Evm Nfts

EVM NFTs

ERC-721 and ERC-1155 are the two NFT standards on EVM chains. ERC-721 represents unique tokens (1-of-1 art, PFPs, deeds), while ERC-1155 represents semi-fungible tokens (game items, editions, tickets). This skill covers secure minting patterns, metadata standards, royalty implementation, and marketplace integration using OpenZeppelin v5.6.1 and Seaport 1.6.

What You Probably Got Wrong

• _safeMint calls onERC721Received on the receiver -- reentrancy vector. The _safeMint function makes an external call to the receiver if it is a contract. A malicious receiver contract can re-enter your mint function and mint more tokens than allowed. Use ReentrancyGuard on ALL mint functions, without exception.

• Allowlist signatures without EIP-712 domain separators are replayed across chains and contracts. Always include block.chainid and address(this) in the domain separator. Signatures MUST include a per-address nonce (tracked onchain in a mapping) and a deadline (block.timestamp expiry). Without nonces, a single valid signature can be replayed indefinitely.

• transferFrom does NOT check onERC721Received -- only safeTransferFrom does. If you send an ERC-721 token to a contract using transferFrom, the contract has no way to react or reject the transfer. The token can be permanently locked. Always use safeTransferFrom when the recipient might be a contract.

• Royalties (ERC-2981) are NOT enforced onchain. ERC-2981 is a read-only interface. Marketplaces query royaltyInfo() and can choose to ignore it. For practical enforcement, use ERC-721C (Limit Break's transfer validator pattern) which hooks into transfer functions to enforce payment.

• ERC-721 has TWO independent approval mechanisms. approve(to, tokenId) grants approval for a single token. setApprovalForAll(operator, true) grants blanket approval for all tokens. These are independent -- revoking one does not affect the other. Users commonly forget setApprovalForAll remains active after individual approvals are cleared.

• tokenURI returns a URI that resolves to JSON metadata, not a URL to an image. The URI points to a JSON document with name, description, image, and optional attributes. The image field inside that JSON is the actual image URL.

• ERC-1155 has no name() or symbol() in the standard. Use uri(id) to get the metadata URI for a specific token ID. OpenZeppelin's ERC1155 implementation does not expose name/symbol by default.

• {id} substitution in ERC-1155 URIs is client-side. The uri() function returns a template like https://api.example.com/token/{id}.json. The {id} placeholder must be replaced by the client with the hex token ID, zero-padded to 64 characters, lowercase, no 0x prefix. Example: token ID 1 becomes 0000000000000000000000000000000000000000000000000000000000000001.

• balanceOf returns a count, not token IDs. For ERC-721, getting the list of owned token IDs requires tokenOfOwnerByIndex (only available if the contract extends ERC721Enumerable). Without enumeration, you must index Transfer events off-chain.

• tokenURI returns empty string for non-existent tokens in some implementations. OpenZeppelin v5.6.1's ERC721URIStorage returns empty string if the token has not been minted. It does NOT revert. Always check _ownerOf(tokenId) != address(0) before returning metadata if you want to revert for non-existent tokens.

• Seaport 1.6 is the current marketplace protocol. Same deterministic address on all EVM chains: 0x0000000000000068F116A894984e2DB1123eB395. Do not use Seaport 1.4 or 1.5 -- they have known issues.

• Reservoir is DEPRECATED. Reservoir shut down in October 2025. Use Seaport directly or the OpenSea API for marketplace integration.

OpenZeppelin v5.6.1 Patterns

OpenZeppelin v5 introduced breaking changes from v4. All code in this skill targets v5.6.1.

