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Add an ERC-721 NFT contract to a Scaffold-ETH 2 project. Use when the user wants to: create an NFT collection, deploy an ERC-721, add NFT minting, build an NFT gallery or transfer UI, or work with non-fungible tokens in SE-2.
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Baseado na classificação ocupacional SOC
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| name | erc-721 |
| description | Add an ERC-721 NFT contract to a Scaffold-ETH 2 project. Use when the user wants to: create an NFT collection, deploy an ERC-721, add NFT minting, build an NFT gallery or transfer UI, or work with non-fungible tokens in SE-2. |
This skill is designed for Scaffold-ETH 2 (SE-2) projects. If the user is not already inside an SE-2 project, use the ethereum-app-builder skill from this same skill package to scaffold one first, then come back here to add ERC-721.
How to check: look for packages/nextjs/ and either packages/hardhat/ or packages/foundry/ in the project root, along with a root package.json with SE-2 workspace scripts (yarn chain, yarn deploy, yarn start).
ERC-721 is the standard interface for non-fungible tokens (NFTs) on Ethereum. This skill covers adding an ERC-721 contract to a Scaffold-ETH 2 project using OpenZeppelin's ERC-721 implementation, along with deployment scripts and a frontend for minting, listing, and transferring NFTs.
For anything not covered here, refer to the OpenZeppelin ERC-721 docs or search the web. This skill focuses on what's hard to discover: SE-2 integration specifics, common pitfalls, and ERC-721 gotchas.
Scaffold-ETH 2 (SE-2) is a yarn (v3) monorepo for building dApps on Ethereum. It comes in two flavors based on the Solidity framework:
packages/hardhat/contracts/, deploy scripts at packages/hardhat/deploy/packages/foundry/contracts/, deploy scripts at packages/foundry/script/Check which exists in the project to know the flavor. Both flavors share:
packages/nextjs/: React frontend (Next.js App Router, Tailwind + DaisyUI, RainbowKit, Wagmi, Viem). Uses ~~ path alias for imports.packages/nextjs/contracts/deployedContracts.ts: auto-generated after yarn deploy, contains ABIs, addresses, and deployment block numbers for all contracts, keyed by chain ID.packages/nextjs/scaffold.config.ts: project config including targetNetworks (array of viem chain objects).package.json: monorepo scripts that proxy into workspaces (e.g. yarn chain, yarn deploy, yarn start).SE-2 uses @scaffold-ui/components for blockchain/Ethereum components (addresses, balances, etc.) and DaisyUI + Tailwind for general component and styling.
The deployment scripts go alongside the existing deploy scripts, and the frontend page goes in the nextjs package. After deployment, deployedContracts.ts auto-generates the ABI and address, so the frontend can interact with the NFT contract using SE-2's scaffold hooks (useScaffoldReadContract, useScaffoldWriteContract, useScaffoldContract).
Look at the actual project structure and contracts before setting things up. Adapt to what's there rather than following this skill rigidly.
OpenZeppelin contracts are already included in SE-2's Hardhat and Foundry setups, so no additional dependency installation is needed. If for some reason they're missing:
@openzeppelin/contracts in packages/hardhat/package.jsonforge install OpenZeppelin/openzeppelin-contracts, with remapping @openzeppelin/contracts/=lib/openzeppelin-contracts/contracts/No new frontend dependencies are required.
The token contract extends OpenZeppelin's ERC721 base. Import path: @openzeppelin/contracts/token/ERC721/ERC721.sol. The constructor takes a token name and symbol.
