Byzantium was a hard fork of the Ethereum network activated on 16 October 2017 at block 4,370,000. It was the first of two upgrades that made up the Metropolis stage of Ethereum's original roadmap, with the second, Constantinople, following in 2019. Byzantium bundled nine Ethereum Improvement Proposals that changed the block reward, delayed the network's difficulty bomb and added features for smart contract developers, including support for cryptographic operations used in zero-knowledge proofs.
Because Byzantium changed the consensus rules, every node had to upgrade to stay on the main chain. It was a planned, uncontroversial fork rather than a split like the one that created Ethereum Classic in 2016. The name refers to the Byzantine Empire, following Ethereum's tradition of naming upgrades after places and stages; it should not be confused with the Byzantine fault tolerance concept in distributed systems or the Byzantine generals problem.
Byzantium fork in plain terms
Think of a city that publishes a new edition of its building code. Builders must follow the new rules from a set date: some old rules are relaxed, some new safety features become possible, and the fee for new permits changes. Everyone who wants their buildings approved has to switch to the new code on the same day.
Byzantium was a new edition of Ethereum's rulebook. From a set block onwards, miners earned less ether per block, a planned slowdown in the network was postponed, and developers gained new tools for building applications. Because everyone switched together, the network kept running as one chain.
Main changes
Byzantium's nine EIPs included:
- Block reward cut. EIP-649 reduced the block reward from 5 ETH to 3 ETH, lowering the rate of new issuance.
- Difficulty bomb delay. The same EIP delayed the "difficulty bomb", a mechanism that gradually makes proof-of-work mining harder, by about a year and a half.
- REVERT opcode. EIP-140 let contracts stop execution and return unused gas with an error message, making failed transactions cheaper and easier to debug.
- STATICCALL. EIP-214 allowed contracts to call others in read-only mode, improving security.
- Return data. EIPs 211 and 140 together made it easier for contracts to handle data returned from other calls.
- Transaction status. EIP-658 added a success or failure flag to transaction receipts, helping wallets and explorers show whether a transaction worked.
- Cryptographic precompiles. EIPs 196, 197 and 198 added built-in support for elliptic curve operations and modular exponentiation, making zk-SNARK verification and RSA-style checks practical on-chain.
- Difficulty adjustment. EIP-100 changed how difficulty responds to uncle blocks.
Background and activation
Ethereum's founders laid out a roadmap with stages called Frontier, Homestead, Metropolis and Serenity. After Homestead in 2016, Metropolis was split into two hard forks to reduce risk. Byzantium was first activated on the Ropsten test network in September 2017, giving developers several weeks to test before the main network upgrade. Client software teams, including those behind Geth and Parity, released compatible versions ahead of the fork. Node operators, miners, exchanges and wallet providers were asked to upgrade before the target block, and exchanges paused deposits and withdrawals around the fork as a precaution. Activation went smoothly.
The upgrade also illustrated how Ethereum governance worked at the time. Proposals were discussed openly in developer calls and forums, client teams agreed which EIPs were ready, and the activation block was chosen to give users time to prepare. There was no on-chain vote; instead, the upgrade succeeded because the large majority of node operators and miners chose to run the new software. This informal process, sometimes called rough consensus, has continued for later forks, though it has been criticised for giving core developers considerable influence over the network's rules.
Impact
The block reward cut reduced Ethereum's annual issuance and was a step in a longer process of tightening supply, which later included EIP-1559 fee burning in 2021 and the move to proof of stake at the Merge in 2022, described under Beacon Chain. The developer features made smart contracts safer and cheaper to run, and the cryptographic precompiles laid groundwork for privacy and scaling tools, including later zero-knowledge rollups. The delay to the difficulty bomb kept mining viable while work on proof of stake continued; the bomb was postponed several more times before the Merge made it irrelevant. Byzantium is often cited as an example of a well-coordinated network upgrade.