Develop Secure Smart Contracts with OpenZeppelin

Fundamental Rule: Prefer Library Components Over Custom Code

Before writing ANY logic, search the OpenZeppelin library for an existing component:

1. Exact match exists? Import and use it directly — inherit, implement its trait, compose with it. Done.
2. Close match exists? Import and extend it — override only functions the library marks as overridable (virtual, hooks, configurable parameters).
3. No match exists? Only then write custom logic. Confirm by browsing the library's directory structure first.

NEVER copy or embed library source code into the user's contract. Always import from the dependency so the project receives security updates. Never hand-write what the library already provides:

• Never write a custom paused modifier when Pausable or ERC20Pausable exists
• Never write require(msg.sender == owner) when Ownable exists
• Never implement ERC165 logic when the library's base contracts already handle it

Methodology

The primary workflow is pattern discovery from library source code:

1. Inspect what the user's project already imports
2. Read the dependency source and docs in the project's installed packages
3. Identify what functions, modifiers, hooks, and storage the dependency requires
4. Apply those requirements to the user's contract

See Pattern Discovery and Integration below for the full step-by-step procedure.

CLI Generators as Reference

Use npx @openzeppelin/contracts-cli to generate reference implementations for pattern discovery: generate a baseline to a file, generate with a feature enabled to another file, diff them, and apply the changes to the user's code. The CLI output is the canonical correct integration — use it as the source of truth for what imports, inheritance, storage, and overrides a feature requires.

See CLI Generators for details on the generate-compare-apply workflow.

If no CLI command exists for what's needed, use the generic pattern discovery methodology from Pattern Discovery and Integration. The absence of a CLI command does not mean the library lacks support — it only means there is no generator.

Pattern Discovery and Integration

Procedural guide for discovering and applying OpenZeppelin contract integration patterns by reading dependency source code. Works for any ecosystem and any library version.

Prerequisite: Always follow the library-first decision tree above (prefer library components over custom code, never copy/embed source).

Step 1: Identify Dependencies and Search the Library

1. Search the project for contract files: Glob for **/*.sol, **/*.cairo, **/*.rs, or the relevant extension from the lookup table below.
2. Read import/use statements in existing contracts to identify which OpenZeppelin components are already in use.
3. Locate the installed dependency in the project's dependency tree:
   • Solidity: node_modules/@openzeppelin/contracts/ (Hardhat/npm) or lib/openzeppelin-contracts/ (Foundry/forge)
   • Cairo: resolve from Scarb.toml dependencies — source cached by Scarb
   • Stylus: resolve from Cargo.toml — source in target/ or the cargo registry cache (~/.cargo/registry/src/)
   • Stellar: resolve from Cargo.toml — same cargo cache locations as Stylus
4. Browse the dependency's directory listing to discover available components. Use Glob patterns against the installed source (e.g., node_modules/@openzeppelin/contracts/**/*.sol). Do not assume knowledge of the library's contents — always verify by listing directories.
5. If the dependency is not installed locally, clone or browse the canonical repository (see lookup table below).

Step 2: Read the Dependency Source and Documentation

1. Read the source file of the component relevant to the user's request.
2. Look for documentation within the source: NatSpec comments (///, /** */) in Solidity, doc comments (///) in Rust and Cairo, and README files in the component's directory.
3. Determine the integration strategy using the decision tree from the Critical Principle:
   • If the component satisfies the need directly → import and use as-is.
   • If customization is needed → identify extension points the library provides (virtual functions, hook functions, configurable constructor parameters). Import and extend.
   • Only if no component covers the need → write custom logic.
4. Identify the public API: functions/methods exposed, events emitted, errors defined.
5. Identify integration requirements — this is the critical step:
   • Functions the integrator MUST implement (abstract functions, trait methods, hooks)
   • Modifiers, decorators, or guards that must be applied to the integrator's functions
   • Constructor or initializer parameters that must be passed
   • Storage variables or state that must be declared
   • Inheritance or trait implementations required (always via import, never via copy)
6. Search for example contracts or tests in the same repository that demonstrate correct usage. Look in test/, tests/, examples/, or mocks/ directories.

Step 3: Extract the Minimal Integration Pattern

From Step 2, construct the minimal set of changes needed:

• Imports / use statements to add
• Inheritance / trait implementations to add (always via import from the dependency)
• Storage to declare
• Constructor / initializer changes (new parameters, initialization calls)
• New functions to add (required overrides, hooks, public API)
• Existing functions to modify (add modifiers, call hooks, emit events)

If the contract is upgradeable, any of the above may affect storage compatibility. Consult the relevant upgrade skill before applying.

Do not include anything beyond what the dependency requires. This is the minimal diff between "contract without the feature" and "contract with the feature."

Step 4: Apply Patterns to the User's Contract

1. Read the user's existing contract file.
2. Apply the changes from Step 3 using the Edit tool. Do not replace the entire file — integrate into existing code.
3. Check for conflicts: duplicate access control systems, conflicting function overrides, incompatible inheritance. Resolve before finishing.
4. Do not ask the user to make changes themselves — apply directly.

Repository and Documentation Lookup Table

Directory Structure Conventions

Where to find components within each repository:

Browse these paths first when searching for a component.

Known Version-Specific Considerations

Do not assume override points from prior knowledge — always verify by reading the installed source. Functions that were virtual in an older version may no longer be in the current one, making them non-overridable. The source NatSpec will indicate the correct override point (e.g., NOTE: This function is not virtual, {X} should be overridden instead).

A known example: the Solidity ERC-20 transfer hook changed between v4 and v5. Read the installed ERC20.sol to confirm which function is virtual before recommending an override.

CLI Generators

The @openzeppelin/contracts-cli package generates reference OpenZeppelin contract implementations from the command line. Use it as the reference source in the generate-compare-apply workflow whenever a command exists for the contract type.

Discovering Commands and Options

Run npx @openzeppelin/contracts-cli --help to list available commands. Each command corresponds to a contract type (e.g., solidity-erc20, cairo-erc721, stellar-fungible). Run npx @openzeppelin/contracts-cli <command> --help to see the available options. Do not rely on prior knowledge of what options exist; check --help at the start of a conversation since the CLI may have been updated.

Generate-Compare-Apply Shortcut

When a CLI command exists for the contract type, pipe generated output to temporary files and diff them to keep generated contract code out of the conversation context:

1. Generate baseline — run with only required options, all features disabled, pipe to a file:

   npx @openzeppelin/contracts-cli solidity-erc20 --name MyToken --symbol MTK > /tmp/oz-baseline.sol
   

2. Generate with feature — run again with the feature enabled, pipe to a second file:

   npx @openzeppelin/contracts-cli solidity-erc20 --name MyToken --symbol MTK --pausable > /tmp/oz-variant.sol
   

3. Compare — diff the two files to identify exactly what changed (imports, inheritance, state, constructor, functions, modifiers):

   diff /tmp/oz-baseline.sol /tmp/oz-variant.sol
   

4. Apply — edit the user's existing contract to add the discovered changes

For interacting features (e.g., access control + upgradeability), generate a combined variant as well.

When No CLI Command Exists or a Feature Is Not Covered

The absence of a CLI command does NOT mean the library lacks support. It only means there is no generator for that contract type. Always fall back to the generic pattern discovery methodology in Pattern Discovery and Integration.

Similarly, when a CLI command exists but does not expose an option for a specific feature, do not stop there. Fall back to pattern discovery for that feature: read the installed library source to find the relevant component, extract the integration requirements, and apply them to the user's contract.