Upgrade Solidity Contracts

SKILL.md

---
name: upgrade-solidity-contracts
description: "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."
license: AGPL-3.0-only
metadata:
  author: OpenZeppelin
---

# Solidity Upgrades

## Contents

- [Proxy Patterns Overview](#proxy-patterns-overview)
- [Upgrade Restrictions Between Major Versions (v4 → v5)](#upgrade-restrictions-between-major-versions-v4--v5)
- [Writing Upgradeable Contracts](#writing-upgradeable-contracts)
- [Hardhat Upgrades Workflow](#hardhat-upgrades-workflow)
- [Foundry Upgrades Workflow](#foundry-upgrades-workflow)
- [Handling Upgrade Validation Issues](#handling-upgrade-validation-issues)
- [Upgrade Safety Checklist](#upgrade-safety-checklist)

## Proxy Patterns Overview

| 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, `TransparentUpgradeableProxy` automatically deploys its own `ProxyAdmin` contract 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-deployed `ProxyAdmin` — do **not** pass an existing `ProxyAdmin` contract address here. Transfer of upgrade capability is handled exclusively through `ProxyAdmin` ownership. This differs from v4, where `ProxyAdmin` was deployed separately and its address was passed to the proxy constructor.

## Upgrade Restrictions Between Major Versions (v4 → v5)

**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.

## Writing Upgradeable Contracts

### Use initializers instead of constructors

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:

```solidity
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:
- Top-level `initialize` uses the `initializer` modifier
- Parent init functions (`__X_init`) use `onlyInitializing` internally — call them explicitly, the compiler does not auto-linearize initializers like constructors
- Always call `_disableInitializers()` in a constructor to prevent attackers from initializing the implementation directly
- Do not set initial values in field declarations (e.g., `uint256 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 intended

### Use the upgradeable package

Import 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.

Complete import block example for v5.5+:

```solidity
// From @openzeppelin/contracts (non-upgradeable package)
import {Initializable} from "@openzeppelin/contracts/proxy/utils/Initializable.sol";
import {UUPSUpgradeable} from "@openzeppelin/contracts/proxy/utils/UUPSUpgradeable.sol";

// From @openzeppelin/contracts-upgradeable (upgradeable base contracts)
import {ERC20Upgradeable} from "@openzeppelin/contracts-upgradeable/token/ERC20/ERC20Upgradeable.sol";
import {OwnableUpgradeable} from "@openzeppelin/contracts-upgradeable/access/OwnableUpgradeable.sol";
```

### Storage layout rules

When upgrading, the new implementation must be storage-compatible with the old one:

- **Never** reorder, remove, or change the type of existing state variables
- **Never** insert new variables before existing ones
- **Only** append new variables at the end
- **Never** change the inheritance order of base contracts

### Namespaced storage (ERC-7201)

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.

```solidity
/// @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:
```solidity
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.

#### Computing ERC-7201 storage locations

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**:

```bash
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.

### Unsafe operations

- **No `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 unsafe
- **No `delegatecall`** to untrusted contracts — a malicious target could `selfdestruct` or corrupt storage

Additionally, avoid using `new` to create contracts inside an upgradeable contract — the created contract won't be upgradeable. Inject pre-deployed addresses instead.

## Hardhat Upgrades Workflow

Install the plugin:

```bash
npm install --save-dev @openzeppelin/hardhat-upgrades
npm install --save-dev @nomicfoundation/hardhat-ethers ethers  # peer dependencies
```

Register in `hardhat.config`:

```javascript
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:
1. Validates the implementation for upgrade safety (storage layout, initializer patterns, unsafe opcodes)
2. Deploys the implementation (reuses if already deployed)
3. Deploys or updates the proxy/beacon
4. Calls the initializer (on deploy)

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.

## Foundry Upgrades Workflow

Install dependencies:

```bash
forge install foundry-rs/forge-std
forge install OpenZeppelin/openzeppelin-foundry-upgrades
forge install OpenZeppelin/openzeppelin-contracts-upgradeable
```

Configure `foundry.toml`:

```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:

```solidity
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:
- Contracts are referenced by name string, not factory object
- No automatic implementation tracking — annotate new versions with `@custom:oz-upgrades-from` or pass `referenceContract` in the `Options` struct
- `UnsafeUpgrades` variant skips all validation (takes addresses instead of names) — never use in production scripts
- Run `forge clean` or use `--force` before running scripts

> Read the installed library's `Upgrades.sol` for the full API and `Options` struct.

## Handling Upgrade Validation Issues

When the plugins flag a warning or error, work through this hierarchy:

1. **Fix the root cause.** Determine whether the code can be restructured to eliminate the concern entirely — remove the problematic pattern or refactor storage. This is always the right first step.
2. **Use in-code annotations if the situation is genuinely safe.** If restructuring isn't appropriate and you've determined the flagged pattern is actually safe, the plugins support annotations that let you document that judgment directly at the source. Check the installed plugin's docs for what's available. These annotations create a clear audit trail of intentional exceptions — use them only after evaluating the safety, not as a shortcut.
3. **Use a narrow flag if an annotation won't work.** Some cases (e.g., a third-party base contract you can't modify) can't be addressed in source. Use the most targeted flag available, scoped to the specific construct.
4. **Broad bypass as a last resort, with full awareness of the risk.** Options like `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.

## Upgrade Safety Checklist

- [ ] **Storage compatibility**: No reordering, removal, or type changes of existing variables. Only append new variables (or add fields to namespaced structs).
- [ ] **Initializer protection**: Top-level `initialize` uses `initializer` modifier. Implementation constructor calls `_disableInitializers()`.
- [ ] **Parent initializers called**: Every inherited upgradeable contract's `__X_init` is called exactly once in `initialize`.
- [ ] **No unsafe opcodes**: No `selfdestruct` or `delegatecall` to untrusted targets.
- [ ] **Function selector clashes**: Proxy admin functions and implementation functions must not share selectors. UUPS and Transparent patterns handle this by design; custom proxies need manual review.
- [ ] **UUPS `_authorizeUpgrade`**: Overridden with proper access control (e.g., `onlyOwner`). Forgetting this makes the proxy non-upgradeable or upgradeable by anyone.
- [ ] **Test the upgrade path**: Deploy V1, upgrade to V2, verify state is preserved and new logic works. Both Hardhat and Foundry plugins can validate upgrades in test suites.
- [ ] **Reinitializer for V2+**: If V2 needs new initialization logic, use `reinitializer(2)` modifier (not `initializer`, which can only run once). The version number must be **strictly incremented** for each upgrade that requires re-initialization: V2 uses `reinitializer(2)`, V3 uses `reinitializer(3)`, and so on. **Reusing the same version number causes silent skipping** — the function executes without error but the body does not run, leaving new state variables at zero/default values (e.g., a new access control role is never set). Always verify in tests that reinitializer logic actually executed by checking the state it was supposed to set.
- [ ] **Unique ERC-7201 namespace ids**: No two contracts in the inheritance chain share the same namespace id. Colliding ids map to the same storage slot, causing silent storage corruption.