Syntax reference for a basic NFT with open minting and IPFS metadata:
// SPDX-License-Identifier: MIT
pragma solidity >=0.8.0 <0.9.0;
import "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import "@openzeppelin/contracts/token/ERC721/extensions/ERC721Enumerable.sol";
contract MyNFT is ERC721Enumerable {
uint256 public tokenIdCounter;
constructor() ERC721("MyNFT", "MNFT") {}
function mintItem(address to) public returns (uint256) {
tokenIdCounter++;
_safeMint(to, tokenIdCounter);
return tokenIdCounter;
}
function tokenURI(uint256 tokenId) public view override returns (string memory) {
_requireOwned(tokenId);
return string.concat(_baseURI(), Strings.toString(tokenId));
}
function _baseURI() internal pure override returns (string memory) {
return "ipfs://YourCID/";
}
}
Adapt the contract based on the user's requirements. Available extensions (all under @openzeppelin/contracts/token/ERC721/extensions/):
ERC721Enumerable: on-chain enumeration of all tokens and per-owner tokens. Enables totalSupply(), tokenByIndex(), tokenOfOwnerByIndex(). Convenient but expensive (see gas section below).ERC721URIStorage: per-token URI storage via _setTokenURI(). Emits ERC-4906 MetadataUpdate events in v5.ERC721Burnable: lets token owners destroy their NFTsERC721Pausable: admin can freeze all transfersERC721Votes: governance checkpoints, each NFT = 1 voteERC721Royalty: ERC-2981 royalty info (see royalties section below)ERC721Consecutive: batch minting during construction (ERC-2309)See OpenZeppelin's ERC-721 extensions for the full list.
If referencing older tutorials or code, note these breaking changes in OpenZeppelin v5:
_beforeTokenTransfer and _afterTokenTransfer hooks are gone. Replaced by a single _update(address to, uint256 tokenId, address auth) override point that handles mint, transfer, and burn.ERC721NonexistentToken, ERC721InsufficientApproval)Ownable requires explicit owner: Ownable(msg.sender) instead of Ownable()ERC721URIStorage now emits ERC-4906 MetadataUpdate events when _setTokenURI is calledDeploy script goes in packages/hardhat/deploy/. SE-2 uses hardhat-deploy, so the script exports a DeployFunction. Use a filename like 01_deploy_my_nft.ts (numbered to control deploy order). The autoMine flag speeds up local deployments.
Add a deploy script in packages/foundry/script/ and wire it into the main Deploy.s.sol. SE-2's Foundry setup uses a ScaffoldETHDeploy base contract and DeployHelpers.s.sol. Import and call the new deploy script from Deploy.s.sol's run function.
ERC-721 metadata follows a standard JSON structure returned by tokenURI():
{
"name": "My NFT #1",
"description": "Description of the NFT",
"image": "ipfs://QmImageCID",
"attributes": [
{ "trait_type": "Color", "value": "Blue" },
{ "trait_type": "Rarity", "value": "Rare" }
]
}
The attributes array is not in the EIP but is the de facto standard used by OpenSea, Blur, and every other marketplace. Without it, traits won't display.
| Factor | On-chain (base64/SVG) | Off-chain (IPFS/Arweave) |
|---|---|---|
| Permanence | Permanent as long as Ethereum exists | Depends on pinning/persistence |
| Gas cost | Very expensive (~128KB payload ceiling) | Cheap (just store a URI string) |
| Mutability | Immutable once deployed | Can disappear if unpinned |
| Best for | Small collections, generative art | Large collections, rich media |
About 20% of sampled NFTs have broken or expired metadata links. Common causes:
ipfs://QmCID (content-addressed, portable) not https://gateway.pinata.cloud/ipfs/QmCID (depends on one gateway staying up)./. OpenZeppelin's tokenURI() concatenates _baseURI() + tokenId.toString(). If the base URI is ipfs://QmCID without a trailing slash, token 42 becomes ipfs://QmCID42 instead of ipfs://QmCID/42.0, 1, 2 etc. (no .json extension) unless you override tokenURI() to append it.For permanent storage, consider Arweave or a paid IPFS pinning service (Pinata, Filebase).
ERC721Enumerable maintains four additional data structures that get updated on every mint and transfer. Concrete gas comparison:
When to use it: Small collections, learning/demos, when you need on-chain enumeration without an indexer.
