erc-721

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.

ERC-721 NFT Integration for Scaffold-ETH 2

Prerequisites

Check if ./packages/nextjs/scaffold.config.ts exists directly in the current working directory (do not search subdirectories). If it doesn't exist, this is not a Scaffold-ETH 2 project. Follow the instructions at https://docs.scaffoldeth.io/SKILL.md to scaffold it first. If it exists, continue directly with building.

Overview

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.

Dependencies

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:

• Hardhat: @openzeppelin/contracts in packages/hardhat/package.json
• Foundry: installed via forge install OpenZeppelin/openzeppelin-contracts, with remapping @openzeppelin/contracts/=lib/openzeppelin-contracts/contracts/

No new frontend dependencies are required.

Smart Contract

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 NFTs
• ERC721Pausable: admin can freeze all transfers
• ERC721Votes: governance checkpoints, each NFT = 1 vote
• ERC721Royalty: 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.

OpenZeppelin v5 changes to be aware of

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.
• Custom errors replaced revert strings (e.g. ERC721NonexistentToken, ERC721InsufficientApproval)
• Ownable requires explicit owner: Ownable(msg.sender) instead of Ownable()
• ERC721URIStorage now emits ERC-4906 MetadataUpdate events when _setTokenURI is called

Metadata: The Part Most People Get Wrong

The metadata JSON schema

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.

On-chain vs off-chain metadata

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

IPFS gotchas

About 20% of sampled NFTs have broken or expired metadata links. Common causes:

• Unpinned data gets garbage collected. IPFS nodes drop data nobody is actively pinning. If the original pinner stops, the data vanishes.
• Gateway URLs vs protocol URIs. Use ipfs://QmCID (content-addressed, portable) not https://gateway.pinata.cloud/ipfs/QmCID (depends on one gateway staying up).
• Base URI must end with /. 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.
• File naming. If using base URI + token ID, metadata files must be named 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: Convenient but Expensive

ERC721Enumerable maintains four additional data structures that get updated on every mint and transfer. Concrete gas comparison:

• Minting 5 tokens with ERC721Enumerable: ~566,000 gas
• Minting 5 tokens with ERC721A: ~104,000 gas (5.5x cheaper)

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 as an alternative

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:

• Requires sequential token IDs (no random IDs)
• First transfer after a batch mint is more expensive (must initialize ownership)
• Not an OpenZeppelin extension; separate dependency from erc721a npm package

Security: The Reentrancy You Didn't Expect

_safeMint and safeTransferFrom call external code

Both _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 permission

setApprovalForAll(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.

Flash loan governance attacks

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.

Royalties (ERC-2981)

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:

• OpenSea: ended mandatory enforcement Aug 2023. Added ERC-721C support Apr 2024 for opt-in on-chain enforcement.
• Blur: enforces only a 0.5% minimum on most collections.

ERC-721C by Limit Break attempted to solve this by restricting transfers to whitelisted operator contracts. Adoption is growing but not universal.

Soulbound Tokens (ERC-5192)

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;
}

Well-Known NFT Contracts (Ethereum Mainnet)

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