forge install OpenZeppelin/openzeppelin-contracts@v5.6.1

ERC-721

Interface

interface IERC721 {
    event Transfer(address indexed from, address indexed to, uint256 indexed tokenId);
    event Approval(address indexed owner, address indexed approved, uint256 indexed tokenId);
    event ApprovalForAll(address indexed owner, address indexed operator, bool approved);

    function balanceOf(address owner) external view returns (uint256);
    function ownerOf(uint256 tokenId) external view returns (address);
    function safeTransferFrom(address from, address to, uint256 tokenId, bytes calldata data) external;
    function safeTransferFrom(address from, address to, uint256 tokenId) external;
    function transferFrom(address from, address to, uint256 tokenId) external;
    function approve(address to, uint256 tokenId) external;
    function setApprovalForAll(address operator, bool approved) external;
    function getApproved(uint256 tokenId) external view returns (address);
    function isApprovedForAll(address owner, address operator) external view returns (bool);
}

Minimal ERC-721 with Mint (OpenZeppelin v5.6.1)

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

import {ERC721} from "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";
import {ReentrancyGuard} from "@openzeppelin/contracts/utils/ReentrancyGuard.sol";

contract SimpleNFT is ERC721, Ownable, ReentrancyGuard {
    uint256 private _nextTokenId;
    uint256 public constant MAX_SUPPLY = 10_000;

    error MaxSupplyReached();

    constructor(address initialOwner)
        ERC721("SimpleNFT", "SNFT")
        Ownable(initialOwner)
    {}

    function mint(address to) external onlyOwner nonReentrant {
        uint256 tokenId = _nextTokenId++;
        if (tokenId >= MAX_SUPPLY) revert MaxSupplyReached();
        _safeMint(to, tokenId);
    }

    function totalSupply() external view returns (uint256) {
        return _nextTokenId;
    }
}

ERC721Enumerable

Adds tokenOfOwnerByIndex and tokenByIndex for on-chain enumeration. Increases gas cost for transfers by ~50%. Only use when on-chain enumeration is a hard requirement.

import {ERC721Enumerable} from "@openzeppelin/contracts/token/ERC721/extensions/ERC721Enumerable.sol";

contract EnumerableNFT is ERC721Enumerable, Ownable, ReentrancyGuard {
    uint256 private _nextTokenId;

    constructor(address initialOwner)
        ERC721("EnumerableNFT", "ENFT")
        Ownable(initialOwner)
    {}

    function _update(address to, uint256 tokenId, address auth)
        internal
        override(ERC721Enumerable)
        returns (address)
    {
        return super._update(to, tokenId, auth);
    }

    function _increaseBalance(address account, uint128 value)
        internal
        override(ERC721Enumerable)
    {
        super._increaseBalance(account, value);
    }

    function supportsInterface(bytes4 interfaceId)
        public
        view
        override(ERC721Enumerable)
        returns (bool)
    {
        return super.supportsInterface(interfaceId);
    }
}

ERC-1155

Interface

interface IERC1155 {
    event TransferSingle(address indexed operator, address indexed from, address indexed to, uint256 id, uint256 value);
    event TransferBatch(address indexed operator, address indexed from, address indexed to, uint256[] ids, uint256[] values);
    event ApprovalForAll(address indexed account, address indexed operator, bool approved);
    event URI(string value, uint256 indexed id);

    function balanceOf(address account, uint256 id) external view returns (uint256);
    function balanceOfBatch(address[] calldata accounts, uint256[] calldata ids) external view returns (uint256[] memory);
    function setApprovalForAll(address operator, bool approved) external;
    function isApprovedForAll(address account, address operator) external view returns (bool);
    function safeTransferFrom(address from, address to, uint256 id, uint256 amount, bytes calldata data) external;
    function safeBatchTransferFrom(address from, address to, uint256[] calldata ids, uint256[] calldata amounts, bytes calldata data) external;
}

ERC-1155 with Supply Tracking

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

import {ERC1155} from "@openzeppelin/contracts/token/ERC1155/ERC1155.sol";
import {ERC1155Supply} from "@openzeppelin/contracts/token/ERC1155/extensions/ERC1155Supply.sol";
import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";
import {ReentrancyGuard} from "@openzeppelin/contracts/utils/ReentrancyGuard.sol";

contract GameItems is ERC1155, ERC1155Supply, Ownable, ReentrancyGuard {
    uint256 public constant SWORD = 0;
    uint256 public constant SHIELD = 1;
    uint256 public constant POTION = 2;

