Upgrade Solidity smart contracts using OpenZeppelin proxy patterns. Use when users need to: (1) make contracts upgradeable with UUPS, Transparent, or Beacon proxies, (2) write initializers instead of constructors, (3) use the Hardhat or Foundry upgrades plugins, (4) understand storage layout rules and ERC-7201 namespaced storage, (5) validate upgrade safety, (6) manage proxy deployments and upgrades, or (7) understand upgrade restrictions between OpenZeppelin Contracts major versions.
| Pattern | Upgrade logic lives in | Best for |
|---|---|---|
UUPS (UUPSUpgradeable) | Implementation contract (override _authorizeUpgrade) | Most projects — lighter proxy, lower deploy gas |
| Transparent | Separate ProxyAdmin contract | When admin/user call separation is critical — admin cannot accidentally call implementation functions |
| Beacon | Shared beacon contract | Multiple proxies sharing one implementation — upgrading the beacon atomically upgrades all proxies |
All three use EIP-1967 storage slots for the implementation address, admin, and beacon.
Transparent proxy — v5 constructor change: In v5,
TransparentUpgradeableProxyautomatically deploys its ownProxyAdmincontract and stores the admin address in an immutable variable (set at construction time, never changeable). The second constructor parameter is the owner address for that auto-deployedProxyAdmin— do not pass an existingProxyAdmincontract address here. Transfer of upgrade capability is handled exclusively throughProxyAdminownership. This differs from v4, whereProxyAdminwas deployed separately and its address was passed to the proxy constructor.
Upgrading a proxy's implementation from one using OpenZeppelin Contracts v4 to one using v5 is not supported.
v4 uses sequential storage (slots in declaration order); v5 uses namespaced storage (ERC-7201, structs at deterministic slots). A v5 implementation cannot safely read state written by a v4 implementation. Manual data migration is theoretically possible but often infeasible — mapping entries cannot be enumerated, so values written under arbitrary keys cannot be relocated.
Recommended approach: Deploy new proxies with v5 implementations and migrate users to the new address — do not upgrade proxies that currently point to v4 implementations.
Updating your codebase to v5 is encouraged. The restriction above applies only to already-deployed proxies. New deployments built on v5, and upgrades within the same major version, are fully supported.
Proxy contracts delegatecall into the implementation. Constructors run only when the implementation itself is deployed, not when a proxy is created. Replace constructors with initializer functions:
import {Initializable} from "@openzeppelin/contracts/proxy/utils/Initializable.sol";
contract MyToken is Initializable, ERC20Upgradeable, OwnableUpgradeable {
/// @custom:oz-upgrades-unsafe-allow constructor
constructor() {
_disableInitializers(); // lock the implementation
}
function initialize(address initialOwner) public initializer {
__ERC20_init("MyToken", "MTK");
__Ownable_init(initialOwner);
}
}
Key rules:
initialize uses the initializer modifier__X_init) use onlyInitializing internally — call them explicitly, the compiler does not auto-linearize initializers like constructors_disableInitializers() in a constructor to prevent attackers from initializing the implementation directlyuint256 x = 42) — these compile into the constructor and won't execute for the proxy. constant is safe (inlined at compile time). immutable values are stored in bytecode and shared across all proxies — the plugins flag them as unsafe by default; use /// @custom:oz-upgrades-unsafe-allow state-variable-immutable to opt in when a shared value is intendedImport from @openzeppelin/contracts-upgradeable for base contracts (e.g., ERC20Upgradeable, OwnableUpgradeable). Import interfaces and libraries from @openzeppelin/contracts. In v5.5+, Initializable and UUPSUpgradeable should also be imported directly from @openzeppelin/contracts — aliases in the upgradeable package will be removed in the next major release.
When upgrading, the new implementation must be storage-compatible with the old one:
The modern approach — all @openzeppelin/contracts-upgradeable contracts (v5+) use this. State variables are grouped into a struct at a deterministic storage slot, isolating each contract's storage and eliminating the need for storage gaps. Recommended for all contracts that may be imported as base contracts.
/// @custom:storage-location erc7201:example.main
struct MainStorage {
uint256 value;
mapping(address => uint256) balances;
}
// keccak256(abi.encode(uint256(keccak256("example.main")) - 1)) & ~bytes32(uint256(0xff))
bytes32 private constant MAIN_STORAGE_LOCATION = 0x...;
function _getMainStorage() private pure returns (MainStorage storage $) {
assembly { $.slot := MAIN_STORAGE_LOCATION }
}
Using a variable from namespaced storage:
function _getBalance(address account) internal view returns (uint256) {
MainStorage storage $ = _getMainStorage();
return $.balances[account];
}
Benefits over legacy storage gaps: safe to add variables to base contracts, inheritance order changes don't break layout, each contract's storage is fully isolated.