When to skip it: Large collections (1k+ tokens), gas-sensitive mints. Use a simple counter for totalSupply() and index token ownership off-chain using Transfer events (via a subgraph or Ponder, both available as SE-2 skills).
ERC721A by Azuki makes batch minting cost nearly the same as minting a single token. A 10-token mint costs ~110,000 gas vs ~1,100,000+ with ERC721Enumerable. It works by lazily initializing ownership: only the first token in a batch gets an ownership record, and later tokens infer ownership by scanning backwards.
Trade-offs:
erc721a npm package_safeMint and safeTransferFrom call external codeBoth _safeMint and safeTransferFrom invoke onERC721Received() on the recipient if it's a contract. This is an external call that happens after the token has been minted/transferred, creating a reentrancy vector.
Real exploit (HypeBears, Feb 2022): The contract tracked per-address minting limits but updated state after _safeMint. An attacker's onERC721Received callback called mintNFT again before the limit was recorded, bypassing the per-address cap entirely.
// VULNERABLE: state update after _safeMint
function mintNFT() public {
require(!addressMinted[msg.sender], "Already minted");
_safeMint(msg.sender, tokenId); // calls onERC721Received on attacker
addressMinted[msg.sender] = true; // too late, attacker already re-entered
}
// SAFE: state update before _safeMint
function mintNFT() public {
require(!addressMinted[msg.sender], "Already minted");
addressMinted[msg.sender] = true; // update state first
_safeMint(msg.sender, tokenId);
}
Mitigations: Update state before _safeMint/safeTransferFrom (checks-effects-interactions pattern), or use OpenZeppelin's ReentrancyGuard (nonReentrant modifier).
setApprovalForAll is a dangerous permissionsetApprovalForAll(operator, true) grants an operator control over all of an owner's NFTs in that collection. Phishing attacks trick users into signing this for malicious operators. Once approved, the attacker can transfer away every NFT the victim owns. Most marketplaces require setApprovalForAll to list NFTs, which is why phishing is so effective.
NFTs used for governance (each NFT = 1 vote) can be manipulated via flash loans: borrow NFTs, vote, return them. Use ERC721Votes with checkpoints and voting delays rather than raw balanceOf() for governance.
ERC-2981 defines a standard royaltyInfo(tokenId, salePrice) function that returns the royalty receiver and amount. OpenZeppelin provides ERC721Royalty to implement this.
The critical thing to know: ERC-2981 is advisory, not enforceable. The standard provides an interface for querying royalty info, but nothing forces marketplaces to honor it. Anyone can transfer an NFT via transferFrom without paying royalties.
Current marketplace stance:
ERC-721C by Limit Break attempted to solve this by restricting transfers to whitelisted operator contracts. Adoption is growing but not universal.
For non-transferable NFTs (credentials, memberships, achievements), ERC-5192 adds a minimal locked(tokenId) interface. In OpenZeppelin v5, the simplest approach is overriding _update:
function _update(address to, uint256 tokenId, address auth)
internal override returns (address)
{
address from = super._update(to, tokenId, auth);
require(from == address(0) || to == address(0), "Non-transferable");
return from;
}
| Collection | Address | Notes |
|---|---|---|
| CryptoPunks (original) | 0xb47e3cd837dDF8e4c57F05d70Ab865de6e193BBB | NOT ERC-721. Pre-dates the standard (June 2017). Custom contract with its own marketplace built in. Had a critical bug where sale ETH was credited to the buyer, not the seller. |
| Wrapped CryptoPunks | 0xb7F7F6C52F2e2fdb1963Eab30438024864c313F6 | ERC-721 wrapper around original punks |
| Bored Ape Yacht Club | 0xBC4CA0EdA7647A8aB7C2061c2E118A18a936f13D | 10,000 apes, standard ERC-721 |
| Azuki | 0xED5AF388653567Af2F388E6224dC7C4b3241C544 | Uses ERC721A for gas-optimized batch minting |
| Pudgy Penguins | 0xBd3531dA5CF5857e7CfAA92426877b022e612cf8 | 8,888 penguins |