    mapping(uint256 id => uint256 cap) public maxSupply;

    error ExceedsMaxSupply(uint256 id);

    constructor(address initialOwner)
        ERC1155("https://api.example.com/items/{id}.json")
        Ownable(initialOwner)
    {
        maxSupply[SWORD] = 1_000;
        maxSupply[SHIELD] = 5_000;
        maxSupply[POTION] = 100_000;
    }

    function mint(address to, uint256 id, uint256 amount) external onlyOwner nonReentrant {
        if (totalSupply(id) + amount > maxSupply[id]) revert ExceedsMaxSupply(id);
        _mint(to, id, amount, "");
    }

    function mintBatch(address to, uint256[] calldata ids, uint256[] calldata amounts)
        external
        onlyOwner
        nonReentrant
    {
        for (uint256 i = 0; i < ids.length; i++) {
            if (totalSupply(ids[i]) + amounts[i] > maxSupply[ids[i]]) revert ExceedsMaxSupply(ids[i]);
        }
        _mintBatch(to, ids, amounts, "");
    }

    function _update(address from, address to, uint256[] memory ids, uint256[] memory values)
        internal
        override(ERC1155, ERC1155Supply)
    {
        super._update(from, to, ids, values);
    }
}

Metadata Standards

Metadata JSON Schema (ERC-721)

{
  "name": "Token #1",
  "description": "Description of the token",
  "image": "ipfs://Qm.../1.png",
  "external_url": "https://example.com/token/1",
  "attributes": [
    { "trait_type": "Color", "value": "Red" },
    { "trait_type": "Level", "value": 5, "display_type": "number" },
    { "trait_type": "Power", "value": 85, "max_value": 100 }
  ]
}

Metadata Storage Comparison

Contract-Level Metadata (contractURI)

OpenSea and other marketplaces read contractURI() for collection-level metadata:

function contractURI() external pure returns (string memory) {
    return "ipfs://QmCollectionMetadataHash";
}

The JSON at that URI:

{
  "name": "Collection Name",
  "description": "Collection description",
  "image": "ipfs://QmCollectionImage",
  "external_link": "https://example.com",
  "seller_fee_basis_points": 500,
  "fee_recipient": "0xRoyaltyRecipient..."
}

ERC-4906: Metadata Update Events

Signal marketplaces to refresh metadata for specific tokens or ranges:

import {IERC4906} from "@openzeppelin/contracts/interfaces/IERC4906.sol";

contract UpdatableNFT is ERC721, IERC4906 {
    function updateMetadata(uint256 tokenId) external onlyOwner {
        emit MetadataUpdate(tokenId);
    }

    function updateAllMetadata() external onlyOwner {
        emit BatchMetadataUpdate(0, type(uint256).max);
    }
}

Royalties (ERC-2981)

ERC-2981 defines a standard royaltyInfo(tokenId, salePrice) function that returns the royalty recipient and amount. Marketplaces query this but are NOT required to enforce it.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

import {ERC721} from "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import {ERC2981} from "@openzeppelin/contracts/token/common/ERC2981.sol";
import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";
import {ReentrancyGuard} from "@openzeppelin/contracts/utils/ReentrancyGuard.sol";

contract RoyaltyNFT is ERC721, ERC2981, Ownable, ReentrancyGuard {
    uint256 private _nextTokenId;

    constructor(address initialOwner, address royaltyReceiver)
        ERC721("RoyaltyNFT", "RNFT")
        Ownable(initialOwner)
    {
        // 5% royalty (500 basis points) on all tokens by default
        _setDefaultRoyalty(royaltyReceiver, 500);
    }

    function mint(address to) external onlyOwner nonReentrant {
        _safeMint(to, _nextTokenId++);
    }

    /// @notice Override for specific token royalty
    function setTokenRoyalty(uint256 tokenId, address receiver, uint96 feeNumerator)
        external
        onlyOwner
    {
        _setTokenRoyalty(tokenId, receiver, feeNumerator);
    }

    function supportsInterface(bytes4 interfaceId)
        public
        view
        override(ERC721, ERC2981)
        returns (bool)
    {
        return super.supportsInterface(interfaceId);
    }
}