When upgrading, never remove a namespace by dropping it from the inheritance chain. The plugin flags deleted namespaces as an error — the state stored in that namespace becomes orphaned: the data remains on-chain but the new implementation has no way to read or write it. If a namespace is no longer actively used, keep the old contract in the inheritance chain. An unused namespace adds no runtime cost and causes no storage conflict. There is no targeted flag to suppress this error; the only bypass is unsafeSkipStorageCheck, which disables all storage layout compatibility checks and is a dangerous last resort.
When generating namespaced storage code, always compute the actual STORAGE_LOCATION constant. Use the Bash tool to run the command below with the actual namespace id and embed the computed value directly in the generated code. Never leave placeholder values like 0x....
The formula is: keccak256(abi.encode(uint256(keccak256(id)) - 1)) & ~bytes32(uint256(0xff)) where id is the namespace string (e.g., "example.main").
Node.js with ethers:
node -e "const{keccak256,toUtf8Bytes,zeroPadValue,toBeHex}=require('ethers');const id=process.argv[1];const h=BigInt(keccak256(toUtf8Bytes(id)))-1n;console.log(toBeHex(BigInt(keccak256(zeroPadValue(toBeHex(h),32)))&~0xffn,32))" "example.main"
Replace "example.main" with the actual namespace id, run the command, and use the output as the constant value.
selfdestruct — on pre-Dencun chains, destroys the implementation and bricks all proxies. Post-Dencun (EIP-6780), selfdestruct only destroys code if called in the same transaction as creation, but the plugins still flag it as unsafedelegatecall to untrusted contracts — a malicious target could selfdestruct or corrupt storageAdditionally, avoid using new to create contracts inside an upgradeable contract — the created contract won't be upgradeable. Inject pre-deployed addresses instead.
Install the plugin:
npm install --save-dev @openzeppelin/hardhat-upgrades
npm install --save-dev @nomicfoundation/hardhat-ethers ethers # peer dependencies
Register in hardhat.config:
require('@openzeppelin/hardhat-upgrades'); // JS
import '@openzeppelin/hardhat-upgrades'; // TS
Workflow concept — the plugin provides functions on the upgrades object (deployProxy, upgradeProxy, deployBeacon, upgradeBeacon, deployBeaconProxy). Each function:
The plugin tracks deployed implementations in .openzeppelin/ per-network files. Commit non-development network files to version control.
Use prepareUpgrade to validate and deploy a new implementation without executing the upgrade — useful when a multisig or governance contract holds upgrade rights.
Read the installed plugin's README or source for exact API signatures and options, as these evolve across versions.
Install dependencies:
forge install foundry-rs/forge-std
forge install OpenZeppelin/openzeppelin-foundry-upgrades
forge install OpenZeppelin/openzeppelin-contracts-upgradeable
Configure foundry.toml:
[profile.default]
ffi = true
ast = true
build_info = true
extra_output = ["storageLayout"]
Node.js is required — the library shells out to the OpenZeppelin Upgrades CLI for validation.
Import and use in scripts/tests:
import {Upgrades} from "openzeppelin-foundry-upgrades/Upgrades.sol";
// Deploy
address proxy = Upgrades.deployUUPSProxy(
"MyContract.sol",
abi.encodeCall(MyContract.initialize, (args))
);
// IMPORTANT: Before upgrading, annotate MyContractV2 with: /// @custom:oz-upgrades-from MyContract
// Upgrade and call a function
Upgrades.upgradeProxy(proxy, "MyContractV2.sol", abi.encodeCall(MyContractV2.foo, ("arguments for foo")));
// Upgrade without calling a function
Upgrades.upgradeProxy(proxy, "MyContractV2.sol", "");
Key differences from Hardhat:
@custom:oz-upgrades-from or pass referenceContract in the Options structUnsafeUpgrades variant skips all validation (takes addresses instead of names) — never use in production scriptsforge clean or use --force before running scriptsRead the installed library's
Upgrades.solfor the full API andOptionsstruct.
When the plugins flag a warning or error, work through this hierarchy:
UnsafeUpgrades (Foundry) or blanket unsafeAllow entries skip all validation for the affected scope. If you use them, comment why, and verify manually — the plugin is no longer protecting you.initialize uses initializer modifier. Implementation constructor calls _disableInitializers().__X_init is called exactly once in initialize.selfdestruct or delegatecall to untrusted targets._authorizeUpgrade: Overridden with proper access control (e.g., onlyOwner). Forgetting this makes the proxy non-upgradeable or upgradeable by anyone.reinitializer(2) modifier (not initializer, which can only run once).