Validate all upgrade targets.
Test upgrades across versions.
Monitor upgrade events (Upgraded).
Use empty data unless initialization is needed.
Ensure new implementations return the correct proxiable UUID.
Enforce proxy context checks — only_proxy() ensures upgrades cannot be called directly on the implementation.

Common Pitfalls

Forgetting access control.
Direct calls to upgrade logic (not via proxy).
Missing proxiable UUID validation.
Changing storage layout without planning.
Sending ETH to constructor without data (will revert).
The VERSION_NUMBER is not increased to the higher value.

Use Cases

Upgradeable tokens standards (e.g. ERC-20, ERC-721, ERC-1155).
Modular DeFi protocols.
DAO frameworks.
NFT marketplaces.
Access control registries.
Cross-chain bridges.

UUPS Proxy

Overview

Context Detection in Stylus

Initialization

Why UUPS?

How It Works

Implementing a UUPS Contract

Implementing the Proxy

Upgrade Safety

1. Access Control

2. Proxy Context Enforcement

3. Proxiable UUID Validation

Initialization

⚠️ Initialization Must Be Explicit (Your Contract State)

Initializing the Proxy

Security Best Practices

Common Pitfalls

Use Cases

Related