Common Minting Patterns

Allowlist with Merkle Proof

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

import {ERC721} from "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import {MerkleProof} from "@openzeppelin/contracts/utils/cryptography/MerkleProof.sol";
import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";
import {ReentrancyGuard} from "@openzeppelin/contracts/utils/ReentrancyGuard.sol";

contract MerkleAllowlistNFT is ERC721, Ownable, ReentrancyGuard {
    uint256 private _nextTokenId;
    bytes32 public merkleRoot;
    mapping(address minter => bool claimed) public hasClaimed;

    error AlreadyClaimed();
    error InvalidProof();

    constructor(address initialOwner, bytes32 _merkleRoot)
        ERC721("AllowlistNFT", "ANFT")
        Ownable(initialOwner)
    {
        merkleRoot = _merkleRoot;
    }

    function allowlistMint(bytes32[] calldata proof) external nonReentrant {
        if (hasClaimed[msg.sender]) revert AlreadyClaimed();

        bytes32 leaf = keccak256(abi.encodePacked(msg.sender));
        if (!MerkleProof.verify(proof, merkleRoot, leaf)) revert InvalidProof();

        hasClaimed[msg.sender] = true;
        _safeMint(msg.sender, _nextTokenId++);
    }
}

Allowlist with EIP-712 Signature (Nonce + Deadline)

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

import {ERC721} from "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import {EIP712} from "@openzeppelin/contracts/utils/cryptography/EIP712.sol";
import {ECDSA} from "@openzeppelin/contracts/utils/cryptography/ECDSA.sol";
import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";
import {ReentrancyGuard} from "@openzeppelin/contracts/utils/ReentrancyGuard.sol";

/// @notice EIP-712 signed allowlist with per-address nonce and deadline to prevent replay
contract SignedAllowlistNFT is ERC721, EIP712, Ownable, ReentrancyGuard {
    using ECDSA for bytes32;

    uint256 private _nextTokenId;
    address public signer;
    mapping(address minter => uint256 nonce) public nonces;

    // EIP-712 domain includes contract address and chainId automatically via EIP712 base
    bytes32 private constant MINT_TYPEHASH =
        keccak256("Mint(address minter,uint256 nonce,uint256 deadline)");

    error InvalidSignature();
    error SignatureExpired();

    constructor(address initialOwner, address _signer)
        ERC721("SignedNFT", "SGNFT")
        EIP712("SignedAllowlistNFT", "1")
        Ownable(initialOwner)
    {
        signer = _signer;
    }

    function allowlistMint(uint256 deadline, bytes calldata signature) external nonReentrant {
        if (block.timestamp > deadline) revert SignatureExpired();

        uint256 currentNonce = nonces[msg.sender];

        bytes32 structHash = keccak256(
            abi.encode(MINT_TYPEHASH, msg.sender, currentNonce, deadline)
        );
        bytes32 digest = _hashTypedDataV4(structHash);
        address recovered = digest.recover(signature);

        if (recovered != signer) revert InvalidSignature();

        // Increment nonce BEFORE external call (_safeMint) -- CEI pattern
        nonces[msg.sender] = currentNonce + 1;
        _safeMint(msg.sender, _nextTokenId++);
    }
}

Dutch Auction

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

import {ERC721} from "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";
import {ReentrancyGuard} from "@openzeppelin/contracts/utils/ReentrancyGuard.sol";

/// @notice Price decreases linearly from startPrice to endPrice over the auction duration
contract DutchAuctionNFT is ERC721, Ownable, ReentrancyGuard {
    uint256 private _nextTokenId;
    uint256 public constant MAX_SUPPLY = 10_000;

    uint256 public immutable auctionStart;
    uint256 public immutable auctionDuration;
    uint256 public immutable startPrice;
    uint256 public immutable endPrice;

    error AuctionNotStarted();
    error MaxSupplyReached();
    error InsufficientPayment();

