---
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."
---

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.

Also read .agents/skills/openzeppelin/SKILL.md since ERC-721 contracts use OpenZeppelin, and that skill has critical guidance on reading the installed source for correct import syntax, override points, and constructor patterns.

Gotchas

Key pitfalls and gotchas to watch for when working with ERC-721.

1. _safeMint Reentrancy

_safeMint and safeTransferFrom invoke onERC721Received() on the recipient if it's a contract. This is an external call after the token has been minted, creating a reentrancy vector.

Real exploit (HypeBears, Feb 2022): State updated after _safeMint allowed attacker to re-enter and bypass per-address minting limits.

// VULNERABLE: state update after _safeMint
function mintNFT() public {
    require(!addressMinted[msg.sender], "Already minted");
    _safeMint(msg.sender, tokenId);
    addressMinted[msg.sender] = true;  // too late
}
 
// SAFE: state update before _safeMint
function mintNFT() public {
    require(!addressMinted[msg.sender], "Already minted");
    addressMinted[msg.sender] = true;  // update first
    _safeMint(msg.sender, tokenId);
}

2. On-Chain SVG Stack-Too-Deep

When generating SVG on-chain, the tokenURI or generateSVG function easily hits Solidity's 16-local-variable stack limit. Split SVG generation into multiple helper functions (e.g., _svgBackground, _svgShapes, _svgText) rather than building the entire SVG in one function.

3. Marketplace Metadata attributes Array

The attributes array in NFT metadata JSON is not in the ERC-721 EIP but is the de facto standard used by OpenSea, Blur, and every marketplace. Without it, traits won't display:

{
  "name": "My NFT #1",
  "description": "...",
  "image": "data:image/svg+xml;base64,...",
  "attributes": [
    { "trait_type": "Color", "value": "Blue" },
    { "trait_type": "Rarity", "value": "Rare" }
  ]
}

4. Required Overrides with ERC721Enumerable

When combining ERC721 + ERC721Enumerable, both define _update and _increaseBalance. You must explicitly override them or the contract won't compile:

function _update(address to, uint256 tokenId, address auth) internal override(ERC721, ERC721Enumerable) returns (address) {
    return super._update(to, tokenId, auth);
}
 
function _increaseBalance(address account, uint128 amount) internal override(ERC721, ERC721Enumerable) {
    super._increaseBalance(account, amount);
}
 
function supportsInterface(bytes4 interfaceId) public view override(ERC721, ERC721Enumerable) returns (bool) {
    return super.supportsInterface(interfaceId);
}

5. IPFS Base URI Trailing Slash

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.