Harmony ONE: The Blockchain, Its NFTs and Building a Marketplace

Harmony ONE is a proof-of-stake, EVM-compatible blockchain, and ONE is the token that pays its gas. This guide explains how the Harmony blockchain works, what changed after the 2022 Horizon bridge hack, where the network stands in 2026, and what building or moving a Harmony ONE NFT marketplace involves.

What the Harmony blockchain is

Harmony launched its mainnet in 2019 as a sharded layer 1 chain with a consensus protocol called Fast Byzantine Fault Tolerance (FBFT). Its staking scheme, Effective Proof-of-Stake (EPoS), was designed to spread stake across many validators.

Shard 0 is the beacon chain, and almost all smart contracts and NFTs run on shard 0. The network has reduced its active shards over time, and in October 2026 the public RPC reports two, shards 0 and 1. Blocks on shard 0 arrive about every 2 seconds.

Harmony runs the Ethereum Virtual Machine, so Solidity contracts deploy on it without changes. A Harmony address can be shown in the one1… format or the familiar 0x… format. The Harmony documentation lists RPC endpoints and network details.

The Horizon bridge hack

In June 2022, attackers took about $100 million from Horizon, Harmony's bridge to Ethereum. A multisig wallet controlled the bridge and reportedly needed only two of five signatures, and the attackers obtained two keys. The FBI later attributed the attack to North Korea's Lazarus Group.

Tokens bridged to Harmony from Ethereum, such as bridged stablecoins, lost their backing after the hack and traded far below face value. The community disputed the recovery proposals, and developer and user activity on Harmony fell steeply.

When two of the bridge's keys were stolen, every asset that relied on the bridge lost value. Marketplaces that accepted bridged tokens as payment were exposed to the same loss.

Harmony ONE in 2026

The network still produces blocks, and the core node software is still maintained. The Harmony release page lists several mainnet releases in 2026. The Bloom hard fork, which activated on July 13, 2026, changed leader rotation, block timestamp checks and cross-shard validation.

The Harmony project has also changed its public focus, and in October 2026 the harmony.one website describes Harmony as a "remix economy for AI video". NFT trading on Harmony stays far below its 2021 level. Several infrastructure services that once supported Harmony no longer support the chain.

Harmony ONE NFTs: HRC-721 and HRC-1155

HRC-721 and HRC-1155 are Harmony's names for ERC-721 and ERC-1155. The interfaces are the same, so a contract written for Ethereum works on Harmony. A Harmony ONE NFT pays gas in ONE and is traded through the same kind of exchange contract as on other EVM chains.

Building an NFT marketplace on Harmony

Some projects still have a reason to build on Harmony, because a community or game already runs there or the remaining users are enough for a small project. The build follows these steps:

1. Check the infrastructure. Confirm that reliable RPC endpoints, a block explorer with source verification, and an indexer or NFT API for Harmony are available.
2. Deploy standard contracts. Use ERC-721 or ERC-1155 with EIP-2981 royalties. Build and test them with Foundry or Hardhat pointed at Harmony's RPC.
3. Use an audited exchange contract. A Seaport-style contract executes signed listings and offers, so the team does not write its own matching code.
4. Accept native ONE. Avoid payment tokens whose backing depends on the old Horizon bridge.
5. Run an indexer if needed. Harmony's RPC is Ethereum-compatible, so standard EVM indexing tools work when no hosted provider covers Harmony.
6. Plan an exit. Store metadata on IPFS or Arweave, and keep a process for taking holder snapshots, so the collection can move to another chain if Harmony declines further.

Harmony compared with other EVM chains

All four chains run the EVM, so the same contracts and most of the same front end can move between them. A project that avoids chain-specific features can change chains later at low cost. The guides to a Polygon NFT marketplace, a BSC NFT marketplace and layer 2 NFTs cover those chains.

Moving a Harmony collection to another chain

A collection owner who wants to leave Harmony has two main options:

• Snapshot and airdrop. The owner records the current holders, airdrops an equal collection on the new chain, and declares the new contract the official one. Every holder receives the new NFT, including holders who no longer use their wallets.
• Burn or lock and claim. Holders burn or lock their NFTs on Harmony and claim the new NFT on the new chain. This method proves that each holder wants to move, but holders who never claim do not receive the new NFT.

With either option, the owner must announce the official contract clearly and ask marketplaces to update their listings. The guides to cross-chain NFT marketplaces and multi-chain NFT support cover the bridge and messaging side.

Points for builders on any chain

• A chain with low fees can still lose its users, so builders should check developer activity and infrastructure support as well as fees and speed.
• Users of a bridge must trust the people or the code that control the bridge. Canonical rollup bridges and audited messaging protocols with large validator sets carry less risk than a small multisig.
• Standard contracts and portable metadata make a move to another chain possible.
• A marketplace should not price listings in bridged tokens whose backing it cannot verify.

Frequently asked questions