    constructor(
        address initialOwner,
        uint256 _auctionStart,
        uint256 _auctionDuration,
        uint256 _startPrice,
        uint256 _endPrice
    )
        ERC721("DutchAuctionNFT", "DANFT")
        Ownable(initialOwner)
    {
        auctionStart = _auctionStart;
        auctionDuration = _auctionDuration;
        startPrice = _startPrice;
        endPrice = _endPrice;
    }

    function currentPrice() public view returns (uint256) {
        if (block.timestamp < auctionStart) revert AuctionNotStarted();
        uint256 elapsed = block.timestamp - auctionStart;
        if (elapsed >= auctionDuration) return endPrice;

        uint256 priceDrop = ((startPrice - endPrice) * elapsed) / auctionDuration;
        return startPrice - priceDrop;
    }

    function mint() external payable nonReentrant {
        uint256 price = currentPrice();
        if (msg.value < price) revert InsufficientPayment();
        uint256 tokenId = _nextTokenId++;
        if (tokenId >= MAX_SUPPLY) revert MaxSupplyReached();

        _safeMint(msg.sender, tokenId);

        uint256 refund = msg.value - price;
        if (refund > 0) {
            (bool sent, ) = payable(msg.sender).call{value: refund}("");
            require(sent);
        }
    }

    function withdraw() external onlyOwner {
        (bool sent, ) = payable(owner()).call{value: address(this).balance}("");
        require(sent);
    }
}

Commit-Reveal for High-Value Mints

Prevents front-running and sniping by splitting mint into two transactions: commit (hash of intent) then reveal (actual mint after delay).

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

import {ERC721} from "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";
import {ReentrancyGuard} from "@openzeppelin/contracts/utils/ReentrancyGuard.sol";

/// @notice Two-phase mint: commit a hash, then reveal after a delay to prevent front-running
contract CommitRevealNFT is ERC721, Ownable, ReentrancyGuard {
    uint256 private _nextTokenId;
    uint256 public constant REVEAL_DELAY = 2;
    uint256 public constant REVEAL_WINDOW = 256;

    struct Commitment {
        uint64 blockNumber;
        bool revealed;
    }

    mapping(bytes32 commitHash => Commitment) public commitments;

    error CommitmentAlreadyExists();
    error CommitmentNotFound();
    error RevealTooEarly();
    error RevealWindowExpired();
    error AlreadyRevealed();

    constructor(address initialOwner)
        ERC721("CommitRevealNFT", "CRNFT")
        Ownable(initialOwner)
    {}

    /// @notice Phase 1: submit keccak256(abi.encodePacked(msg.sender, salt))
    function commit(bytes32 commitHash) external {
        if (commitments[commitHash].blockNumber != 0) revert CommitmentAlreadyExists();
        commitments[commitHash] = Commitment({
            blockNumber: uint64(block.number),
            revealed: false
        });
    }

    /// @notice Phase 2: reveal with original salt after REVEAL_DELAY blocks
    function reveal(bytes32 salt) external nonReentrant {
        bytes32 commitHash = keccak256(abi.encodePacked(msg.sender, salt));
        Commitment storage c = commitments[commitHash];

        if (c.blockNumber == 0) revert CommitmentNotFound();
        if (c.revealed) revert AlreadyRevealed();
        if (block.number < c.blockNumber + REVEAL_DELAY) revert RevealTooEarly();
        // blockhash only available for last 256 blocks
        if (block.number > c.blockNumber + REVEAL_WINDOW) revert RevealWindowExpired();

        c.revealed = true;
        _safeMint(msg.sender, _nextTokenId++);
    }
}

Free Claim (One Per Address)

