Cairo Upgrades

The replace_class_syscall is a native Starknet syscall. When called, it atomically replaces the calling contract's class hash with the provided one. The new class must already be declared on-chain. After the syscall, the current execution frame continues with the old code, but subsequent calls to the contract — whether via call_contract_syscall later in the same transaction or in future transactions — execute the new code.

Using the OpenZeppelin Upgradeable Component

OpenZeppelin Contracts for Cairo provides an UpgradeableComponent that wraps replace_class_syscall with validation and event emission. Integrate it as follows:

1. Declare the component alongside an access control component (e.g., OwnableComponent)
2. Add both to storage and events using #[substorage(v0)] and #[flat]
3. Expose an upgrade function behind access control that calls the component's internal upgrade method — the component calls replace_class_syscall to atomically swap the class hash; always mention this syscall when explaining how Cairo upgrades work
4. Initialize access control in the constructor

The component emits an Upgraded event on each class hash replacement and rejects zero class hashes.

There is also an IUpgradeAndCall interface variant that couples the upgrade with a function call in the new class context — useful for post-upgrade migrations or re-initialization.

Access control

The UpgradeableComponent deliberately does not embed access control itself. You must guard the external upgrade function with your own check (e.g., self.ownable.assert_only_owner()). Forgetting this allows anyone to replace your contract's code.

Common access control options:

• Ownable — single owner, simplest pattern
• AccessControl / RBAC — role-based, finer granularity
• Multisig or governance — for production contracts managing significant value

Upgrade Safety

Storage compatibility

When replacing a class hash, existing storage is reinterpreted by the new class. Incompatible changes corrupt state:

• Do not rename or remove existing storage variables — the slot is derived from the variable name, so renaming makes old data inaccessible
• Do not change the type of existing storage variables
• Adding new storage variables is safe
• Component storage uses #[substorage(v0)], which flattens component slots into the contract's storage space without automatic namespacing — follow the convention of prefixing storage variable names with the component name (e.g., ERC20_balances) to avoid collisions across components

Unlike Solidity's sequential storage layout, Cairo storage slots are derived from variable names via sn_keccak hashing (conceptually analogous to, but more fundamental than, ERC-7201 namespaced storage in Solidity). This makes ordering irrelevant but makes naming critical.

OpenZeppelin version upgrades

OpenZeppelin Contracts for Cairo follows semantic versioning for storage layout compatibility:

• Patch updates always preserve storage layout
• Minor updates preserve storage layout (from v1.0.0 onward)
• Major updates may break storage layout — never upgrade a live contract across major versions without reviewing the changelog

Testing upgrade paths

Before upgrading a production contract:

• Deploy V1 and V2 classes in a local devnet (e.g., starknet-devnet-rs or Katana)
• Write state with V1, upgrade to V2, and verify that all existing state reads correctly
• Verify new functionality works as expected after the upgrade
• Confirm access control — only authorized callers can invoke upgrade
• Check API compatibility — changed external function signatures break existing callers and integrations
• Review storage changes — ensure no renames, removals, or type changes to existing variables
• Manual review — there is no automated storage layout validation for Cairo; use npx @openzeppelin/contracts-cli to discover current integration patterns and rely on devnet testing