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

import {ERC721} from "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";
import {ReentrancyGuard} from "@openzeppelin/contracts/utils/ReentrancyGuard.sol";

contract FreeClaimNFT is ERC721, Ownable, ReentrancyGuard {
    uint256 private _nextTokenId;
    uint256 public constant MAX_SUPPLY = 5_000;
    bool public claimActive;

    mapping(address claimer => bool claimed) public hasClaimed;

    error ClaimNotActive();
    error AlreadyClaimed();
    error MaxSupplyReached();

    constructor(address initialOwner)
        ERC721("FreeClaimNFT", "FREE")
        Ownable(initialOwner)
    {}

    function setClaimActive(bool active) external onlyOwner {
        claimActive = active;
    }

    function claim() external nonReentrant {
        if (!claimActive) revert ClaimNotActive();
        if (hasClaimed[msg.sender]) revert AlreadyClaimed();
        uint256 tokenId = _nextTokenId++;
        if (tokenId >= MAX_SUPPLY) revert MaxSupplyReached();

        hasClaimed[msg.sender] = true;
        _safeMint(msg.sender, tokenId);
    }
}

Marketplace Integration (Seaport 1.6)

Last verified: March 2026

Seaport 1.6 is OpenSea's marketplace protocol, deployed at the same deterministic address on all EVM chains: 0x0000000000000068F116A894984e2DB1123eB395.

Approving Seaport

Before listing, the NFT owner must approve Seaport as an operator:

nftContract.setApprovalForAll(0x0000000000000068F116A894984e2DB1123eB395, true);

Creating a Listing (TypeScript with viem)

import { createWalletClient, http, type Address } from "viem";
import { mainnet } from "viem/chains";
import { privateKeyToAccount } from "viem/accounts";

const SEAPORT = "0x0000000000000068F116A894984e2DB1123eB395" as const;

// Seaport order components -- ERC-721 listing for ETH
interface OrderParameters {
  offerer: Address;
  zone: Address;
  offer: Array<{
    itemType: number;    // 2 = ERC721, 3 = ERC1155
    token: Address;
    identifierOrCriteria: bigint;
    startAmount: bigint;
    endAmount: bigint;
  }>;
  consideration: Array<{
    itemType: number;    // 0 = ETH, 1 = ERC20
    token: Address;
    identifierOrCriteria: bigint;
    startAmount: bigint;
    endAmount: bigint;
    recipient: Address;
  }>;
  orderType: number;     // 0 = FULL_OPEN
  startTime: bigint;
  endTime: bigint;
  zoneHash: `0x${string}`;
  salt: bigint;
  conduitKey: `0x${string}`;
  totalOriginalConsiderationItems: bigint;
}

See examples/marketplace-listing/ for a complete working example with order signing and fulfillment.

Seaport Item Types

Security Checklist for NFT Contracts

• ReentrancyGuard on ALL mint functions (_safeMint makes external calls)
• Supply cap enforced with require(tokenId < MAX_SUPPLY) or equivalent
• Per-wallet mint limit to prevent single-wallet hoarding
• Commit-reveal for high-value mints to prevent front-running/sniping
• Metadata freeze function (emit BatchMetadataUpdate then disable further changes)
• Never use tx.origin for authorization -- always msg.sender
• EIP-712 domain separators on all signature-based allowlists
• Nonce tracking for signature-based mints to prevent replay
• Deadline/expiry on all signed messages
• Ownable with explicit initial owner (OZ v5 requires constructor arg)
• supportsInterface correctly overridden when combining ERC721 + ERC2981 + ERC4906
• Withdrawal function for ETH from paid mints (owner-only, pull pattern)
• No hardcoded royalty recipient if it needs to be updatable

Related Skills

• openzeppelin -- contract library used for all implementations in this skill
• solidity-security -- comprehensive Solidity security patterns and audit checklist
• eip-reference -- detailed EIP specifications including ERC-721, ERC-1155, ERC-2981
• foundry -- testing and deployment framework for Solidity contracts
• viem -- TypeScript library for EVM interaction used in marketplace examples

References

• ERC-721: Non-Fungible Token Standard
• ERC-1155: Multi Token Standard
• ERC-2981: NFT Royalty Standard
• ERC-4906: EIP-721 Metadata Update Extension
• ERC-6551: Non-fungible Token Bound Accounts
• OpenZeppelin Contracts v5 Docs
• OpenZeppelin v5 Migration Guide
• Seaport Protocol Documentation
• Seaport 1.6 Source (ProjectOpenSea)
• OpenSea Metadata Standards
• Limit Break ERC-721C